Vaisala FD12P User Manual

Weather Sensor
FD12P
USER'S GUIDE
M210296en-A
May 2002
PUBLISHED BY
Vaisala Oyj Phone (int.): +358 9 8949 1 P.O. Box 26 Fax: +358 9 8949 2227 FIN-00421 Helsinki Finland
Visit our Internet pages at http://www.vaisala.com/
© Vaisala 2002
No part of this manual may be reproduced in any form or by any means, electronic or mechanical (including photocopying), nor may its contents be communicated to a third party without prior written permission of the copyright holder.
The contents are subject to change without prior notice.
Please observe that this manual does not create any legally binding obligations for Vaisala towards the customer or end user. All legally binding commitments and agreements are included exclusively in the applicable supply contract or Conditions of Sale.
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Table of Contents
CHAPTER 1
GENERAL INFORMATION ..........................................................................11
About This Manual..................................................................11
Contents of This Manual......................................................11
Version Information................................................................12
Related Manuals .....................................................................12
Safety .......................................................................................12
General Safety Considerations............................................12
Product Related Safety Precautions ...................................13
Safety Summary ............................................................. 13
Ground the Equipment ...................................................13
Radio Frequency Interference Statement (USA) ...........15
ESD Protection .................................................................... 15
Trademarks .............................................................................16
Warranty ..................................................................................16
CHAPTER 2
PRODUCT OVERVIEW ................................................................................ 17
Introduction.............................................................................17
Hardware Structure .............................................................17
Sensing Elements ..........................................................19
Electronics Enclosure .....................................................20
Structural Elements ........................................................ 20
Operating Principle ..............................................................21
Using FD12P .......................................................................22
Equipment Nomenclature ......................................................23
Specifications .........................................................................24
Mechanical Specifications ...................................................24
Electrical Specifications.......................................................24
Optical Specifications ..........................................................25
Capabilities and Limitations ..................................................26
Visibility Measurement Specifications .................................26
Weather Sensing Specifications..........................................26
Environmental Specifications ..............................................27
CHAPTER 3
INSTALLATION ............................................................................................ 29
Organizing Installation ...........................................................29
Location and Orientation .......................................................30
Grounding and Lightning Protection....................................32
Equipment Grounding..........................................................32
Internal Grounding ............................................................... 34
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Grounding for Testing Purposes......................................... 34
Grounding Remote Units and Communication Cable......... 34
Cable Selection ...................................................................... 35
Line Power Cabling............................................................. 35
Communication Cable......................................................... 35
Unloading and Unpacking..................................................... 36
Unpacking Procedure ......................................................... 36
Storage Information ............................................................ 36
Installation Procedures ......................................................... 37
Constructing the Foundation............................................... 37
Mounting When Casting the Pad................................... 38
Mounting to an Existing Surface.................................... 38
Assembling the FD12P ....................................................... 40
Attaching the DTS14B Temperature Sensor to the Mast ... 41
Connecting Cables.............................................................. 43
Basic Wiring ................................................................... 43
Communication Cable EMC-shielding........................... 46
Connecting a Background Luminance Sensor or a
Day/Night Switch to FD12P ........................................... 48
Communication Options ..................................................... 50
Serial Communications Settings.................................... 50
Serial Transmission RS-232.......................................... 50
Serial Multipoint Transmission RS-485 ......................... 51
Modem DMX21 .............................................................. 52
Indicators and Manual Controls..................................... 54
Indicators .................................................................. 54
Manual Controls ....................................................... 54
Analog Transmission ..................................................... 55
Connecting the Maintenance Terminal.......................... 55
Startup Testing.................................................................... 56
Initial Settings...................................................................... 56
CHAPTER 4
OPERATION ................................................................................................ 59
Introduction ............................................................................ 59
User Commands in Normal Operation................................. 59
Markings Used in This Manual ............................................. 61
Entering/Exiting the Command Mode .................................. 61
OPEN Command ................................................................ 61
CLOSE Command .............................................................. 62
Automatic Message Sending ................................................ 62
Message Types................................................................... 63
Message 0 ..................................................................... 64
Message 1 ..................................................................... 64
Message 2 ..................................................................... 65
Message 3 ..................................................................... 65
Message 4 ..................................................................... 66
Messages 5 and 6 ......................................................... 66
Message 7 ..................................................................... 68
Message Polling ..................................................................... 69
FD12P Command Set............................................................. 70
HELP Command ................................................................. 70
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MES Command ...................................................................71
AMES Command............................................................71
Weather Related Commands ..............................................73
WPAR Command ........................................................... 73
WSET Command ...........................................................73
Precipitation Limit......................................................74
Weather Update Delay..............................................74
Haze Limit .................................................................74
Rain Intensity Scale................................................... 75
Violent Rain Limit ......................................................75
Heavy Rain Limit .......................................................75
Light Rain Limit.......................................................... 75
Drizzle Limit...............................................................75
Heavy Drizzle Limit....................................................76
Light Drizzle Limit ......................................................76
Snow Limit.................................................................76
Heavy Snow Limit......................................................76
Light Snow Limit ........................................................76
Snow Pellets Limit .....................................................77
Snow Grains Limit .....................................................77
Ice Crystals Limit .......................................................77
Hail Limit.................................................................... 77
DRD Scale................................................................. 77
Warm Limit ................................................................77
PRW Command..............................................................77
CLRS Command ............................................................78
WHIS Command.............................................................78
System Configuration Commands....................................... 79
PAR Command............................................................... 79
CONF Command............................................................80
BAUD Command ............................................................84
BLSC Command.............................................................84
Maintenance Commands..................................................... 85
STA Command ...............................................................85
CAL Command ...............................................................87
TCAL Command.............................................................87
CLEAN Command .......................................................... 88
CHEC Command............................................................89
FREQ Command ............................................................ 89
DRY and WET Commands ............................................89
AN Command .................................................................90
Analog Output Commands ..................................................91
Analog Output Calibration ..............................................91
Data Scaling ................................................................... 92
Hardware Check.............................................................92
Other Commands ................................................................93
TIME Command .............................................................93
DATE Command ............................................................93
RESET Command .......................................................... 94
CHAPTER 5
FUNCTIONAL DESCRIPTION .....................................................................95
General.....................................................................................95
Optical Measurement .............................................................96
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Optical Arrangement ........................................................... 96
FDT12B Transmitter Unit .................................................... 96
FDR12 Receiver Unit .......................................................... 98
Additional Measurements ..................................................... 99
General ............................................................................... 99
DRI21 Interface Board ........................................................ 99
DRD12 Rain Detector ....................................................... 100
DTS14B Temperature Sensor .......................................... 101
FDP12 Control Unit .............................................................. 101
Measurement Signal Processing........................................ 103
Optical Signal Processing................................................. 103
DRD12 Signal Processing ................................................ 104
Algorithm Description ......................................................... 105
Visibility ............................................................................. 105
Detecting Precipitation ...................................................... 106
Precipitation Intensity........................................................ 106
Precipitation Accumulation ............................................... 107
Present Weather ............................................................... 108
Precipitation Types ...................................................... 108
Liquid Precipitation ................................................. 109
Frozen Precipitation................................................ 110
Mixed Precipitation ................................................. 111
Unknown Precipitation ............................................ 111
Visibility Types ............................................................. 111
Fog.......................................................................... 111
Haze and Mist......................................................... 112
Weather Classes ......................................................... 112
Weather Code Selection .............................................. 113
Applications.......................................................................... 113
Internal Monitoring .............................................................. 114
Built-in Tests ..................................................................... 114
Memory Tests ................................................................... 115
Signal Monitoring .............................................................. 115
Hardware Monitoring......................................................... 115
Contamination Monitoring................................................. 116
CHAPTER 6
MAINTENANCE ......................................................................................... 117
General.................................................................................. 117
Cleaning ................................................................................ 118
Cleaning Lenses and Hoods............................................. 118
Cleaning DRD12 Rain Detector........................................ 118
Calibration ............................................................................ 119
General ............................................................................. 119
Visibility Calibration........................................................... 119
Calibration Check Procedure....................................... 120
Calibration Procedure .................................................. 123
Calibrating the DTS14B Temperature Sensor............. 124
Removing and Replacing.................................................. 126
Removing and Replacing Optical Units ....................... 126
Removing and Replacing the DRD12 Rain Detector .. 129
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Calibrating FD12P Weather Sensor after Unit
Replacement ................................................................130
CHAPTER 7
TROUBLESHOOTING................................................................................133
Warnings ...............................................................................133
Troubleshooting Examples..................................................133
Message Indicating Warning or Alarm .............................133
Message Missing ............................................................... 134
Visibility Value is Missing...................................................135
Visibility Value is Continuously Too Good ......................... 135
Visibility is Constantly Too Low .........................................136
FDP12 Reports Precipitation When There Is None........... 136
FD12P Reports Frozen Precipitation during Rain ............. 136
Have Jumper Settings Been Changed? ............................137
Values for Internal Monitoring.............................................137
Getting Help ..........................................................................140
Return Instructions...............................................................140
APPENDIX A
NWS AND WMO CODES USED IN FD12P ...............................................143
The NWS Codes ....................................................................143
APPENDIX B
JUMPER SETTINGS AND INTERNAL WIRING........................................147
CPU Board.............................................................................149
DC-Regulator.........................................................................149
DRI21 Interface Board ..........................................................149
APPENDIX C
TRANSMITTER AND RECEIVER TEST POINTS .....................................151
INDEX .........................................................................................................153
List of Figures
Figure 1 FD12P Weather Sensor Site ....................................................18
Figure 2 DRD12 Rain Detector and DTS14B Temperature Sensor .......19
Figure 3 FD12P Block Diagram ..............................................................21
Figure 4 Recommended Location for the FD12P ................................... 31
Figure 5 FD12P Equipment Grounding ..................................................33
Figure 6 Casting a Concrete Foundation................................................37
Figure 7 Constructing the FD12P Foundation ........................................ 39
Figure 8 Tilting the Pole Mast ................................................................. 41
Figure 9 DTS14B and the Sensor Holder Assembly to Mast .................42
Figure 10 Connecting Internal Grounding.................................................43
Figure 11 Cabling Principle.......................................................................44
Figure 12 Line Voltage and ON/OFF Switches.........................................45
Figure 13 Electronics Enclosure Feedthroughs........................................46
Figure 14 Cable Grounding Instructions ...................................................47
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Figure 15 Wiring the Connector for the LM11 Background Luminance
Meter ........................................................................................ 49
Figure 16 Wiring the Day/Night Photo Switch .......................................... 50
Figure 17 Communication Option ............................................................ 51
Figure 18 RS-485 Communication Option ............................................... 52
Figure 19 Wiring the Modem.................................................................... 53
Figure 20 Analog Current Loop Option .................................................... 55
Figure 21 FD12P Optical System............................................................. 96
Figure 22 FDT12B Transmitter Block Diagram........................................ 97
Figure 23 FDR12 Receiver Block Diagram .............................................. 98
Figure 24 DRI21 Block Diagram in the FD12P Application.................... 100
Figure 25 DRD12 Block Diagram........................................................... 101
Figure 26 FDP12 Control Unit Block Diagram ....................................... 102
Figure 27 Optical Raw Data (in Rain) .................................................... 103
Figure 28 Optical Signal Amplitude Distribution (in Rain) ...................... 104
Figure 29 DRD12 Surface Signal (Heavy Rain Beginning).................... 104
Figure 30 Precipitation Type Determination Principle ............................ 109
Figure 31 Assembling the FDA13 Calibrator.......................................... 122
Figure 32 DTS14 Sensor Holder Assembly to Mast .............................. 125
Figure 33 Removing the Optical Units ................................................... 127
Figure 34 Replacing the Optical Units.................................................... 128
Figure 35 Removing the DRD12 Rain Detector ..................................... 129
Figure 36 Basic Electronics Enclosure Wiring ....................................... 148
Figure 37 Test Points, Transmitter......................................................... 151
Figure 38 Test Points, Receiver ............................................................. 152
List of Tables
Table 1 Manual Revisions..................................................................... 12
Table 2 Related Manuals ...................................................................... 12
Table 3 Basic Set .................................................................................. 23
Table 4 Options..................................................................................... 23
Table 5 Mains Cable Selection ............................................................. 35
Table 6 Communication Cable Lengths................................................ 35
Table 7 Transmit Frequencies of the DMX21 Modem Board ............... 53
Table 8 LED Indicators of the DMX21 Modem ..................................... 54
Table 9 Manual Controls of the DMX21 Modem................................... 54
Table 10 Default Communication Settings.............................................. 57
Table 11 Commands for Changing the Default Settings ........................ 57
Table 12 Commands for Displaying and Changing the Parameters ...... 57
Table 13 Settings and Corresponding Commands................................. 60
Table 14 Routine Command for Maintenance ........................................ 60
Table 15 Status Report Command ......................................................... 60
Table 16 Transmitter Status Correspondence between MITRAS and
FD12P ...................................................................................... 67
Table 17 Receiver Status Correspondence between MITRAS and FD12P67
Table 18 HELP Command Sets .............................................................. 70
Table 19 Commands for Displaying or Setting Weather Analysis
Parameters............................................................................... 73
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Table 20 Commands for Displaying System Parameters and Editing the
Current System Configuration ..................................................79
Table 21 Maintenance Commands..........................................................85
Table 22 Hardware Error Texts ...............................................................86
Table 23 Hardware Warning Texts ..........................................................86
Table 24 Parameters for Optical Measurement.....................................131
Table 25 Parameters for DRD12 Precipitation Detector .......................131
Table 26 Parameters for DTS14 Temperature Sensor..........................131
Table 27 Updating Parameters..............................................................131
Table 28 Parameters and Commands...................................................132
Table 29 Values for Internal Monitoring.................................................138
Table 30 Internal Weather Types, NWS Code .....................................143
Table 31 WMO SYNOP Codes (4680, W Table 32 WMO SYNOP Codes (4680, W Table 33 WMO SYNOP Codes (4680, W
) ......................................143
aWa
) ......................................144
aWa
) ......................................144
aWa
Table 34 WMO Code Table 4678. Codes Used by FD12P ...................145
Table 35 CPU Board Jumpers...............................................................149
Table 36 CPU Board Connectors ..........................................................149
Table 37 DC-regulator Connectors........................................................149
Table 38 DRI21 Interface Board Jumpers ............................................. 149
Table 39 DRI21 Interface Board Connectors ........................................150
Table 40 Electronics Enclosure/Transducer Cable Signals ..................150
Table 41 Transmitter Test Points........................................................... 151
Table 42 Receiver Test Points............................................................... 152
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Chapter 1 _________________________________________________________ General Information
CHAPTER 1

GENERAL INFORMATION

About This Manual

This manual is a general information source as well as a detailed
operational guide to the FD12P Weather Sensor.

Contents of This Manual

This manual consists of the following chapters:
- Chapter 1, General Information, provides important safety, revision history, contact, and warranty information for the product.
- Chapter 2, Product Overview, introduces the FD12P Weather Sensor features, advantages, and the product nomenclature.
- Chapter 3, Installation, provides you with information to help you install this product.
- Chapter 4, Operation, contains information needed to operate this product.
- Chapter 5, Functional Description, gives a functional description on the product.
- Chapter 6, Maintenance, describes the overall maintenance of the product.
- Chapter 7, Troubleshooting, deals with troubleshooting information.
- Appendix A, NWS and WMO Codes Used in FD12P
- Appendix B, Jumper Settings and Internal Wiring
- Appendix C, Transmitter and Receiver Test Points
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Version Information

Table 1 Manual Revisions
Manual Code Description
FD12P-U106en-1.2 Weather Sensor, User's Guide M210296en-A This manual.

Related Manuals

Table 2 Related Manuals
Manual Code Manual Name
DMX21T0496-1.1 DMX21 CCITT Modem LM11T0545-1.2 LM11 Background Luminance Meter

Safety

WARNING
CAUTION

General Safety Considerations

Throughout the manual, important safety considerations are highlighted as follows:
Warning alerts you to a serious hazard. If you do not read and follow instructions very carefully at this point, there is a risk of injury or even death.
Caution warns you of a potential hazard. If you do not read and follow instructions carefully at this point, the product could be damaged or important data could be lost.
NOTE
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Note highlights important information on using the product.
Chapter 1 _________________________________________________________ General Information

Product Related Safety Precautions

The FD12P Weather Sensor delivered to you has been tested for safety and approved as shipped from the factory. Note the following precautions:
WARNING
CAUTION
WARNING
Ground the product, and verify outdoor installation grounding periodically to minimize shock hazard.
Do not modify the unit. Improper modification can damage the product or lead to malfunction.
Safety Summary
The following are general safety precautions must be observed during all phases of installation, operation and maintenance.
Neglecting to follow these precautions or specific warnings and cautions elsewhere in this manual violates safety standards of design, manufacture and intended use of the instrument. Vaisala Oyj. and its Subsidiaries do not answer for the consequences if the customer neglects to follow these requirements.
Ground the Equipment
To minimize the hazard of electrical shock, follow accurately the installation procedure in Chapter 3, Installation, on page 29.
NOTE
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Note that the chassis of the FD12P Weather Sensor must be connected to a good electrical earth. The instrument is equipped with a three-conductor AC power cable. Be sure that the earth wire of the cable is connected to an electrical ground.
There is also a grounding clamp at the bottom of the electronics enclosure of Weather Sensor FD12P. Good grounding with a 16-mm
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cable must be provided. Besides increasing safety, this also protects the Weather Sensor against lightning induced voltages.
To prevent operator injury or damage to the Weather Sensor, check that the LINE VOLTAGE SETTING is correct before connecting the line power (See Figure 12 on page 45.) Also ensure that the line power outlet is provided with a protective ground contact.
WARNING
WARNING
WARNING
Do not operate in an explosive atmosphere.
Do not operate the equipment in the presence of flammable gases or fumes. Operation of any electrical instrument in such an environment constitutes a definite safety hazard.
Do not service or adjust alone.
Do not attempt internal service or adjustment unless another person, capable of rendering first aid and resuscitation, is present.
Keep away from live circuits.
Component replacement or internal adjustments must be made by qualified maintenance personnel. Operating personnel must not remove instrument covers. Do not remove or replace any components with the power cable connected. Under certain conditions, dangerous voltages may exist even with the power cable disconnected. To avoid injuries disconnect power, and discharge all circuits before touching them.
WARNING
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Do not substitute parts or modify the instrument.
Because of the danger of introducing additional hazards, do not modify or substitute parts in the instrument. Contact Vaisala or its authorized representative for repairs to ensure that safety features are maintained.
Chapter 1 _________________________________________________________ General Information
CAUTION
The component boards including CMOS microchips should be transported and stored in conductive packages. Although new CMOS devices are protected against overvoltage damages caused by static electric discharge of the operator, careful handling is recommended: the operator should be properly grounded. Unnecessary handling of component boards should be avoided.
Radio Frequency Interference Statement (USA)
The United States Federal Communications Commission (in 47 CFR
15.838) has specified that the following notice must be brought to the
attention of users of this kind of a product in the USA:
Federal communications commission radio frequency interference statement
This equipment generates and uses radio frequency energy and if not installed and used properly, that is in strict accordance with the manufacturer's instructions, may cause interference to radio and television reception. The Weather Sensor is designed to provide reasonable protection against such interference in an airport installation. However, there is no guarantee that interference will not occur in a particular installation. If this equipment causes interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures:
- reorient the receiving antenna
- relocate this device with respect to the receiver
- move this device away from the receiver
If necessary, the user should consult the dealer or an experienced radio/television technician for additional suggestions.

ESD Protection

Electrostatic Discharge (ESD) can cause immediate or latent damage to electronic circuits. Vaisala products are adequately protected against ESD for their intended use. However, it is possible to damage the product by delivering electrostatic discharges when touching, removing, or inserting any objects inside the equipment housing.
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To make sure you are not delivering high static voltages yourself, take the following precautions:
- Handle ESD sensitive components on a properly grounded and protected ESD workbench. When this is not possible, ground yourself to the equipment chassis before touching the boards. Ground yourself with a wrist strap and a resistive connection cord. When neither of the above is possible, touch a conductive part of the equipment chassis with your other hand before touching the boards.
- Always hold the boards by the edges and avoid touching the component contacts.

Trademarks

Intel® is a registered trademark of the Intel Corporation in the U.S. and other countries.

Warranty

For certain products Vaisala normally gives a limited one-year warranty. Please observe that any such warranty may not be valid in case of damage due to normal wear and tear, exceptional operating conditions, negligent handling or installation, or unauthorized modifications. Please see the applicable supply contract or conditions of sale for details of the warranty for each product.
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Chapter 2 ___________________________________________________________Product Overview
CHAPTER 2

PRODUCT OVERVIEW

This chapter introduces the FD12P Weather Sensor features, advantages, and the product nomenclature.

Introduction

The FD12P Weather Sensor is an intelligent, multi-variable sensor for automatic weather stations and airport weather observing systems. The sensor combines the functions of a forward scatter visibility meter and a present weather sensor. In addition, the sensor can measure the intensity and amount of both liquid and solid precipitation.
The FD12P can be used to automatically determine the visibility and precipitation related weather codes in the World Meteorological Organization (WMO) standard SYNOP and METAR messages. The sensor can also be employed as an observer's aid in a semi-automatic weather observing system. The sensor is also suitable for other weather observing systems providing valuable information, for example, to road and harbor authorities.
The versatility of the FD12P is achieved with a unique operating principle. The FD12P measures precipitation water content with a capacitive device and combines this information with optical scatter and temperature measurements. These three independent measurements together provide data sufficient for an accurate evaluation of current visibility and weather type.

Hardware Structure

The structural basis of the FD12P is the pole mast that supports the transducer crossarm (FDC115). The crossarm contains the optical
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units, FDT12B Transmitter and FDR12 Receiver. The DRD12 Rain Detector is fastened to the crossarm. The electronics enclosure with the main data processing and interface units is mounted to the pole mast as seen in Figure 1 below.
0201-085
Figure 1 FD12P Weather Sensor Site
The following numbers are related to Figure 1 above:
1 = Transducer crossarm 2 = DRD12 Rain Detector 3 = DTS14 Temperature Sensor 4 = Pole mast 5 = Electronics enclosure
The FD12P Weather Sensor consists of three parts: sensing elements, electronics enclosure, and structural elements. They are described in detail on the next page.
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Chapter 2 ___________________________________________________________Product Overview
Sensing Elements
The FDT12B Transmitter emits pulses of near infrared light. It is permanently tilted 16.5º downwards. The optical power is stabilized by a closed hardware loop. The unit also includes a receiver circuit for monitoring lens contamination.
The FDR12 Receiver measures the scattered part of the FDT12B light beam. The FDR12 contains also an additional light transmitter for monitoring lens contamination. Like the transmitter, the receiver is also tilted 16.5º downwards. Therefore, the receiver unit measures light scattered at an angle of 33°.
The DRD12 Rain Detector outputs a signal proportional to the amount of water on two RainCap™ sensing elements. These elements consist of thin wires protected by an insulating glass coating. The presence of water changes the capacitance of the elements. The combined capacitance of the plates is measured by the DRD12 electronics. Integrated heating resistors keep the elements dry when, for example, fog and melt snow fall on them. The Rain Detector is protected by a windshield to decrease the effect of wind on the measurement results. The DRD12 is illustrated in Figure 2 below.
The DTS14B Temperature Sensor is a Pt100 thermistor that is used to measure the crossarm temperature. See Figure 2 below.
0201-086
Figure 2 DRD12 Rain Detector and DTS14B Temperature
Sensor
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The following numbers refer to Figure 2 on page 19:
1 = Two RainCapTM elements 2 = DRD12 Rain Detector 3 = Wind shield 4 = Assembly clamp 5 = DTS14 Temperature sensor
Electronics Enclosure
The FDP12 Control Unit is the main data processor and communication unit of the FD12P.
The DRI21 Interface Board is a Vaisala, general-purpose sensor interface, with several analog and digital input channels. In the FD12P, one of the DRI21 Interface Board channels is used for measuring the crossarm temperature and the DRD12 analog signal. In addition, the DRI21 controls the DRD12 heating and reads the precipitation ON/OFF status.
The FDW13 Mains Power Supply converts the mains voltage to 24 VAC power for the FDS12 regulator and the heater elements. The FDW13 includes also the mains voltage selector and the mains ON/OFF switch, which also functions as an automatic fuse.
The FDS12 DC Voltage Regulatorconverts the AC or DC input voltage (min. 18 V) to 12 VDC power used by FD12P electronics. The FDS12 also includes one relay used to control heater power.
The DMX21 Modem (optional) is a standard, 300-baud modem used only in the leased line mode with the FD12P.
The FDE12 Backup Temperature Sensor is included.
Structural Elements
The structural elements include the pole mast with a standard height of 2 meters and the FDC115 Transducer Crossarm with a length of
1.5 meters, which is also the total width of the FD12P.
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Chapter 2 ___________________________________________________________Product Overview

Operating Principle

The FD12P Weather Sensor is a microprocessor controlled, intelligent sensor combining optical forward scatter measurement, capacitive precipitation sensing, and temperature measurement. The main units of the FD12P are shown in Figure 3 below.
9502-091
Figure 3 FD12P Block Diagram
The FD12P evaluates Meteorological Optical Range (MOR) by measuring the intensity of infrared light scattered at an angle of 33°. The scatter measurement is converted to the visibility value (MOR) after a careful analysis of the signal properties. Special processing is used in case of precipitation.
The FD12P software detects precipitation droplets from rapid changes in the scatter signal. The droplet data is used to estimate optical precipitation intensity and amount. In addition to the optical signal, the analog output of the DRD12 Rain Detector is used to estimate the precipitation intensity and type.
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The output of the DRD12 is proportional to the water amount on the capacitive sensing surfaces while the optical intensity is proportional to the total volume of the reflecting particles. The ratio of optical and capacitive intensities is used to determine the basic precipitation type.
The crossarm temperature (TS) is measured with the DTS14B Temperature Sensor connected to the DRI21 interface card. The temperature data together with the optical signal profile and the DRD12 surface sensor data are used to determine the actual weather code.
The software performs all signal analyses in the FD12P except the DRD12 Rain ON/OFF status, which is hardware-based and is used as an auxiliary parameter. The FD12P has a fixed program that is divided into tasks executed under control of a real-time operating system kernel. Each task is like an endless loop with a limited function. The operating system kernel controls the timing of the tasks and the interactions between the tasks.

Using FD12P

The FD12P is typically used as a component of a weather observing system. The final weather message (SYNOP, METAR) is then coded in the central unit of a weather observation system (for example, Vaisala MILOS 500) or by a human observer using the FD12P as an observation aid.
The FD12P output is a digital serial interface, which can be configured into two different operating modes: the sensor can be set to send a data message automatically at selected intervals, or the FD12P can be polled by the host computer. The same serial line is also used as an operator interface.
The operator controls and checks the operation of the FD12P by using a maintenance terminal. A set of built-in commands and test routines is provided for configuring and monitoring the multiple functions of the FD12P.
The standard data messages contain a status character for indicating faults detected by the internal diagnostics. If the error status is set, the operator can view a special status message. It contains detailed results of the diagnostics and a written description of the fault. Using this information, the operator can take corrective action or provide the maintenance personnel with valuable advice.
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Chapter 2 ___________________________________________________________Product Overview

Equipment Nomenclature

The standard equipment nomenclature and common names are listed in Table 3 and Table 4 below.
Table 3 Basic Set
Type Name Description
FDC115 Transducer Crossarm Optics, analog, monitoring
assembly FDT12B Transmitter FDR12 Receiver 16614ZZ Crossarm Cable FDB12 Electronics Enclosure Power, conversions, interfacing
assembly FDP12 Processor Board FDS12 DC Voltage Regulator FDW13 Mains Power Supply DRI21 Interface Board 16615ZZ Transducer Cable 16737ZZ I/O Bus Cable DRD12 Rain Detector FD30513 Pole Mast Standard 2-m mast 13145 Base Plate and
Installation Set
Table 4 Options
Type Name Description
FDA13 Visibility Calibration Set FD45094 Maintenance cable RS232 cable with 9-pin D-
connector. Termbox-48 Mains and Signal
Junction Box
FD12MODEM Modem Option For remote communication. FD12PLM11 LM11 Option For ambient light
16616ZZ Extended Transducer
Cable
Adapting/extending the local
cable. Contains heavy-duty
transient protection circuitry.
measurement.
For optional high-mast
mounting.
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Specifications

Mechanical Specifications

- Dimensions: 2.3 m × 1.6 m × 0.6 m (H × W × D)
- Weight: 35 kg, excluding the installation plate for the pole mast
- Mounting: on a concrete foundation with three ∅16-mm bolts
- Material: anodized aluminum, natural gray

Electrical Specifications

- Mains supply: 115/230 VAC ± 20 %, 45 ... 65 Hz
- Maximum power consumption: 35 W + 100 W defrosting heaters (in cold weather)
The sensor electronics:
- Lock-in amplifier
- LED power stabilizer
- Contamination monitor
- Lens heater
The control unit:
- Intel 8031 microprocessor
- Program memory, 64 Kbytes
- Read/write memory, 32 Kbytes
Outputs:
- Serial data line may be used either as RS-232 level signals or interfaced via an optional data modem
- RS-485 (2-wire)
- 4 - 20 mA analog current (sink) output
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The output data:
- Automatic or polled data message
- Visibility, present weather, precipitation and status data
- Automatic message type and interval is selectable at 15
seconds to n × 15 seconds (n < 18) intervals
The analog visibility output:
- Selectable range and mode (linear or logarithmic)
- Status control bit for remote alarm relay, etc.
- Alarms and warnings (hardware failures, visibility limits)

Optical Specifications

Operating principle:
- Forward scatter at an angle of 33o and capacitive rain sensor.
The light transmitter:
- Light source: near-infrared LED
- Peak wavelength: 875 nm
- Modulation frequency: 2.3 kHz
- Transmitter lens diameter: 71 mm
- Reference photodiode: for light source control
- Backscatter photodiode: for contamination and blockage measurement
The light receiver:
- Photodiode: PIN 6 DI
- Spectral response: max. responsivity at 850 nm, 0.55 A/W (in range 550 ... 1050 nm over 0.3 A/W)
- Reception lens diameter: 71 mm
- Backscatter light source: near-infrared LED for contamination and blockage measurement
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Capabilities and Limitations

Visibility Measurement Specifications

Measurement range of Meteorological Optical Range (MOR):
- 10 ... 50 000 m according to a 5 % Contrast Threshold Definition
Accuracy:
- ± 10 %, range 10 ... 10 000 m
- ± 20 %, range 10 000 ... 50 000 m
Instrument consistency:
- + 4 %
Update interval:
- 15 seconds

Weather Sensing Specifications

Precipitation detection sensitivity:
- 0.05 mm/h or less, within 10 minutes
Weather type identification:
- 11 different types of precipitation
- Fog (mist) and haze (smoke, sand)
Weather type reporting:
- WMO code table 4680 (with some additions from code table
4677)
- Code letters for precipitation, NWS
- WMO code table 4678 (supported codes are shown in Table 34 on page 145).
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Precipitation intensity measurement:
- Range 0.00 ... 999 mm/h
- Accuracy ± 30 % (range 0.5 ... 20 mm/h, liquid precipitation)

Environmental Specifications

Operating temperature range:
- 40 ... +55 oC
Operating humidity range:
- Up to 100 % RH
Wind speed:
- Up to 60 m/s (standard mast)
Sun orientation:
- Direct and reflected sunlight into the light receiver must be avoided.
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CHAPTER 3

INSTALLATION

This chapter provides you with information to help you install this product.
NOTE
Before installation, read section Product Related Safety Precautions on page 13.

Organizing Installation

Before you begin to install the FD12P Weather Sensor, make a plan of the installation steps. The following is an example of how to organize the installation process.
1. Surveying the site:
- Find the most representative measurement site.
- Determine orientation of the Weather Sensor.
2. Cabling plan is required for the following:
- Grounding cabling layout and cable type.
- Power supply cabling layout and cable type.
- Modem/signal cabling layout and cable type.
3. Ordering the construction materials and cables.
4. Digging for cables and foundation.
5. Casting the concrete:
- Prepare concrete blocks by using a casting mold.
- Cast the fixing bolts in their places at the same time.
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6. Installing the base plate and the pole mast:
- Install the base plate with the bolts on the concrete block.
- Level the plate.
- Mount the pole mast on the base plate.
- Mount the junction box to the pole mast (optional). Junction boxes are available from Vaisala.
7. Connecting cables:
- Connect the mains and signal cables of the site to the junction box or have them ready for direct connection to the sensor.
8. Final installation:
- Install the electronics enclosure and the crossarm of the FD12P to the pole mast.
- Connect the power and signal cables of the FD12P.
- Connect the modem/signal line to the host computer, display, etc.
9. Start-up tests for the system.

Location and Orientation

The main requirements for the location of the FD12P are as follows:
1. Place the FD12P at a site where the measurements will be representative of the surrounding weather conditions.
The ideal site has a minimum clearance of 100 meters from all large buildings and other constructions that generate heat and/or obstruct precipitation droplets. Also avoid shading of trees as this may cause changes in the microclimate.
2. Make sure the site is free of obstacles and reflective surfaces, which disturb the optical measurements and act as obvious sources of contamination.
There must not be any obstacles in the line-of-sight of the transmitter and receiver units (see Figure 4 on page 31). If the transmitter beam is reflected from obstacles back to the receiver unit, the sensor will indicate too low MOR values as the reflected signal cannot be distinguished from the real scatter signal. Reflections are detected by rotating the sensor crossarm. They will change depending on the crossarm orientation. Also the visibility reading will change accordingly.
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The receiver and transmitter optics should not point towards powerful light sources or, in bright daylight, reflective surfaces such as snow or sand. The receiver should point north in the Northern Hemisphere and south in the Southern Hemisphere. The receiver circuit may become saturated in bright light, and the built-in diagnostics will indicate a warning. Intense light can generate false contamination alarms from the transmitter unit. Bright daylight will also increase the noise level in the receiver.
The transmitter and receiver should face away from any obvious source of contamination such as spray from passing vehicles. Dirty lenses will cause the sensor to report too high visibility values. Excessive contamination is automatically detected by the sensor.
Harmful reflections are typically avoided if the transmitter beam is directed towards a surface, which will reflect most of the light away from the sensor. The distance of 6 meters shown in Figure 4 below is only for guidance; it is not an absolute requirement.
There should be no flashing lights near the sensor. A flashing light can cause errors in detecting precipitation towards
No obstacles or reflecting surfaces
Figure 4 Recommended Location for the FD12P
The following numbers refer to Figure 4 above.
0110-178
1 = Transmitter 2 = Receiver
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3. Power supply and communication lines must be available.
When the site for the FD12P is selected, take into consideration the available power supply and communication lines. This influences the amount of work and accessories needed and thus, the actual installation costs.

Grounding and Lightning Protection

Equipment Grounding

Equipment grounding protects the electrical modules of the FD12P, for example, against lightning and prevents radio frequency interference. The FD12P equipment is grounded using a jacketed grounding cable and conductive grounding rod(s).
The FD12P must be grounded by means of the grounding clamp, which is located under the cable flange (See Figure 5 on page 33). A 16-mm² jacketed grounding cable is connected to the clamp. Depending on the need, one to four copper-sheathed steel rods are driven into the ground. If several rods are needed, the alignment from the foot of the base plate must be radial.
The grounding principles are the following:
- The grounding rod must be isntalled as close to the pole mast
as possible to minimize the length of the grounding cable. The grounding cable can be also cast inside the concrete base.
- The length of the grounding rod depends on the local
groundwater level. The lower end of the grounding rod must continuously touch moist soil.
The grounding quality can be checked with a georesistance meter. The resistance must be less than 10 ohms. This way the lowest possible resistance is achieved.
The junction box must be also grounded via the grounding cables in the same way as the electronics enclosure (Figure 5 on page 33). The junction box is optional.
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0111-001
Figure 5 FD12P Equipment Grounding
The following numbers refer to Figure 5 above:
1 = Electronics enclosure 2 = Junction box (optional) 3 = Mains cable 4 = Cable tubing 5 = Grounding rods 6 = Signal cable 7 = 16 mm² grounding cables 8 = Grounding clamp
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Internal Grounding

The electronics enclosure and the bottom plate of the FD12P are secured by a 1.5-mm², yellow-green ground cable and the crossarm is grounded through the transducer cable shield. The other parts of the crossarm are in galvanic contact with each other.
CAUTION
When installing the FD12P, the grounding flat connector must be plugged to the ground terminal socket, which is located beside the MIL-connector in the crossarm. See instructions in section Assembling the FD12P on page 40 and Figure 10 on page 43.

Grounding for Testing Purposes

The FD12P is provided with a two-meter mains cable. The cable has a grounded plug. The plug must be connected only to an outlet that has a ground terminal. This grounding is sufficient when the instrument is used indoors, for example, for testing purposes.

Grounding Remote Units and Communication Cable

Remote units, such as, the PC data logger, must be grounded and protected against lightning.
WARNING
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A lightning strike through a communication wire can cause a voltage surge dangerous to life at remote sites if the remote units are not properly grounded.
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Cable Selection

Line Power Cabling

The FD12P is supplied with a two-meter power cable. If a local terminal for 115/230 VAC power supply is not available, use an extended mains cable from the FD12P to the nearest power source. This cable should be armored and of underground type. The armored reinforcing acts as a mechanical shield and also provides protection against lightning. Ground the cable screen at both ends.
The recommended mains wire cross sections are shown in Table 5 below for mains voltage 230 VAC. For 115 VAC, divide the maximum distances by four.
Table 5 Mains Cable Selection
Maximum Distance from Voltage Source
2 km 1.5 mm 4 km 2.5 mm 8 km 4.0 mm
One-wire Cross-section Area
2 2 2
Nearest AWG-gauge
No 15 AWG 10 mm No 13 AWG 14 mm No 11 AWG 18 mm
Typical Non­armored Cable Diameter
NOTE
Cables with diameters more than 12 mm require a separate junction box which is also available from Vaisala.

Communication Cable

The FD12P provides the RS-232C, RS-485, CCITT V.21 modem, and analog transmission interfaces. Consider your needs for communication before the installation. The communication method depends on the distance between the computer or display and the FD12P and the number of the FD12P sensors. Table 6 below describes the possibilities.
Table 6 Communication Cable Lengths
Cable Length One FD12P Several FD12Ps on line
< 150 m RS-232 RS-485, modem < 500 m RS-485, modem RS-485, modem > 500 m Modem Modem
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For a modem and RS signal cable, use a screened, 2 × 0.22-mm² twisted pair cable with a minimum diameter of 5 mm. For details, see section Communication Options on page 50.

Unloading and Unpacking

The contents of the delivery in question are specified in the packing list included with the delivery documents. The FD12P equipment is normally delivered in three cases containing the following parts:
- Crossarm FDC115 containing the optics.
- Electronics enclosure FDB12 with radiation shield.
- Pole mast.
Two persons can easily move the cases from a truck to the installation site.
NOTE
Handle gently the case containing the optical parts. Do not drop either end of the case.

Unpacking Procedure

1. Read the packing list supplied within the delivery documents. Compare the packing list against the purchase order to make sure that the shipment is complete.
2. Open the covers.
3. In case of any discrepancies or damage, contact the supplier.
4. Place the packing materials and covers back in the cases and store them for possible reshipment.

Storage Information

Store the FD12P in its packages in dry conditions, not in the open air. The storage conditions are as follows:
- Temperature −40 oC ... 70 oC.
- Relative humidity up to 95 %.
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Installation Procedures

Constructing the Foundation

Cast a concrete foundation or use an existing construction that is level and rigid. The recommended minimum dimensions for the foundation are illustrated in Figure 6 below. It is easiest to mount the foundation screws while casting the pad. If the pad was casted earlier, drill three holes into the concrete for the wedge bolts.
0110-179
Figure 6 Casting a Concrete Foundation
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The following numbers and letters refer to Figure 6 on page 37:
A = Watertight plastic for conducting rainwater away
(recommended) 1 = Concrete block 2 = Surface horizontal to ±0.5° 3 = Ground level 4 = Reinforcing steel 5=
The Installation Set included in the FD12P delivery contains the required equipment both for mounting when casting the pad and mounting to an existing surface. Use the triangle shaped template as an auxiliary device and remove it before mounting the base plate.
Reinforcing steel or use steel mesh 150 × 150 mm
Mounting When Casting the Pad
1. Fasten the three reinforcing plates to the lower end of the foundation screws with six M16 nuts. See Figure 7 (C, top view) on page 39.
2. Fix the template to the upper ends of the foundation screws with six nuts.
3. Embed the assembly in the concrete foundation as shown in Figure 7 on page 39.
4. After the concrete has set, remove the template.
Mounting to an Existing Surface
1. Drill three, 20-mm holes using the template, minimum depth 65 mm. Refer to Figure 7 on page 39.
2. Remove the template.
3. Clean the holes.
4. Fasten the foundation screws to the wedge bolts by hand.
5. Protect the tops of the screws with two nuts tightened together.
6. Then place the wedge bolt and foundation screw combinations in the holes, wedge bolts down, and hammer the combinations down.
7. Tighten the foundation screws as tight as possible.
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0110-180
Figure 7 Constructing the FD12P Foundation
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The following numbers and letters refer to Figure 7 on page 39:
A = Mounting when casting the pad 1 = A10.5 DIN125, 4 pieces 2 = M10 x 30 DIN933, 4 pieces 3 = A17 DIN125, 3 pieces 4 = M16 DIN934, 3 pieces 5 = Template 6 = Foundation screw M16 x 250, 3 pieces 7 = Base level B = Mounting to an existing surface 6 = Foundation screw M16 x 250, 3 pieces 8 = Template 9 = Wedge bolt M16, 3 pieces C = Top view 10 = M10, 4 pieces 11 = Baseplate

Assembling the FD12P

1. Mount the base plate and level it with the six M16 nuts.
2. Mount the pole mast pedestal and the tilting support on the base plate with four M10 bolts (Figure 7 on page 39, C, top view).
3. Attach the electronics enclosure to the pole mast with two clamps and four M6 Allen screws.
4. Tilt the mast. See Figure 8 on page 41.
5. Feed the crossarm cable and temperature sensor DTS14B cables inside the pole mast.
6. Check that a thin rubber gasket is on the insertion neck of the crossarm.
7. Connect the crossarm cable plug to the MIL-connector (see Figure 10 on page 43).
8. Connect the grounding flat connector to the other pin of the ground terminal socket as shown in Figure 10 on page 43.
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0110-181
Figure 8 Tilting the Pole Mast
The following numbers refer to Figure 8 above:
1 = Pole mast. For tilting, loosen the upper and remove the lower
M10 × 100 bolts.
2 = Tilting supporters. To be installed under the fastening screws.

Attaching the DTS14B Temperature Sensor to the Mast

To attach the DTS14B Temperature Sensor to the mast, do the following:
1. Pull the DTS14B temperature sensor out of the side hole of the
mast.
2. Then attach the holder to the mast in the following way:
- Open the fixing screw fully (part 3 below).
- Push the screw head to the holder with you finger.
- Put the holder to the hole in the mast pole.
- Slide the holder upwards as long as it goes and hold it there.
- Tighten the fixing screw firmly.
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- Insert the crossarm to the pole mast and lock it in the right
position with two 8-mm bolts.
- Erect the mast.
- Lift the DRD12 Rain Detector to an upright position. Tighten
the clamp.
0201-088
Figure 9 DTS14B and the Sensor Holder Assembly to Mast
The following numbers refer to Figure 9 above:
1 = Sensor holder 2=DTS14 3 = Fixing screw
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0110-182
Figure 10 Connecting Internal Grounding
The following numbers refer to Figure 10 above:
1 = Crossarm 2 = Ground terminal socket 3 = Grounding flat connector 4 = Pole mast 5 = MIL-connector

Connecting Cables

Basic Wiring
To do the basic wiring, do the following:
1. The electronics enclosure includes a power cable. Remove the
plug.
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If you use another, longer cable, make sure to connect the wires in a correct way, especially the protective ground wire (usually yellow-green). Refer to Figure 11 below.
2. Connect the power cord to the screw terminals in a junction box or bring the power line directly to the electronics enclosure. The selected method depends on the thickness of the power cable, which should be checked before the installation. The electronics enclosure has a cable outlet with a diameter of 10 - 12 mm.
3. Feed Neutral N (normally blue) and protective earth PE (normally yellow-green) via separate conductors.
4. Feed the communication cable through one of the two cable feedthroughs. For cable shield connections, see instructions in section Communication Cable EMC-shielding on page 46.
5. Wire the communication cable according to instructions in section Communication Options on page 50.
9509-011
Figure 11 Cabling Principle
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NOTE
If the line voltage used differs from 230 V (the initial setting at the factory), check the voltage setting of the FDW13 Mains Power Supply (alternatives 115 VAC and 230 VAC). You can find the line voltage setting switch on the left side of the FDW13 unit (see Figure 12 below).
0110-183
Figure 12 Line Voltage and ON/OFF Switches
The following numbers are related to Figure 12 above:
1 = Electronics enclosure 2 = Pole mast 3 = DMX21 modem 4 = DC regulator FDW13 5 = ON/OFF switch 6 = Line voltage setting 7 = FD12P control unit
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0110-184
Figure 13 Electronics Enclosure Feedthroughs
The following numbers refer to Figure 13 above:
1 = Grounding 2 = DTS14 cable feedthrough 3 = Temperature sensor (TE) 4 = Cap (Pg 13.5) of optional opening for the LM11
background luminance meter 5 = Main power cable 6 = FDC115 transducer cable feedthrough 7 = Standard communication cable feedthrough
Communication Cable EMC-shielding
The electronics enclosure has one cable outlet for a cable diameter from Ø7 to Ø10 mm, which is reserved for a signal or modem cable. Although the shielding of the cable may be just grounded after cable inlet, an efficient procedure against RF-interference requires special care. Ground the cable gland to keep EMI levels within specifications.
For a proper RF-grounding of any jacketed cable, the instructions are the following:
1. Lead the signal cable through the cable inlet. If the field cable is thicker than 10 mm, use a separate signal junction box. See Figure 14 on page 47.
2. Strip 80 mm of the cable sheath leaving approximately 40 mm of the shield.
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3. Remove the cap of the cable gland, including the rubber cylinder
and the metal rings. Slide the cap with rubber cylinder onto the cable.
4. For a thin cable, add a shrinkable tube to increase cable
diameter.
5. Slide two metal rings on the shielding and squeeze it evenly
between the rings.
6. Secure with a shrinkable tube.
7. Tighten the cable with the cable gland and proceed with the
wiring.
8. Connect the signal cable to the screw terminals in the electronics
enclosure.
9. Ground the signal cable with the same method at both ends.
0205-006
Figure 14 Cable Grounding Instructions
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Connecting a Background Luminance Sensor or a Day/Night Switch to FD12P
The FD12P Weather Sensor supports two different methods for ambient light sensing. The Background Luminance Meter LM11 can be connected to the FD12P for accurate ambient light measurement. The LM11 sensor and necessary wiring are included in option FD12PLM11 (see Figure 15 on page 49 for the wiring details). The background luminance measurement is typically used in the RVR systems.
The LM11 output frequency is measured with the DRI21 interface board and then converted into background luminance by the FD12P software. The conversion uses a scaling factor, which needs to be configured by the user. For details, see section BLSC Command on page 84.
In certain applications it is necessary to calculate night visibility separately using a formula that differs from MOR. In these cases a simple day/night photo switch is sufficient for discerning between day and night ambient light conditions. The switch can be connected to the serial line control input on the FDP12 processor board. For wiring details, see Figure 16 on page 50.
Positive voltage is interpreted as a night condition and the background luminance value in the FD12P output message is set to 0. Negative voltage or an open circuit is interpreted as a day condition and the luminance value is set to 1. For details, see section BLSC Command on page 84.
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9610-006
Figure 15 Wiring the Connector for the LM11 Background
Luminance Meter
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NOTE
9610-007
Figure 16 Wiring the Day/Night Photo Switch

Communication Options

Serial Communications Settings
The factory default settings of the FD12P serial communications port are 300 baud. Even parity is 7 data bits, 1 stop bit.
Serial Transmission RS-232
For the RS-232 communication, connect the signal wires to screw terminal X18 (CTR lines not needed) at CPU board FDP12. See Figure 17 on page 51.
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9509-013
Figure 17 Communication Option
The Vaisala recommendation for the maximum length of the RS-232 cable is 150 m (500 ft).
Serial Multipoint Transmission RS-485
The RS-485 transmission standard allows several FD12Ps to communicate (half duplex) with the host computer using a single twisted pair. For the RS-485 communication, connect the signal wires to 4-pin screw connector X21 at the CPU board. See Figure 18 on page 52.
In the multidrop configuration, where several FD12P Weather Sensors are on the same communication line, units are differentiated by an ID. Set a different unit ID to each FD12P with the CONF command. Set FD12 P to the polling mode with the AMES 0 2 command. The host system must ask data messages by polling each FD12P.
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9509-014
Figure 18 RS-485 Communication Option
Modem DMX21
The Modem DMX21 is a CCITT V.21 modem, operating at 300 bps. Connect the signal wires to MODEM LINE 1 and 2, and screw terminals 7 and 9 on Interface board 16127FD. See the wiring diagram in Figure 19 on page 53.
In the multidrop configuration, where several FD12P Weather Sensors are on the same modem line, the units are differentiated by an ID. Set a different unit ID to each FD12P with the CONF command. In the multidrop configuration, only one FD12P modem carrier can be active at the time. To set the modem carrier under the FD12P software control, set jumper X2 to position 1-3 in the modem interface board as shown in Figure 19 on page 53. If the X2 jumper is in position 3-4 and if the unit ID is not set, FD12P keeps the modem carrier signal on all the time. Set the FD12P to the polling mode with the AMES 0 2 command. The host system must ask data messages by polling each FD12P.
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9509-012
Figure 19 Wiring the Modem
Usually, the modem of the FD12P operates in the answer mode, and the modem of the host computer in the originate mode. In the standard FD12P system, the S3 switch on the DMX21 board is in the DOWN position and the answer mode is selected. When the switch is in the UP position, the originate mode is selected. The transit frequencies of the DMX21 modem are presented in Table 7 below.
Table 7 Transmit Frequencies of the DMX21 Modem Board
Originate Mode Answer Mode
TXD 0 1 0 1 CCITT 1180 980 1850 1650
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Indicators and Manual Controls
This section describes the alternatives of the indicators and manual controls available in the FD12P DMX21 modem.
Indicators
The LED indicators of the DMX21 modem are listed and described in Table 8 below.
Table 8 LED Indicators of the DMX21 Modem
LED Indicator Description
LED V16 R Ring indicator, normally off. LED V17 ON LINE Normally lit when the S2 switch is in the UP position.
The line, however, is permanently connected by jumper connections. The V17 LED may also be off although the modem is online. The S2 switch in the DOWN position switches the V17 LED off.
LeV18 CD Indicates when a carrier frequency is detected. To
make date interchange possible, the modems must first detect the carrier frequency.
LED V19 TxD Transmitted data stream when the data is 1, the LED
is lit.
LED V20 RxD Received data stream. When the received data is 1,
the LED is lit.
Manual Controls
The manual controls and their positions are listed and described in Table 9 below.
Table 9 Manual Controls of the DMX21 Modem
Control Position Description
S2
S1
S3
¹
The ORIG mode may also be used depending on the host computer modem. When they operate in the ORIGINATE mode, the modem of FD12 P should be set in the ANSWER mode and vice versa.
UP Line relay permanently on (recommended
position). MIDDLE Line relay controlled by software (switched lines). DOWN Line relay permanently off. UP MIDDLE Software-readable switch (recommended
position). DOWN UP ORIG mode permanently on. MIDDLE ORIG/ANSWER modes under software control. DOWN ANSWER mode permanently on (normally
used).¹
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Analog Transmission
For 4-20 mA analog visibility measurement only two wires are needed. Do the following:
1. Connect the voltage supply either from remote or internal supply (from +Vb = 12 V or +V 100 ohm).
2. Connect the signal wire to screw connector X20 pin 3 "sink" at the CPU board. In the drawing, a remote voltage supply is used and the return signal is wired from pin 4 "gnd". See Figure 20 below.
For more information of the analog output port functioning and configuring, see section Analog Output Commands on page 91.
= 23 V) to resistor R (for example,
bb
9607-007
Figure 20 Analog Current Loop Option
Connecting the Maintenance Terminal
Any computer equipped with a terminal emulation software or a VT100 compatible terminal with the RS-232 serial interface can be used as a maintenance terminal for the FD12P. The optional maintenance cable provides a 9-pin D-connector for the computer and a 3-pin connector for the FD12P.
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To connect the maintenance terminal, do the following:
1. Disconnect the serial line screw connector or modem interfacing cable (or the RS-485) from X18.
2. Plug the maintenance cable into X18.
3. Refer to Figure 17 on page 51 (RS-232).

Startup Testing

Before closing the cover of the electronics enclosure, a short startup must be done as follows:
1. Connect a terminal via serial line to the sensor (See sections Serial Transmission RS-232 on page 50 or Connecting the Maintenance Terminal on page 55). Set the terminal baud rate to 300 and set the data frame to contain 7 data bits and 1 stop bit, even parity.
2. Turn on the main switch at the power supply FDW13.
3. Check that the red LED on the CPU board is lit for a few seconds, after which the green LED should start blinking. If not, continue with troubleshooting.
4. After startup, the FD12P outputs:
VAISALA FD 12P V1.XX 19YY-MM-DD SN
(ID is also included, if configured.)
5. Wait for one minute and enter the command mode with the OPEN command. Check with the STA command that no hardware errors or warnings are detected.
6. Enter the automatic message mode by typing CLOSE and check that a message appears every 15 seconds in the display.

Initial Settings

The FD12P Weather Sensor is typically interfaced to a host computer or a data logger in an automatic weather observing system. After the physical connection has been made, the communication details can be configured in the FD12P software. Suitable communication settings depend on the implementation of the whole system.
By default the sensor transmits a new ASCII data message through the serial line every 15 seconds. The user can change the interval and
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message type. The sensor can also be used in a polled mode, that is, a data message is only sent when the host computer requests one with a special command. In addition, the baud rate of the serial line can be changed to a higher value. The default communications settings are listed in Table 10 below.
Table 10 Default Communication Settings
Setting Default
Baud rate 300 baud (7E1) Polled or automatic mode, message type Sensor ID No ID set
Automatic mode, message 2 interval 15
In multipoint communication where several sensors share the same communication line, the FD12P should be used in the polled mode and individual sensors must have distinct identifiers (ID).
The baud rate should not be changed if the optional 300-baud modem is used.
The commands for changing the default settings are listed in Table 11 below. Detailed descriptions of the commands can be found in Chapter 4 on page 59.
Table 11 Commands for Changing the Default Settings
Operation Command
Baud rate selection BAUD Polled or automatic mode, message type setting AMES Sensor ID configuration CONF
The FD12P has also several changeable parameters, which control the operation of the present weather algorithm and precipitation measurement. The factory set parameter values have been found appropriate in tests and usually do not need to be changed. However, there may be conditions where other parameter values give better results.
The commands for displaying and changing the parameters are listed in Table 12 below.
Table 12 Commands for Displaying and Changing the
Parameters
Operation Command
Parameter listing WPAR Parameter change WSET
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Local practice may require changes, especially in the precipitation intensity limits and the haze threshold value. For details, see the description of the WSET command in section WSET Command on page 73.
When the precipitation intensity and amount measurement is not factory calibrated, higher accuracy can be achieved by adjusting a scaling factor with the WSET command. The new scaling factor can be calculated by comparing the FD12P against a reference rain gauge. For details, see the description of the WSET command in section WSET Command on page 73.
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CHAPTER 4

OPERATION

This chapter contains information needed to operate this product.

Introduction

The FD12P Weather Sensor is a fully automatic instrument for continuous weather measurement. Normally, the FD12P Weather Sensor is either set to send a data message automatically or it is polled by a host computer. In addition, a set of user commands is provided for configuring and monitoring the system performance. These commands can be given in the command mode. See section Entering/Exiting the Command Mode on page 61.
The FD12P Weather Sensor has seven different standard message formats for data message output. The FD12P presents the weather type using the World Meteorological Organization (WMO) code table 4680 (WaWa Present Weather reported from an automatic weather station). Code numbers 77 (snow grains), 78 (ice crystals), and 89 (hail) are from the code table 4677 because the types are not included in the code table 4680. In addition, the United States National Weather Service (NWS) abbreviations are available. The list of NWS and WMO codes is presented in Appendix A on page 143.

User Commands in Normal Operation

User intervention is not required in the normal operation of the FD12P Weather Sensor. Operator commands are used only in the initial set­up and during routine maintenance. Several commands are also available for troubleshooting.
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When the sensor has been installed, the user may need to change some of the default settings. For details, see section Initial Settings on page
56.
Table 13 below lists the settings and the corresponding commands.
Table 13 Settings and Corresponding Commands
Operation Command
Baud rate BAUD Polled or automatic mode, message type setting Sensor ID CONF Weather algorithm parameters WSET
AMES
Table 14 below lists the routine maintenance commands.
Table 14 Routine Command for Maintenance
Operation Command
Sensor cleaning CLEAN
(optional) Visibility calibration CHEC, CAL Temperature calibration FREQ, TCAL Weather algorithm parameters WSET
The standard output messages contain a status character, which presents the results of the internal diagnostics to the host computer or the user. If the sensor indicates a warning or an alarm in a standard output message, the host computer or the user can obtain a detailed status report by using a special command (STA). The status report can also be polled (message 3) in place of the standard data message. Usually, the detailed status information is sufficient for locating the fault.
Table 15 below lists the status report command.
Table 15 Status Report Command
Operation Command
Getting a status report STA
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Markings Used in This Manual

The general format of the command is the following:
COMMAND parl...parn
where
Command = An FD12P command given by the user parl...parn = Possible parameters of the command
↵↵↵↵
= Symbolizes pressing the ENTER key
NOTE
All the command parameters are separated from each other by a space character. Every user command must be ended with ENTER, illustrated in this manual by ↵↵↵.
The system output is illustrated as Courier type font, for example,
BACKSCATTER INCREASED

Entering/Exiting the Command Mode

Before any commands can be given to the FD12P, the communication line in the FD12P has to be assigned to the operator. Otherwise, it is assigned to automatic messages or polled communication. The user assigns the command mode with the OPEN command.

OPEN Command

If no device identifier (id) is defined, type
OPEN
↵↵
If id is defined, for example as A, type
OPEN A
If id is defined but forgotten, type
OPEN ^C
where ^ is the control key.
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↵.
↵↵
↵↵
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The FD12P answers
LINE OPENED FOR OPERATOR COMMANDS
If no input is given within 60 seconds, the FD12P closes the line automatically.

CLOSE Command

Line can be released to automatic data messages by typing
CLOSE
The FD12P answers
LINE CLOSED
↵↵

Automatic Message Sending

In the automatic (CLOSEd) mode, the FD12P sends the predefined message at selected intervals. The automatic message is selected by the AMES command.
AMES Message_number Message_interval ↵↵↵
where
Message_number
- The valid range is 0 ... 7, refers to section Message Types on page 63.
- Selects the corresponding message. Any negative message number is converted to 0.
- If only the message number is given, the previous message interval setting is used.
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Message_interval
- Given in multiples of 15 seconds (= measuring interval). Therefore, intervals 15, 30, 45 ... are valid. Other intervals are converted to multiples of 15 seconds. The maximum sending interval is 255 seconds (4 minutes 15 seconds).
For example, typing
AMES 1 60
selects message number 1 to be sent once in a minute.
Messages can also be displayed in the command mode with the MES command, described in section MES Command on page 71.

Message Types

All the data messages are of fixed length and the data is in fixed fields. Message 2 is intended to be used as the standard present weather message. The length of the status message depends on the possible alarm and warning states.
The FD12P adds frame strings to the polled and automatic messages. The content of the frame strings is presented in the following:
!FD id"message body#-*
where
!
FD = FD12P sensor identifier
id = Unit identifier 2 characters, if ID is not defined
" #
-*
= Start of heading (ASCII 1, non-printable character, in
terminal screen typically seen as the mark)
= Space character
characters space and 1 are shown = Start of text (ASCII 2, non-printable character) = End of text (ASCII 3, non-printable character) = CR + LF (ASCII 13 + ASCII 10)
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NOTE
The contents of messages 0, 1, 2, 5, 6, and 7 are illustrated as follows:
10 6800 110.96 <- The first row is the output.
↓↓
--------- offset frequency ìììì Field
------- one minute average visibility íííídescriptions
- 1=hardware error, 2= hardware warning îîîî
- 1= alarm 1 2= alarm 2
Example with frames
FD 110 6800 110.96
!FD 1"10 6800 110.96#-* 012345678901234567890123456
NUMBERS mark the character positions
.
Message 0
Message 0 displays only the one-minute average visibility truncated to 19900 and the offset frequency of the optical measurement hardware.
00 680 0 126.87
------- offset frequency
------- one minute average visibility
- 1=hardware error, 2= hardware warning
- 1= visibility alarm 1, 2= visibility alarm 2
An example with frames is as follows:
FD 100 6800 126.87
!FD 1"00 6800 126.87#-* 012345678901234567890123456
Message 1
Message 1 displays the instant precipitation type and the optical (volume) intensity. The intensity can be integrated to rain sum, but not snow.
00 6839 52 0.3
------- visi bility one minute average, max 50000m
- 1=hardware error, 2= hardware warning
- 1= visibility ala rm 1, 2= visibility alarm 2
------ precipitation (volume) intensity, mm/h
--- instan t precipitation type, 0 ... 99
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An example with frames is as follows:
FD 100 6839 52 0.3
!FD 1"00 6839 52 0.3#-* 0123456789012345678901234567
Message 2
Message 2 is intended as the standard present weather message used in data loggers or display units and it is set as default at the factory.
00 6839 7505 L 52 61 61 0 .33 12.16 0
------- c umulative water
------- p recipitation (volume) intensity,mm/h
--- one hour present weather code, 0...99
--- 15 minute p resent weather code, 0...99
--- ins tant present weather code, 0 ... 99
---- instan t present weather, NWS codes
------ visi bility ten minute average, max 50000m
------ visib ility one minute average, max 50000m
- 1=hardware error, 2= hardware warning
- 1= visibility ala rm 1, 2= visibility alarm 2
-----cumulative snow sum,0...999mm
sum,0...999mm
An example with frames is as follows:
FD 100 6839 75 05 L 52 61 61 0.33 12.16 0
!FD 1"00 6839 7505 L 52 61 61 0.3 3 12.16 0#-* 012345678901234567890123456789012345 67890123456789012345
Message 3
Message 3 is the same as the status message obtained by using the STA command. Refer to Table 22 on page 86 for possible error texts.
FD12P STATUS
SIGNAL 0. 39 OFFSET 126.83 DR IFT 0.14 REC. BACKSCATT ER 1281 CHANGE -1 TR. BACKSCATTE R 10.3 CHANGE 0.1 TE 2.7 VBB 19.4 VH 0.6 LEDI 5.6 P15 15.1 M15 -15.0 BGND -0.1 AMBL 0.1 D UTY 1.6 DRI21 MEASUREM ENTS TS 1.8 DRD INST 811 DRY 915.6 HARDWARE :
OK
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Message 4
Message 4 is for hardware monitoring. The same data line is obtained by using the FREQ command.
0.51 126.82 0.91 15 3 5 2.7 5.6 1280 19.5 1.8 819
>FREQ
SIGNAL+ OFFSET DIST SWID MAXI O WID TE LEDI BACKS VBB TS DRD
0.59 126.83 0.91 17 2 6 2.7 5.6 1280 19.4 1.8 816
817
0.59 126.83 0.91 17 2 6 2.7 5.6 1280 19.4 1.8
Messages 5 and 6
Messages 5 and 6 are for MITRAS transmissometer message emulation in VAISALA RVR systems.
An example of MES 5 is provided below:
"ID 1 V 1050 CV ////// B ///// S0101 -*#
--- <CR> <LF><ETX>
-- receive r 2 status (two spa ces)
-- receiver 1 sta tus
-- trans mitter status
-- S, status hea ding
------ bac kground luminance value cd/m2(option)
-- B, background luminance heading
------- (contamination) compensated visibility (reserved, not supported by FD12P)
--- CV, compensated visibility heading
------- non-compensated visibility m
-- V, visibility heading
-- unit ID (one character on ly)
-- ID, start indic ator
- <STX>
"ID 1 V 1050 CV ////// B ///// S0101 -*# 123456789012345678901234567890123456 7890123
An example of MES 6 is provided below:
"ID 1 V 4550 B ///// S4101 -*#
--- <C R><LF><ETX>
-- receiv er 2 status
-- receive r 1 status
-- tran smitter status
-- S, status h eading
------ ba ckground luminance value cd/m
-- B, background luminance heading
------- non-compensated visibility m
-- V, visibility heading
-- unit ID (one character on ly)
-- ID, start indic ator
- <STX>
2
"ID 1 V 4550 B ///// S4101 -*# 123456789012345678901234567890123
The status is in hexadecimal notation.
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The status bits emulate the MITRAS status as shown in Table 16 and Table 17 below.
Table 16 Transmitter Status Correspondence between
MITRAS and FD12P
BIT MITRAS FD12P
0 MEAS MODE ON 1 ECON MODE OFF 2 OPTICAL SURFACE TRB warning or alarm 3 POWER SUPPLY VBB (power supply) 4 HEATING VH (lens heater current) 5 FLASH LAMP LEDI (LED intensity control) 6 BL METER BL meter interface (DRI21)
connected
7 MEASUREM. LOOP SIGNAL -
Table 17 Receiver Status Correspondence between
MITRAS and FD12P
BIT MITRAS FD12P
0 MEAS MODE ON 1SPARE 2 OPTICAL SURFACE REC. BACKSCATTER
warning or alarm 3 POWER SUPPLY ±15 V status 4 HEATING
OK = OFF 5 CALIBRATION AMBL 6 TEST
OK = OFF 7SPARE
The MITRAS polling command is the following:
P<sp><ID><cr><lf>
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Message 7
Message 7 is meant mainly for aviation specific purposes. The message contents are as follows:
00 6839 7505 L 52 61 61 0.33 12.16 0 23.4 12345
------background luminance cd/m
------ surface temperature
----- cumulative snow sum
------- cumulative water sum
------- precipitation water intensity,mm/h (1 minute average)
--- one hour present weather code, 0 ... 99
--- 15 minute present weather code, 0 ... 99
--- instant present weather code, 0 ... 99
---- instant present weather, NWS codes
------ visibility ten minute average, max 50000m
------ visibility one minute average, max 50000m
- 1=hardware error, 2= hardware warning
- 1= visibility alarm 1, 2= visibility alarm 2
-RA , instant METAR weather codes
RERA , recent METAR weather (RE criteria used)
An example with frames is as follows:
FD 100 6839 7505 R 61 61 61 0.33 12.16 0 23.4 /////
-RA RERA
!FD 1"00 6839 7505 R 61 61 61 0.33 12.16 0 23.4 /////-*
-RA-* RERA-*
#-*
2
Message 7 consists of four lines. METAR present weather codes are output on the second and third lines. These lines are not of fixed length because METAR codes can be combined in many ways. The METAR codes may also be left out but the lines of the message are always terminated by a carriage return and line feed characters.
The background luminance value displays the measured luminance in cd/m², if the Vaisala LM11 Background Luminance Meter is attached to the FD12P (option FD12PLM11). If a day/night switch is connected to the processor board, the background luminance value displays the switch state (1 = day, 0 = night).
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Message Polling

In the polled (CLOSEd) mode, the FD12P sends a data message when the host computer transmits a polling command. The message polling mode is selected by the following command:
AMES Message_number Message_interval ↵↵↵
where
Message_number
- The valid range is 0 ... 7, refer to section Message Types on page 63.
- Selects the corresponding message as the default polled message. Any negative message number is converted to 0.
Message_interval
- Negative or zero interval is used to disable the automatic sending. This is used when messages are polled.
For example, AMES 0 0 selects message 0, and cancels the automatic sending.
The polling command format is the following:
<ENQ> FD <SP> id <SP> message_number <CR>
where
<ENQ> = ASCII character 05 hex <SP> = ASCII character 20 hex (space) id = Selected in the configuration message_
= Optional
number
If only one FD12P unit in on the line and no id is set, the command format is the following:
<ENQ> FD <CR>
When the FD12P unit number two (id = 2) is polled for message number 3, the command format is the following:
<ENQ> FD <SP> 2 <SP> 3 <CR>
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This format is also used when several devices are on the same line. Use character 1 as the id if the id has not been set but a specific message type is polled.
The FD12P does not echo the polling character string.
The answer message format is the following:
<SOH> FD <SP> id <STX> text <ETX><CR><LF>
The id field always contains two characters. If only one character has been set as the id, the sensor will output an <SP> as the first character in the field.
When there are several devices on the same line, the polled unit turns the modem (DMX21) carrier on after it has acknowledged the request. When the carrier is switched on, additional characters will appear before the <SOH> (01 hex) character. The FD12P waits about 100 ms after turning the carrier on before it starts to send the message. When the FD12P has sent the message, it turns the carrier off. This will also generate additional characters, which have to be ignored by the host.

FD12P Command Set

HELP Command

The operator receives information about available commands by typing
HELP
The HELP command sets are listed in Table 18 below.
Table 18 HELP Command Sets
Command Description
OPEN Assigns the line for operator commands CLOSE Releases the line for automatic messages MES Displays data message AMES Number Interval
STA Displays status PAR Parameter message WPAR Weather parameter message PRW Present weather message CONF Password CLEAN Sets clean references
Automatic message (with parameters Number and Interval)
Updates configuration
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Command Description
CHEC Displays average signal CAL Calibrator_frequency CLRS Clears precipitation sums ACAL Analog output calibration TIME hh mm ss DATE yyyy mm dd BAUD rate par
AN channel
DAC data RESET Hardware reset by watchdog WHIS Present weather history WSET PRW reference values DRY ON Sets DRD dry offset WET ON Sets DRD wet scale BLSC Sets/Displays background luminance scale
Calibration
Sets/displays system time Sets/displays system date Baud rate setting (Rate 300, 1200, 4800, 9600) (Par E(7E1) or N(8N1) Analog channel (0,1,3,8 ... 15 or ANALOG ID) (Without DATA = SWEEP)

MES Command

After opening the line for operator commands (see section Entering/Exiting the Command Mode on page 61), a data message can be displayed using the MES command. There are eight messages available for different uses and they numbered from 0 to 7. Refer to section Message Types on page 63 for message type descriptions.
The command format is the following:
MES Message_number ↵↵↵
with a valid range from 0 to 7. For example, when choosing the data message number 2, type
MES 2
AMES Command
The AMES command defines the message, which the FD12P transmits as the automatic message or as the default polled message. Messages can also be displayed by the MES command, described in MES Command on page 71.
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The format of the AMES command is the following:
AMES Message_number Message_interval
where
Message_number
- The valid range is 0 to 7.
- Selects the corresponding message. Any negative message number is converted to 0.
- The message number is also the default number for the MES command and polling.
- If only the message number is given, the previous interval setting is used.
Message_interval
- Given in multiples of 15 seconds (= the measuring interval). Therefore intervals 15, 30, 45 ... are valid. Other intervals are converted to integer multiples of 15 seconds. The maximum sending interval is 255 seconds (4 minutes 15 seconds).
- Negative or zero interval ignores the automatic sending. This is used when messages are polled. Refer to section Message Polling on page 69 for details.
For example, typing
AMES 1 60
selects message number 1 to be sent once in a minute.
Typing
AMES 0 0
selects message 0, and cancels the automatic sending.
The AMES command without parameters displays the current selection and it is the following:
AMES ↵↵↵
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Weather Related Commands

To display or set the weather analysis parameters and results, use the commands listed in Table 19 below.
Table 19 Commands for Displaying or Setting Weather
Analysis Parameters
Command Parameters and Results
WPAR Weather parameter message WSET Prw reference values PRW Present weather message CLRS Clear precipitation sums WHIS Present weather history
WPAR Command
Use the WPAR command to display the present weather analysis parameters. Typing WPAR displays the following output:
WEATHER PARAMETERS PRECIPITATION LIMIT 40
WEATHER UPDATE DELAY 6 HAZE LIMIT 9000 RAIN INTENSITY SCALE 0.80 VIOLENT RAIN LIMIT 50 HEAVY RAIN LIMIT 8.0 LIGHT RAIN LIMIT 2.0 DRIZZLE LIMIT 15 HEAVY DRIZZLE LIMIT 30 LIGHT DRIZZLE LIMIT 3 WARM LIMIT 8.0 SNOW LIMIT 5.0 HEAVY SNOW LIMIT 600 LIGHT SNOW LIMIT 1200 SNOW PELLETS LIMIT 30 SNOW GRAINS LIMIT 20 ICE CRYSTALS LIMIT 40 HAIL LIMIT 300 DRD SCALE 1.5 DRD DRY OFFSET 900.0 DRD WET SCALE 0.0016
WSET Command
The WSET command is used to modify the present weather analysis parameters.
The command asks for one parameter at a time, showing the parameter name and the current setting. Accept the current value by pressing ENTER. You can give a new value by typing the value before pressing ENTER.
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Type
WSET
and the output is as follows:
SET PRESENT WEATHER PARAMETERS PRECIPITATION LIMIT ( 40) WEATHER UPDATE DELAY ( 6) HAZE LIMIT ( 9000) RAIN INTENSITY SCALE ( 0.80) VIOLENT RAIN LIMIT ( 50) HEAVY RAIN LIMIT ( 8) LIGHT RAIN LIMIT ( 2) DRIZZLE LIMIT ( 15) HEAVY DRIZZLE LIMIT ( 30) LIGHT DRIZZLE LIMIT ( 3) SNOW LIMIT ( 5.0) HEAVY SNOW LIMIT ( 600) LIGHT SNOW LIMIT ( 1200) SNOW PELLETS LIMIT ( 30) SNOW GRAINS LIMIT ( 20) ICE CRYSTALS LIMIT ( 40) HAIL LIMIT ( 300) DRD SCALE ( 1.5) WARM LIMIT ( 8.0)
The parameters are described in detail below.
Precipitation Limit
The Precipitation limit parameter is the threshold of accumulated particle magnitudes (in FD12P internal units) that reports the precipitation on state. The typical parameter value is 40. The smaller value represents a more sensitive operation and faster response at the beginning of an event.
Weather Update Delay
The Weather update delay parameter is a time as multiple of 15 seconds, during which the instant precipitation type is not changed. The intensity may change faster.
Haze Limit
The Haze limit parameter specifies the visibility threshold for reporting haze or mist. When the visibility is between 1000 m and the Haze limit, the FD12P will report either haze or mist depending on the air humidity.
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Rain Intensity Scale
The Rain intensity scale parameter is multiplied by the measured raw intensity, which gives the reported precipitation intensity (optical). The rain amount is scaled with the same coefficient because the amount is a direct integral of 15-second intensities.
The typical value for the Rain intensity scale is 0.8. as the optimal value is complex to determine; it depends on the optical, opto­electronic, and electronic parameters. No applicable factory calibration method has been developed yet.
The precipitation measurement can be calibrated by comparing the FD12P rain amount to measurements made with a suitable reference rain gauge. Make the comparison after a few rainfalls with 5 mm or more of total accumulated rain. A new scaling factor can be calculated using the following formula:
Newscale = Oldscale × (Ref_Amount/FD12P_Amount)
where
Oldscale = Old value of Rain Intensity Scale Ref_Amount = Amount measured with the reference rain gauge FD12P_Amount = Corresponding amount measured by the FD12P
Violent Rain Limit
The Violent rain limit parameter defines the minimum rain intensity (mm/h), when the intensity is violent.
Heavy Rain Limit
The Heavy rain limit parameter defines the minimum rain intensity (mm/h), when the intensity is heavy.
Light Rain Limit
The Light rain limit parameter specifies the maximum rain intensity (mm/h), when the intensity is light. If the rain intensity is between the above heavy and light limits, it is moderate.
Drizzle Limit
The Drizzle limit parameter refers to the maximum drop size (in FD12P internal units), which can be detected as drizzle. The typical value is 15, which has been found to be the optical signal from a 0.5 mm diameter droplet measured by typical FD12P hardware. The
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parameter value relates to the square of droplet radius. The relationship is the following:
X = 240× R
2
where
X = Parameter value R = Droplet radius
Parameter value 30 would correspond to about a 0.7-mm droplet diameter.
Heavy Drizzle Limit
The Heavy drizzle limit parameter refers to the minimum number of drizzle droplets detected in 15 seconds. They must be detected before drizzle becomes heavy (dense).
Light Drizzle Limit
The Light drizzle limit parameter defines the maximum number of droplets detected in 15 seconds, when drizzle is light.
Snow Limit
The Snow limit parameter specifies the minimum ratio of optical precipitation intensity to surface sensor (DRD12) precipitation intensity, when the precipitation is snow. A half of this value is used for separating sleet and ice pellets.
The typical value for Snow limit is 5. A smaller value directs the FD12P to report more wet precipitation as snow.
Heavy Snow Limit
The Heavy snow limit parameter defines the minimum visibility (m) on a two-minute average in heavy snow.
Light Snow Limit
The Light snow limit parameter defines the maximum visibility (m) on a two-minute average in light snow. If snow is detected and the two­minute visibility average is between the above heavy and light snow limits, snow intensity is moderate.
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Snow Pellets Limit
The Snow pellets limit parameter specifies the minimum particle size (in FD12P internal units), which is detected as snow pellets. (Additional internal criteria are used before the precipitation type is determined to be snow pellets.)
Snow Grains Limit
The Snow grains limit parameters refers to the maximum particle size (in FD12P internal units), which is detected as snow grains.
Ice Crystals Limit
The Ice crystals limit parameters defines the maximum particle size (in FD12P internal units), which is detected as ice crystals. (Additional internal criteria are used before the precipitation type is determined to be ice crystals.)
Hail Limit
The Hail limit parameters refers to the minimum particle size (in FD12P internal units), which is detected as hail. (Additional internal criteria are used before the precipitation type is determined to be hail.)
DRD Scale
The DRD scale parameter is the scaling factor for the calculated intensity of the DRD12 surface sensor. The typical value for this parameter is 1.5. The value is also good for a very clean DRD12. When the DRD12 becomes dirty after some precipitation events, it becomes more sensitive, especially for light rain. Thus, a smaller value of the scale could be used.
Warm Limit
The Warm limit parameter defines a more flexible, maximum snow reporting temperature limit, which is required in some areas. The nominal value is +8 °C.
PRW Command
The Present Weather command (PRW) command, displays a verbal format message.
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When you type
PRW
the system output is the following:
PRESENT WEATHER MODERATE DRIZZLE
VISIBILITY 7161 m AVE 10 MIN 7533 RAIN INTENSITY 0.16 mm/h CUMULATIVE SUM 12.16 SNOW INTENSITY 0.0 mm/h CUMULATIVE SUM 0 TEMPERATURE 2.7 TS 1.8 DRD SUM 22.04
CLRS Command
The CLRS command resets (to 0.00) the cumulative sums of precipitation. This resetting can also be done in the protocol mode by the host computer, using the following command format:
<ESC> FD id C <CR>
Then the FD12P responds to the accepted command with the following ASCII character:
<ACK> (06 hex)
WHIS Command
The WHIS command displays the instant precipitation type codes (NWS) for one hour.
Type
WHIS
to get the results shown on the next page.
PRW HISTORY
LLLLLLLLLLLLLLLLR-R-R-R-R-R-R-R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R- R R R R R R R- R- R- R- R- R- R- R- R- R- R- R- R - R- R- R- R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R-R-R-R-R-R-RRRRRRR-R-R-R-R-R-R-R-R-R-R-R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R­R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R-
>
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System Configuration Commands

Table 20 below lists the commands that can be used to display system parameters and to edit current system configuration.
Table 20 Commands for Displaying System Parameters and
Editing the Current System Configuration
Command Description
PAR Parameter message CONF password BAUD rate par Set baud rate (rate 300,1200,2400,4800,9600)
BLSC Set/display background luminance scale
Update configuration
Par E (7E1), N (8N1)
PAR Command
The current system parameters can be displayed by using the PAR, the System Parameters, command.
When you type
PAR
the system output is shown on the next page.
SYSTEM PARAMETERS VAISALA FD12P V 1.83 1999-11-19 SN: 46401 ID STRING: AUTOMATIC MESSAGE 2 INTERVAL 15 ALARM LIMIT 1 0 ALARM LIMIT 2 0 OFFSET REF 130.50 CLEAN REFERENCIES TRANSMITTER 8.9 RECEIVER 1769 CONTAMINATION WARNING LIMITS TRANSMITTER 1.5 RECEIVER 200 CONTAMINATION ALARM LIMITS TRANSMITTER 5.0 RECEIVER 500 SIGNAL SCALE 1 1.485 SCALE 0 0.000 TE OFFSET 58.5 TS SCALE 1 0.058 SCALE 0 -58.969 ANALOG VISIBILITY MAX 20000 MIN 10
LINEAR MODE
ANALOG OUTPUT SCALE 1 0.143 SCALE 0 713.00
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CONF Command
The configuration command, CONF, is used to set or update system parameters and to adjust certain calibrations, reference values, and limits. You can limit the use of this command by protecting it with a password. New parameter values are saved in the non-volatile memory (EEPROM).
The CONF command displays the parameters one by one and asks for a new value. In most cases, the current value is shown as the default value. The parameter is not updated if the user only presses the ↵↵↵ key.
You can modify the following system parameters using the CONF command:
- Vis Alarm Limits
- Offset Freq Reference
- Temperature TE Scale
- Password Characters
- Unit Id Characters (2)
- References And Limits for Contamination Monitoring
- Analog Output Minimum Visibility
- Analog Output Maximum Visibility
- Analog Output Lin/Log
To prevent unauthorized change of the system parameters, a four-
character password can be set at the beginning of the CONF setting. You can also modify the password then. When you do not want to set or modify the password, press ↵↵↵ .
When a password has been set in the previous session, the command format is the following:
CONF password
To change the password, type
CONF password N (N stands for new).
NOTE
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You must also know the previous password.
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When no password has been set, the command format is the following:
CONF
The system response to the CONF command is presented below (The bold text refers to user actions.)
CONF. PASSWORD (4 CHARS MAX)
UPDATE CONFIGURATION PARAMETERS UNIT ID (2 CHAR) ( 1) SET REFERENCE PARAMETERS ( 25.9) TE OFFSET OFFSET REFERENCE UPDATED ALARM LIMIT 1 ( 1000) ALARM LIMIT 2 ALARM LIMIT 2 UPDATED TRANSMITTER CONTAMINATION LIMITS WARNING LIMIT WARNING LIMIT UPDATED ALARM LIMIT ( 5.0) RECEIVER CONTAMINATION LIMITS WARNING LIMIT ( 100) ALARM LIMIT ALARM LIMIT UPDATED ANALOG OUTPUT MODE 0 = LINEAR1=LN(0) ANALOG OUTPUT RANGE MAX VISIBILITY ( 10000) MIN VISIBILITY ( 50) END OF CONFIGURATION
( 127.48) Y
( 200) 300
( 1.0) 1.5
( 500) 600
The questions asked by the system are described below.
First the system asks for a new password:
CONF. PASSWORD (4 CHARACTERS MAX)
This question is asked when there is no valid password or the existing password is updated. If updating is requested by the N parameter and an empty line is given for an answer, the password is removed. Otherwise, the user gives a new password to the system.
The system asks the following:
UPDATE CONFIGURATION PARAMETERS
UNIT ID (2 CHAR)( 1)
If the FD12P unit is named by one- or two-character ID codes, the OPEN and POLLING commands use it as a parameter. The ID code is
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also included in the data message heading. ID 1 is used as a default in the message heading if no other ID is given. The current ID can be removed by pressing " - " as an answer to the question.
In the multidrop configuration, where several FD12 Weather Sensors are on the same communication line, the units are differentiated by the ID.
The next CONF parameters are hardware- or system-dependent. They can be changed from the factory set values for better performance or maintenance purposes. The example configuration session is explained in the following.
The single point calibration of the TE backup temperature measurement can be done by giving the temperature.
SET REFERENCE PARAMETERS
TE ( 25.9)
The default value is the current temperature. If it is not correct, a new value must be typed as the answer. The new value is used to correct the internal TE scaling factor. The TE temperature is used as a backup in FD12P. The temperature is used in the visibility measurement to control the precipitation effect correction algorithm. Snow and rain have a different kind of effect on the scattering signal when it is used for the visibility calculation.
The currently measured offset value (not a parameter) is shown in the brackets (see next page).
OFFSET ( 127.48) Y
OFFSET REFERENCE UPDATED
After receiving Y as an answer, the system accepts the offset frequency to be the reference parameter for hardware monitoring. The parameter value is further compared with the current value to detect drift or other failure in the optical signal measurement electronics.
The visibility alarm limits are checked. Limit 1 is expected to be higher than Limit 2. The limit values are expressed in meters.
ALARM LIMIT 1 ( 1000) ALARM LIMIT 2 ALARM LIMIT 2 UPDATED
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( 200) 300
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In the above example, alarm Limit 2 receives a new value, 300 m. When the visibility now weakens below Limit 2, then the data message (0 to 2) data status is set to 2. The visibility alarm is not shown in the status message.
The backscatter/contamination control is done by comparing the current values of backscatter signal with the reference values given with the CLEAN command. The limits given here are limits for the change in backscatter signals.
TRANSMITTER CONTAMINATION LIMITS WARNING LIMIT WARNING LIMIT UPDATED ALARM LIMIT ( 5.0)
( 1.0) 1.5
The transmitter values are expressed in volt (V). The measurement range is 0 to 13 V, where 0 V is a blocked lens. The limit value is given as a positive value although the signal becomes smaller when contamination increases.
A contamination change of 5 V represents about a 10 % decrease in the transmitter's lens transmittance (as also does the same increase in the visibility indication).
RECEIVER CONTAMINATION LIMITS WARNING LIMIT ( 300) ALARM LIMIT ALARM LIMIT UPDATED
( 500) 600
The receiver values are expressed in hertz (Hz). The measurement range is from 0 to 10000 Hz, where 10000 Hz is a blocked lens. A contamination change of 500 Hz represents about a 10 % decrease in the receiver's lens transmittance.
The analog output mode and visibility range are set last. In the logarithmic mode, the minimum visibility must be different from 0 as LN(0) is not defined.
ANALOG OUTPUT MODE 0 = LINEAR1=LN(0)
ANALOG OUTPUT RANGE
MAX VISIBILITY ( 10000) MIN VISIBILITY ( 50) END OF CONFIGURATION
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BAUD Command
The baud rate and communication type can be changed by typing following the operator command:
BAUD value communication_ type
The baud rates are 300, 1200, 2400, 4800, and 9600. The communication types are E (7E1) and N (8N1).
The new value is saved in EEPROM and it is used also after reset or power up. The default baud rate set at the factory is 300 baud (7E1). Defining the communication type is optional. It does not change if the baud rate is changed. Other baud rates than 300 baud are not allowed with the DMX21 modem.
The BAUD command displays the current baud rate and communication type. For an example, see the following:
BAUD RATE: 300 E71
BLSC Command
The Vaisala LM11 Background Luminance sensor can be connected to the FD12P for ambient light measurement. Each LM11 sensor has an individual scaling coefficient, which is defined at the factory. The scaling coefficient is written on a label in the LM11 sensor. This coefficient should be configured to the FD12P for correct scaling of the measured background luminance values.
The BLSC command is used to set or display the background luminance scale.
When you type
BLSC
↵↵
it displays the current background luminance scale.
When you type
BLSC Scaling_factor
↵↵
it sets the new background luminance scale.
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If the LM11 is not connected, the scaling factor should be negative. Value -1.0 has been set at the factory as the default value. If a positive value is used, the sensor expects a signal from the LM11.
For an example, see the following:
>blsc
BL SCALE -1.000
>blsc 10.4
BL SCALE 10.400
If a day/night switch is connected to the serial line control input on the FDP12 processor board, the FD12P can read the switch state and report it as a background luminance value of 1 (day) or 0 (night). The FD12P firmware will read the switch if the background luminance scaling factor is set to 0.

Maintenance Commands

The maintenance commands are listed in Table 21 below.
Table 21 Maintenance Commands
Command Description
STA
CAL Calibrator_frequency
TCAL
CLEAN
CHEC
FREQ
DRY ON
WET ON
AN CHANNEL
STA Command
The STA command displays the results from the built-in test system as a status message. Message 3 gives the same status message as the STA command.
Displays status. Calibration. Temperature measurement calibration. Sets clean references. Displays average signal. Displays internal signals. Sets DRD12 dry offset. Sets DRD wet scale. Analog channel (0,1,3,8 ... 15 or ANALOG ID).
When you type
STA
↵↵
the system output is the following:
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SIGNAL 0.39 OFFSET 126.83 DRIFT 0.14 REC. BACKSCAT TER 1281 CHANGE -1 TR. BACKSCATT ER 10.3 CHANGE 0.1 TE 2.7 VBB 19.4 VH 0.6 LEDI 5.6 P15 15.1 M15 -15.0 BGND -0.1 AMBL 0.1 DUTY 1.6 DRI21 MEASURE MENTS TS 1.8 DRD INST 811 DRY 915.6 HARDWARE :
OK
An asterisk (*) before a value indicates an exceeded limit.
In the end, there are verbal comments on the combined errors detected. These comments can be one or many of the following listed in Table 22 below.
Table 22 Hardware Error Texts
Error text Description
BACKSCATTER HIGH
TRANSMITTER ERROR
+15 V POWER ERROR
OFFSET ERROR
SIGNAL ERROR
RECEIVER ERROR
DATA RAM ERROR EEPROM ERROR
The receiver or transmitter contamination signal has increased more than the ALARM limit given in the configuration. The LEDI signal is more than 7 V or less than
-8 V. The receiver/transmitter power is less than 14 V or more than 16 V. The offset frequency is zero (cable is disconnected). The signal frequency is less than 50 % of the offset frequency. Too low signal detected in the receiver backscatter measurement. The error is in RAM read/write check. This is an EEPROM checksum error.
The hardware warning texts are listed in Table 23 below.
Table 23 Hardware Warning Texts
Warning text Description
BACKSCATTER INCREASED
TRANSMITTER INTENSITY LOW RECEIVER SATURATED OFFSET DRIFTED
LENS HEATER OFF DRI21 NOT
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The receiver or transmitter contamination signal has increased more than the WARNING limit selected in the configuration. The LEDI signal is less than -3 V.
The AMBL signal is less than -9 V.
The offset has drifted more than ±5 Hz from the reference value. No current flowing to lens heaters. The DRI21 board cannot be detected.
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Warning text Description
CONNECTED
TS SENSOR ERROR
DRD12 ERROR
LUMINANCE SENSOR
ERROR
TE SENSOR ERROR
VISIBILITY NOT
CALIBRATED
The DTS14B measurement is off limits. The DRD12 analog signal is close to zero. The LM11 signal is zero (not checked if the BLSC is negative). Box temperature sensor TE measurement is off limits. The visibility calibration coefficient has not been changed from the default value.
CAL Command
The CAL command is used to calibrate the visibility measurement. The calibration is done by using opaque glass plates with known scatter properties.
The command type is the following:
CAL Calibrator_signal_value ↵↵↵
Type, for example, the following:
CAL 985
↵↵
The calibrator signal value is printed on the labels of the glass plates. Typically, the signal is close to 1000 Hz. The FD12P calculates a new scaling factor and stores it in the non-volatile memory (EEPROM). Refer to section Calibration on page 119 for instructions.
TCAL Command
The TCAL command is used to calibrate the sensor crossarm temperature (TS) measurement. Only 0 °C temperature is important in its accuracy because it is used in the identification of freezing rain.
When you type
TCAL
↵↵
the command displays the current scaling factors.
Without a parameter, the command displays the current scaling factors and current TS.
When you type
TCAL TS
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the command initializes the two-point calibration sequence, where two temperatures must be simulated.
When you type
TCAL TS DTS14B_temperature
↵↵
a single-point calibration to the TS is made. That is, the scaling factor TS 0 is adjusted by the command routine.
The following command
TCAL TS 0.0
↵↵
makes a zero calibration, if the temperature sensor DTS14B is in an ice bath or otherwise at a temperature of 0 °C .
The following command
TCAL TS 0.0581 -59.0
↵↵
sets both scaling factors.
The system output is as follows:
DRI TEMPERATURE SCALES
TS 1 0.0581 TS 0 -59.0000 TS 2.8
CLEAN Command
The CLEAN command has no parameters and it is used to set the clean references for contamination control. This command is given during maintenance procedures after cleaning the lenses or after replacing the transmitter or receiver board.
When you type
CLEAN
the FD12P output is as follows:
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↵↵
CLEAN REFERENCES TRANSMITTER 12.0 RECEIVER 1402 UPDATED
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CHEC Command
The CHEC command is used in the visibility calibration procedure to display the two-minute average signal frequency in hertz. The command has no parameters.
The display is terminated by pressing ESC. Pressing any other key will pause the display. In the beginning, the eight-location buffer is filled with the first value. The buffer is used to calculate the average
When the FDA13 calibrator is installed, the value displayed in the message should be the same as printed on the calibrator glass plate. In clear air the value should be near zero.
When you type
CHEC
↵↵
the output is the following:
SCALED FREQUENCY AVE (2 MIN)
999.9938
999.9880
>
FREQ Command
The FREQ command is for hardware monitoring. Message 4 gives the same data line as the FREQ command.
An example of the output is the following:
>freq
SIGNAL+ OFFSET DIST SWID MAXI OWID TE LEDI BACKS VBB TS DRD
0.03 129.79 1.0042424.4 5.3 1303 19.5 23.1 900
0.03 129.79 1.0042424.4 5.3 1303 19.5 23.1 900
A new line is printed every 15 seconds. The command output is terminated by pressing the ESC key. The first line is a title line with the signal names.
DRY and WET Commands
The DRY and WET commands are used to check and adjust the operation of the Rain Detector DRD12 analog signal measurement.
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The DRY command is used to set the dry signal end of the DRD12 signal normalization calculation.
When you type
DRY
↵↵
the output is, for example, the following:
DRD DRY OFFSET 915.6
The DRY OFFSET value must be between 850 and 980 when the DRD12 hardware operates normally. The DRY command shows this parameter. The parameter is set by the DRY ON command.
When you type
DRY ON
↵↵
the WET command without a parameter shows the scaling factor that normalizes the DRD12 signal change from the dry state to the wet state to 1.00. A typical value is 0.0015. An example is shown in the following:
WET
DRD WET SCALE 0.00169
The WET ON command is used to set the parameter. The DRD12 measuring surfaces must be coated with a wet cloth or immersed in water, when the WET ON command is given. An example of the command is given below:
WET ON
AN Command
The AN command can be used continuously to display the selected analog monitor channel. The channel ID can be used as a parameter, instead of the channel number. Thus, the AN AMBL command is the same as AN 12.
The message consists of the raw binary number from the A/D converter and the corresponding scaled and filtered value.
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Type
AN AMBL
WAITING FOR MULTIPLEXER
ANALOG INPUT, AMBL
118 0.1
119 0.3
↵↵

Analog Output Commands

Analog Output Calibration
The DAC output voltage is converted to current, 0 to 22 mA unscaled. This current is then software-calibrated to give 4 mA at the minimum visibility and 20 mA at the maximum visibility. The minimum and maximum visibility values are set in the configuration session. A hardware error is indicated by 0 mA.
The ACAL command sets two-bit values, 4000 and 800, to the digital-to-analog converter. The corresponding currents measured by a multimeter must be given as answers to the questions asked in the commands. The analog output scaling factors, which define the bits/mA relation, are then calculated by the software. The scaling factors are Scale 0 and Scale 1. The FD12 calculates them as follows:
Scale 0 = 4×((4000-800)/(high current - low current))
The bit value that gives 4 mA. Scale 1 depends on the mode.
In linear mode:
Scale 1 = bits16mA / (maximum vis - minimum vis)
In logarithmic mode:
Scale 1 = (ln(max vis) - ln(min vis)) / bits16mA
bits16mA = (3200 / (high current - low current)) × 16
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When you type
ACAL
↵↵
the command gives, for example, the following output:
MEASURED CURRENT (mA) 22.16
MEASURED CURRENT (mA) 4.52
Data Scaling
The FD12 scales the visibility value to a binary number for the DAC ( = DACBITS) so that the minimum visibility corresponds to the 4 mA-calibrated value and maximum visibility to the 20 mA­calibrated value.
In linear mode:
DACBITS = (visibility - min visibility) × scale 1 + scale 0
In logarithmic mode:
DACBITS = (ln(visibility) - ln(min visibility)) × scale 1 + scale 0
If visibility is less than minimum visibility then
DACBITS = bits4mA = scale 0
If visibility is more than maximum visibility then
DACBITS = bits 20mA
Hardware Check
The DAC bit value from 0 to 4095 can be given as a parameter. The value does not change until you press ESC. When the DAC command has been given without a parameter, the analog output sweeps slowly from 0 to maximum and from 0 until you press the ESC key.
For example:
DAC 800
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Other Commands

TIME Command
The TIME command is useful for maintenance purposes.
To display the current system time, type
TIME
↵↵
The system output is, for example, the following:
10:11:12
To set the time, use the following command:
TIME hh mm ss ↵↵↵
NOTE
where
hh = Hours mm = Minutes ss = Seconds
Reset the time after a power break.
DATE Command
The DATE command is used to display the current date.
Type
DATE
To set a new system date, use the command:
DATE yyyy mm dd↵↵↵
↵↵
where
yyyy = Year mm = Month dd = Day
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RESET Command
The RESET command makes the hardware reset by the watchdog circuitry.
The command format is the following:
RESET
↵↵
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CHAPTER 5

FUNCTIONAL DESCRIPTION

This chapter gives a functional description on the product.

General

The FD12P Weather Sensor is an optical sensor that measures visibility (meteorological optical range, MOR, and precipitation intensity and type. The FD12P measures visibility using the forward scatter measurement principle. Light scatters from particles whose diameter is in the order of magnitude of the light wavelength. The amount of scatter is proportional to the attenuation of the light beam.
Larger particles behave as reflectors and refractors and their effect on the MOR must be handled separately. Usually, these particles are precipitation droplets. The FD12P optical arrangement allows for individual droplets to be detected from rapid signal changes. The FD12P software calculates the precipitation intensity by analyzing the amplitudes of these changes. This intensity estimate is proportional to the volume of the precipitation droplets.
The optical signal also contains some information about the precipitation type but not enough for reliable identification. Additional information is needed, especially in conditions where the precipitation is very light or the weather is windy. As the extra parameter, the FD12P measures an estimate of the water content of precipitation with the DRD12 rain detector. In rain, the water equivalent and the volume are equal. However, in snow the optical volume estimate is about ten times larger. This difference of approximately one decade is used to discern between rain and snow.
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Optical Measurement

Optical Arrangement

0110-185
Figure 21 FD12P Optical System
The following numbers refer to Figure 21 above:
1 = FDT12 transmitter 2 = FDR12 receiver 3 = Sample volume
The FD12P measures light scattered at an angle of 33°. This angle produces stable response in various types of natural fog. Precipitation droplets scatter light in a different manner than fog. Thus, their contribution to visibility must be analyzed separately. The FD12P can detect and measure precipitation droplets from the optical signal and use this information in processing the scatter measurement results.
The FD12P has a small sample volume of about 0.1 liters (see Figure 21 above). This enables independent particles to be measured even at quite heavy precipitation intensities. The signal levels from even the smallest droplets can also be detected.

FDT12B Transmitter Unit

The transmitter unit consists of an infrared LED, control and triggering circuits, LED intensity monitor, backscatter receiver, and analog multiplexer.
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9611-002
Figure 22 FDT12B Transmitter Block Diagram
The transmitter unit electronics pulses the IR-LED at a frequency of
2.3 kHz. One PIN-photodiode monitors the transmitted light intensity. The transmit level measurement is used to automatically keep the LED's intensity at a preset value. The "LEDI" feedback voltage is channeled through the analog multiplexer to the CPU for monitoring.
The feedback loop compensates for temperature and aging effects of the light-emitting diode. On the other hand, the active compensation slightly accelerates the LED aging. For this reason, the initial LED current is set to a value, which guarantees several years of maintenance-free operation.
A reset pulse (RES) from the FDR12 Receiver synchronizes the IR­LED timing with the receiver's lock-in amplifier. The CPU can also delay the transmitter firing for a special out-of-phase measurement. This feature is used in measuring the internal noise level (offset) of the circuitry.
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An extra photodiode measures the light scattered backwards from the lens, other objects, or contaminants. This signal as well as several internal signals are monitored via MUX-line.
The CPU board supplies only one voltage Vb = 10 - 13 V for both the transmitter and receiver. This is used for heating the lenses, for the transmitter LED heating and for producing both +5 V digital and +15 V analog supplies. The +15 V supply is located on the FDT12B board.

FDR12 Receiver Unit

The Receiver Unit consists of a light receiver, preamplifier, voltage to frequency converter, backscatter measurement light source LED, and some control and timing electronics.
9611-003
Figure 23 FDR12 Receiver Block Diagram
The receiving PIN photodiode senses the transmitted light pulses scattered from the aerosol particles. The signal voltage is filtered and detected by a phase-sensitive, lock-in amplifier synchronized with the transmitter.
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The lock-in circuits take two samples of the background level and one sample of the active signal level while the transmitter LED is lit. The difference between the sampled voltages is amplified and then converted into frequency.
The frequency signal is buffered by a differential line driver and sent to the CPU board for accurate counting.
An ambient light level as high as 30 kcd/m2 does not influence the detection of the photo diode, neither does it saturate the A4 pre­amplifier. The Ali signal (proportional to the ambient light) is led to the CPU for monitoring.
An extra IR-LED is needed for backscatter or contaminant measurement. The light level is sampled and converted into frequency using the same method of detection described with the scattering signal measurement.

Additional Measurements

General

The FD12P includes the DRD12 Rain Detector for estimating the water content of precipitation and the DTS14B Temperature Sensor for measuring the sensor crossarm temperature (TS). Both additional sensors are measured using the DRI21 Interface Board, which is coupled on the FD12P PICOBUS.

DRI21 Interface Board

The DRI21 is a Vaisala general-purpose sensor interface with several analog and digital input channels. One of the DRI21 temperature input channels (Pt100) is used to measure the crossarm temperature (DTS14B). One 10-bit analog input channel is used to measure the DRD12 analog signal. In addition, the DRI21 controls the DRD12 heating and reads the rain ON/OFF status.
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9807-031
Figure 24 DRI21 Block Diagram in the FD12P Application

DRD12 Rain Detector

The DRD12 analog signal is proportional to the water amount on the sensing surfaces. Water on the DRD12 changes the capacitance of the sensor elements. The capacitance of the elements controls the output frequency of an oscillator. This frequency is amplified and also converted into a voltage signal for direct analog measurement. With dry surfaces, the DRD12 outputs about 3 V and with totally wet surfaces 1 V. Refer to Figure 29 on page 104, section DRD12 Signal Processing on page 104.
A droplet detector monitors the voltage signal. When a new droplet hits the DRD12 sensing surface, the voltage changes rapidly and the detector circuit reacts. The detector triggers a delay circuit, which controls the precipitation ON/OFF output. When new droplets are detected often enough, the delay circuit output will stay constantly on.
The voltage signal is measured once a second by an analog channel of the DRI21 interface board. In addition, the precipitation (ONN/OFF signal) is read with a digital input.
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