Decagon Devices VP-4 Operator's Manual

Operator’s Manual

Decagon Devices, Inc.

Version: July 10, 2017 — 12:32:13

VP-4

Decagon Devices, Inc.

2365 NE Hopkins Court

Pullman WA 99163

Phone: [phone]

Fax: [phone]

Website: www.decagon.com

Email: [e-mail] or [e-mail]

c

2017 Decagon Devices, Inc.

ii

VP-4 CONTENTS

Contents

1 Introduction 1

1.1 Customer Support . . . . . . . . . . . . . . . . . . . . 1

1.2 Warranty . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . 2

2 About VP-4 3

2.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . 3

3 Theory 8

4 RH Sensor 10

4.1 RH Sensor Conditioning . . . . . . . . . . . . . . . . . 11

5 Connecting Sensors 12

5.1 Connecting to an Em50 Series Logger . . . . . . . . . 12

5.2 Connecting to a Non-Decagon Logger . . . . . . . . . 13

5.3 Pigtail End Wiring . . . . . . . . . . . . . . . . . . . . 13

6 Communication 15

6.1 Digital Communication for Data Logger . . . . . . . . 15

7 Installation 18

8 Troubleshooting 20

9 References 21

10 Declaration of Conformity 22

iii

VP-4 1 INTRODUCTION

1 Introduction

1.1 Customer Support

If you ever need assistance with your VP-4, or if you just have ques­tions or feedback, there are several ways to contact us. Customer ser­vice representatives are available to speak with you Monday through
Friday, between 7am and 5pm Pacific time.

Note: If you purchased your VP-4 through a distributor, please con­tact them for assistance.

Email:
[e-mail] or [e-mail]

Phone:
509-332-5600

Fax:
509-332-5158

If contacting us by email or fax, please include as part of your mes­sage your instrument serial number, your name, address, phone, fax
number, and a description of your problem or question.

Please read these instructions before operating your sensor to en­sure that it performs to its full potential.

1.2 Warranty

The sensor has a 30-day satisfaction guarantee and a one-year war­ranty on parts and labor. Your warranty automatically validates
upon receipt of the instrument.

1

1 INTRODUCTION VP-4

1.3 Seller’s Liability

Seller warrants new equipment of its own manufacture against de­fective workmanship and materials for a period of one year from the
date of receipt of equipment.

Note: We do not consider the results of ordinary wear and tear,
neglect, misuse, accident as defects.

The Seller’s liability for defective parts shall in no event exceed the
furnishing of replacement parts “freight on board” the factory where
originally manufactured. Material and equipment covered hereby
which is not manufactured by Seller shall be covered only by the
warranty of its manufacturer. Seller shall not be liable to Buyer for
loss, damage or injuries to persons (including death), or to property
or things of whatsoever kind (including, but not without limitation,
loss of anticipated profits), occasioned by or arising out of the instal­lation, operation, use, misuse, nonuse, repair, or replacement of said
material and equipment, or out of the use of any method or process
for which the same may be employed. The use of this equipment con­stitutes Buyer’s acceptance of the terms set forth in this warranty.
There are no understandings, representations, or warranties of any
kind, express, implied, statutory or otherwise (including, but with­out limitation, the implied warranties of merchantability and fitness
for a particular purpose), not expressly set forth herein.

2

VP-4 2 ABOUT VP-4

2 About VP-4

The VP-4 measures air temperature, relative humidity (RH), vapor
pressure, and barometric pressure. A microprocessor within the VP­4 calculates vapor pressure from the RH and temperature measure­ments. The VP-4 uses a sensor chip to measure both air temperature
and RH and a secondary chip to measure barometric pressure. Each
VP-4 has calibrations coefficients stored on board. The calibration
coefficients are applied before data are output. Air temperature, rel­ative humidity, vapor pressure, and barometric pressure are output
from the sensor using an RS232 (TTL) string and the common SDI­12 communication protocol.

The VP-4 sensor is packaged in a rugged Delrin housing with the
sensor electronics potted in marine grade polyurethane encapsulant.
The RH/T sensor chip is protected by a hydrophobic porous Teflon
filter that is water and dust proof, but has an extremely high vapor
conductance allowing fast sensor equilibration with the surrounding
atmosphere. An additional stainless steel screen protects the Teflon
filter and RH/T sensor from impact and abrasion. The rugged design
allows the VP-4 to withstand long term exposure to hostile condi­tions, making it ideal for a wide range of applications including stan­dard meteorological monitoring, evapotranspiration measurement,
greenhouse monitoring and control, concrete moisture monitoring,
and building humidity monitoring for mold prevention and remedia­tion.

2.1 Specifications

Relative Humidity

Range: 0 to 100% RH

Resolution: 0.1% RH

Accuracy: Sensor measurement accuracy is variable across a range

of RH. Refer to the chart in Figure 1 to determine the accuracy
specification for the VP-4 sensor:

3

2 ABOUT VP-4 VP-4

Equilibration Time (τ, 63%): <40 s (response time in 1 m/s air

stream)

Hysteresis: <1% RH typical

Long term Drift: <0.5% RH/year typical

Figure 1: Humidity Accuracy Chart

Temperature

Range: −40◦C to 80◦C

Resolution: 0.1◦C

Accuracy: Sensor measurement accuracy is variable across a range

of temperatures. Refer to the chart in Figure 2 to determine
the accuracy specification for the VP-4 sensor:

Equilibration Time (τ, 63%): < 400 s (response time in 1 m/s air

stream)

Long term drift: < 0.04◦C/year typical

4

VP-4 2 ABOUT VP-4

Figure 2: Temperature Accuracy Chart

Vapor Pressure

Range: 0 to 47 kPa

Resolution: 0.001 kPa

Accuracy: Sensor measurement accuracy is variable across a range

of temperatures and RH. Refer to the chart below to determine
the accuracy specification for the VP-4 sensor:

5

2 ABOUT VP-4 VP-4

Figure 3: Vapor Pressure Accuracy

Barometric Pressure

Range: 49 to 109 kPa

Resolution: 0.01 kPa

Accuracy: 0.4 kPa

General

Dimensions: 1.96 cm (dia) x 5.4 cm (h)

Power requirements: 3.6 to 15 VDC, 0.03 mA quiescent, 4 mA dur-

ing 300 ms measurement

Response (Measurement) Time: 300 ms

Output: Decagon Digital or SDI-12

Operating Temperature: −40 to 80◦C

Connector types: 3.5 mm (stereo) plug or stripped & tinned lead

wires (Pigtail)

6

VP-4 2 ABOUT VP-4

Cable Length: 5 m standard; custom cable length available upon

request

Data Logger Compatibility (not exclusive):

Decagon: Em50, Em50R, Em50G (Firmware 2.19+)
Campbell Scientific: Any logger with serial I/O (CR10X,

CR850, 1000, 3000, etc.)

Handheld Reader Compatibility: ProCheck (rev 1.57+)

Software Compatibility

ECH2O Utility (rev 1.74+)
DataTrac (rev 3.11+)

7

3 THEORY VP-4

3 Theory

Relative Humidity

The VP-4 utilizes a capacitance type RH sensor to measure the rela­tive humidity of the surrounding air. Relative humidity is measured
at the same location as the temperature sensor. For this to be an ac­curate representation of the atmospheric humidity, it is critical that
the humidity sensor be at air temperature. For most measurement
scenarios, the VP-4 should be housed in a radiation shield with ad­equate air flow to allow the sensor to come into equilibrium with
air temperature. This is not as critical for non-greenhouse, indoor
monitoring applications where radiation loading is small.

The VP-4 sensor provides a RH measurement that is referenced to
saturation vapor pressure over liquid water, even at temperatures be­low freezing where ice is likely to be present instead of super cooled
water. Although this is the standard way to define RH (WMO, 2008),
it has the disadvantage of providing incorrect RH values below freez­ing when referenced to ice. The figure below shows the maximum RH
the VP-4 measures at saturation, 100% RH and with temperatures
below zero. RH values below saturation can be corrected using the
correction shown in the figure for a given temperature.

Figure 4: RH Value Corrections

8

VP-4 3 THEORY

Temperature

The VP-4 has a band gap temperature sensor integrated into the
sensor electronics. The temperature sensor is located with the RH
sensor and accurately measures the sensor temperature. Sensor tem­perature should remain close to air temperature if you adequately
shield and aspirate the VP-4 radiation shield.

Vapor Pressure

Vapor pressure is calculated from the primary measurements of sen­sor RH and sensor temperature. The saturation vapor pressure (es)
is calculated from the sensor temperature using the Magnus-Tetens
equation for calculating saturation vapor pressure over liquid water
formulated by Murray (1967)

es= aexp



bT

T + c



(1)

with coefficients described by Buck (1981): a = 0.611 kPa, b =

17.502, c = 240.97◦C, and T is temperature in◦C. Vapor pressure
is simply the product of saturation vapor pressure and RH, with RH
expressed as a unitless ratio ranging from 0 to 1.

Vapor Pressure = es× RH (2)

Unlike relative humidity, vapor pressure does not depend on temper­ature, and is relatively conservative over small spatial scales. This
means that the vapor pressure of the atmosphere near the VP-4 is
the same as the vapor pressure at the VP-4 sensor, even if the VP-4
is not at the same temperature as the atmosphere. Additionally, it
is the vapor pressure of the atmosphere (not RH) that controls the
rate of vapor phase water transport (e.g. evaporation, transpiration,
and distribution of water vapor). As discussed above, RH measure­ments below a temperature of 0◦C introduce errors introduced due
to the use of liquid water as the reference. However, because the
Buck (1981) formulation for liquid water is used to calculate vapor
pressure over the full temperature range, VP-4 vapor pressure output
values are correct over the full temperature range.

9

4 RH SENSOR VP-4

4 RH Sensor

RH Sensor Stability

Each VP-4 sensor is verified as accurate before leaving our facility.
However, all capacitance RH sensors drift over long periods of ex­posure to environmental conditions. The VP-4 RH sensor typically
drifts less than 0.5% RH per year. We recommend that VP-4 sensors
be calibrated every one to two years under normal use conditions to
ensure best possible accuracy. To have your VP-4 fully calibrated or
calibrate contact [e-mail] and ask about our calibra­tion service. See Section 4.1 for more information about calibration.

The polymer RH sensing element in the VP-4 can also be “poisoned”
by exposure to volatile organic compounds, solvents, and other chem­icals. The effects of exposure to these chemicals can range from
subtle loss of accuracy to catastrophic failure. If you suspect that
your VP-4 has suffered chemical exposure or notice questionable RH
measurements, you can check the sensor accuracy using known RH
conditions.

A convenient method for generating known RH conditions is through
the use of salt solutions. For an initial check, we recommend prepar­ing a saturated NaCl solution, which has an equilibrium RH of 0.75
(75%). To prepare the salt solution, start with lab grade NaCl and
mix in enough water that there is a thin layer of liquid water present
over a thick slurry of NaCl crystals. The VP-4 sensor can either be
sealed into a chamber or bell jar with the salt solution or the open­ing of the VP-4 can be sealed into a small chamber that contains the
salt solution. Whatever method is used, it is critical that the VP-4
sensor be at the same temperature as the salt solution or large errors
in the measured RH occur.

Salt solutions prepared at a wide range of RH are available from
Decagon (see chart below). It is possible to prepare your own solu­tions using the mixing ratios shown below, but great care and preci­sion are required to obtain accurate results. It is especially important
that the salt used be pure and dry. Decagon salt solutions are speci-

10

VP-4 4 RH SENSOR

fied accurate to within ±0.3% RH. As mentioned above, it is critical
that the VP-4 sensor be at the same temperature as the salt solution
or large errors in the measured RH occur.

Table 1: Salt Solutions

Equilibrium RH Salt Molality (m)

% Saturation (mol salt/kg water)

25% LiCl 13.41 m
50% LiCl 8.57 m
76% NaCl 6.00 m
92% NaCl 2.33 m

4.1 RH Sensor Conditioning

If a VP-4 sensor has been exposed to solvents or other chemicals,
the following conditioning procedure may bring the sensor back to
the original calibration state. First, bake the sensor in dry heat at
100 to 105◦C for 10 hours. Then re-hydrate the sensors at 20 to
30◦C under˜75% RH for 12 hours. A 75% RH environment can be
conveniently established by sealing the sensor in a headspace over
saturated NaCl prepared as described above.

RH Sensor Calibration

Decagon offers a service to calibrate VP-4 sensors (contact Decagon
Support at [e-mail] for more information). Prior to
shipping the RH sensors are verified over salt solutions at 25%, 50%,
and 76% RH, to ensure that they are properly functioning.

We recommend that VP-4 sensors be re-calibrated every one to two
years under normal use conditions to ensure best possible accuracy.
For safety-critical or especially high accuracy applications, more fre­quent calibration is recommended. Additionally, if sensors have been
poisoned by chemical exposure and conditioning fails to restore ac­curate measurements, the sensors should be sent back to Decagon
for evaluation and possible calibration.

11

5 CONNECTING SENSORS VP-4

5 Connecting Sensors

We designed the VP-4 sensor to work most efficiently with Decagon
Em50, Em50R, and Em50G data loggers, or the ProCheck handheld
reader. The standard sensor with 3.5 mm stereo connector quickly
connects to and is easily configured with a Decagon data logger or
ProCheck.

The VP-4 sensor incorporates several features that also make it an
excellent sensor for use with third party loggers. The sensor may be
purchased with stripped and tinned wires (pigtail) for terminal con­nections. Visit our website at www.decagon.com/support/literature
to get extensive directions on how to integrate the VP-4 sensor into
third party loggers.

The VP-4 sensor comes standard with a five meter cable. Sensors
may be purchased with custom cable lengths for an additional fee
(on a per-meter fee basis). Decagon has tested its digital sensor suc­cessfully up to 1,000 meters (3,200 ft). This option eliminates the
need for splicing the cable (a possible failure point).

5.1 Connecting to an Em50 Series Logger

We designed the VP-4 to work seamlessly with the Em50 series data
loggers. Simply plug the 3.5 mm “stereo plug” connector into one of
the five sensor ports on your Em50 series data logger.

Figure 5: 3.5 mm Stereo Plug Wiring

Once the VP-4 has been connected to your Em50 series data logger,
configure the logger port for the VP-4 and set the measurement inter-

12

VP-4 5 CONNECTING SENSORS

val. Logger configuration may be done using either ECH2O Utility
or DataTrac 3 (see respective manuals). Please check your software
version to ensure it supports the VP-4. To update your software to
the latest version, please visit Decagons free software download site:
www.decagon.com/support/downloads. The following firmware and
software support the VP-4 sensor:

Em50, Em50R, Em50G Firmware version 2.19 or greater
ProCheck Firmware version 1.57 or greater
ECH2O Utility 1.74 or greater
DataTrac 3.11 or greater

To download data from the logger to your computer, you need to
use the ECH2O Utility, DataTrac 3 or a terminal program on your
computer.

5.2 Connecting to a Non-Decagon Logger

The VP-4 sensor may be purchased for use with non-Decagon data
loggers. These sensors typically come configured with stripped and
tinned (pigtail) lead wires for use with screw terminals. Refer to
your distinct logger manual for details on wiring. Our Integrator’s
Guide gives detailed instructions on connecting the VP-4 sensor to
non-Decagon loggers. Please visit www.decagon.com/support/litera

ture to reference the complete Integrator’s Guide.

5.3 Pigtail End Wiring

Figure 6: Pigtail Wiring

Connect the wires to the data logger as shown, with the supply wire
(white) connected to the excitation, the digital out wire (red) to a

13

5 CONNECTING SENSORS VP-4

digital input, the bare ground wire to ground as illustrated below.

Figure 7: Wiring Diagram

Note: The acceptable range of excitation voltages is from 3.6 to 15
VDC. If you wish to read the VP-4 with the Campbell Scientific Data
Loggers, you must power the sensors off a 12 V or switched 12 V port.

If your VP-4 has a standard 3.5 mm plug, and if you wish to connect
it to a non-Decagon data logger, you have two options. First, you
can clip off the plug on the sensor cable, strip and tin the wires,
and wire it directly into the data logger. This has the advantage of
creating a direct connection with no chance of the sensor becoming
unplugged; however, it then cannot be easily used in the future with
a Decagon readout unit or data logger. The other option is to obtain
an adapter cable from Decagon. The 3-wire sensor adapter cable has
a connector for the sensor jack on one end, and three wires on the
other end for connection to a data logger (this type of wire is often
referred to as a “pigtail adapter”). Both the stripped and tinned
adapter cable wires have the same termination as seen above; the
white wire is excitation, red is output, and the bare wire is ground.

14

VP-4 6 COMMUNICATION

6 Communication

The Decagon VP-4 sensor can communicate using two different meth­ods, Decagon serial string or SDI-12 communication protocol. In this
chapter we briefly discuss the specifics of each of these communica­tion methods. Please visit www.decagon.com/support/literature for
the complete Integrator’s Guide, which gives more detailed explana­tions and instructions.

6.1 Digital Communication for Data Logger

SDI-12 Communication

The communications between the VP-4 and the Decagon logger is
automatic and only requires you to select the correct sensor configu­ration for the sensor port. When a Decagon logger applies excitation
voltage, the VP-4 sensor makes a measurement. Within about 550 ms
of excitation three measurement values transmit to the data logger
as a serial stream of ASCII characters complete with checksum and
sensor identification. Please see the VP-4 Integrators Guide.docx
document available from www.decagon.com/support/literature for
more information on this communication protocol.

SDI-12 Commands

Table 2 provides SDI-12 commands that the VP-4 sensor responds
to. The sensor address is shown as a. If a ? is substituted for a all
addresses respond.

15

6 COMMUNICATION VP-4

Table 2: SDI-12 Commands

Send

Identification

aI! a13DECAGON VP-4 385<CR><LF>

Change Address aAb! b<CR><LF>(b is new address)
Address Query ?! a<CR><LF>

Start

Measurement

aM! a0013<CR><LF>

Send Data aD0!

a+1.170 + 21.5 + 0.457
+92.25<CR><LF>(4 values)

SDI-12 Sensor Bus

Up to 62 sensors can be connected to the same 12 V supply and
communication port on a data logger. This simplifies wiring because
no multiplexer is necessary. However, one sensor problem can bring
down the entire array (through a short circuit or incorrect address
settings). If you use an SDI-12 sensor bus we recommend that you
make an independent junction box with wire harnesses where all sen­sor wires attach to lugs so sensors can be disconnected individually
if a problem arises. A single three-wire cable can be run from the
junction box to the data logger.

SDI-12 Address

The SDI-12 protocol requires that all sensors have a unique address.
VP-4 sensors come from the factory with an SDI-12 address of 0.
To add more than one SDI-12 sensor to a system, the sensor address
must be changed. Address options include 0...9, A...Z, a...z. The best
and easiest way to change an address is to use Decagon’s ProCheck
(if the option is not available on your ProCheck, please upgrade to
the latest version of firmware). SDI-12 addressing can be accessed
in the CONFIG menu by selecting SDI-12 Address. Addresses may
then be changed by simply pressing the up or down arrows until you
see the desired address and pushing Enter.

The SDI-12 communication protocol is supported by Campbell Sci­entific data loggers like the CR10X, CR200, CR1000, CR3000, etc.

16

VP-4 6 COMMUNICATION

Direct SDI-12 communication is supported in the Terminal Emulator
mode under the “Tools” menu on the Connect screen. Detailed in­formation on setting the address using CSI data loggers can be found
on our website at http://www.decagon.com/support/downloads.

Power

The VP-4 is an extremely low power sensor. When continuously
powered, but not making a measurement or communicating, it uses
30µA. When using the sensor as part of an SDI-12 bus, it is recom­mended that the sensors be excited continuously to avoid issues with
initial sensor startup interfering with the SDI-12 communications.

Reading

When reading the VP-4 in SDI-12 mode, the first number output
by the sensor is vapor pressure in kPa, the second number is temper­ature in Celsius, the third number is relative humidity in a unitless
ratio from 0 to 1 (0 to 100%), and the fourth number is barometric
pressure in kPa.

17

7 INSTALLATION VP-4

7 Installation

Installation in a Radiation Shield

Relative humidity is measured at the temperature of the humidity
sensor. For this to be an accurate representation of the atmospheric
humidity, it is critical that the humidity sensor be at air temperature.
For most outdoor and greenhouse measurement scenarios, house the
VP-4 in a radiation shield with adequate air flow to allow the sensor
to come into equilibrium with air temperature. In general, tem­perature and humidity measurements become more accurate as wind
speed increases. A radiation shield is not critical for non-greenhouse,
indoor monitoring applications where radiation loading is small.

Decagons radiation shield comes with a mounting bracket and seven
discs that prevent direct sunlight from coming into coming contact
with the sensor. This isolation from solar radiation prevents false
readings of elevated temperatures, allowing for accurate measure­ment of ambient air temperature. To install the sensor in the radia­tion shield follow the directions 1 through 4.

1. Gently pry the white cap from the bottom center hole of the
radiation shield.

2. Slide the white cap onto the cable of the sensor.

3. Insert the sensor into the bottom of the shield and snap the
cap back into place.

4. The unit can then be mounted on the desired surface for your
study.

18

VP-4 7 INSTALLATION

Note: Be sure to fasten the sensor cord to your mounting post
to help support the weight of the cable in order to prevent it
from being pulled out.

19

8 TROUBLESHOOTING VP-4

8 Troubleshooting

If you encounter problems with the VP-4 sensor, they most likely
manifest themselves in the form of no reading from communication
problems, catastrophic sensor failure, or highly inaccurate measure­ments due to sensor poisoning by volatile chemicals. Before contact­ing Decagon about the sensor, do the following:

Data Logger

1. Check to make sure the connections to the data logger are both
correct and secure.

2. Ensure that your data logger batteries are not dead or weak­ened.

3. Check the configuration of your data logger in ECH2O Utility
or ECH2O DataTrac to make sure you have selected VP-4.

Sensors

1. Check sensor cables for nicks or cuts that could cause a mal­function.

2. Check your screen and filter to make sure that they are not con­taminated or blocked. Airflow must not be restricted through
the filter.

3. In the case of inaccurate readings, consider evaluating sensor
accuracy and/or reconditioning the sensor as described in the
RH Sensor section.

20

VP-4 9 REFERENCES

9 References

Buck, A.L.. (1981). New equations for computing vapor pressure
and enhancement factor. Journal of Applied Meteorology, 20, 1527-

1532.

Goff, J.A. and S. Gratch. (1946). Low-pressure properties of wa­ter from −160◦to 212◦F. Transactions of the American society of
heating and ventilating engineers, 51, 125-164.

Murray, F.W.. (1967). On the computation of saturation vapor
pressure. Journal of Applied Meteorology, 6, 203-204.

WMO. (2008). Guide to meteorological instruments and methods
of observation. World Meteorological Organization, 7, 8, Geneva,
Switzerland.

21

10 DECLARATION OF CONFORMITY VP-4

10 Declaration of Conformity

Application of Council Directive: 2004/108/EC and 2011/65/EU

Standards to which conformity is
declared:

EN 61326-1:2013 and

EN 50581:2012

Manufacturer’s Name: Decagon Devices, Inc. 2365 NE

Hopkins Ct. Pullman, WA 99163
USA

Type of Equipment: Temperature, Humidity, Pressure

Sensor

Model Number: VP-4

Year of First Manufacture: 2015

This is to certify that the VP-4 manufactured by Decagon Devices,
Inc., a corporation based in Pullman, Washington, USA meets or ex­ceeds the standards for CE compliance as per the Council Directives
noted above. All instruments are built at the factory at Decagon and
pertinent testing documentation is freely available for verification.

22

Index

CE Compliance, 22
Connection

Em50 Logger, 12

Non-Decagon Logger, 13
Contact Information, ii, 1
Customer Support, 1

Data Logger

Digital Communication, 15
Declaration of Conformity, 22

Email, 1

Fax, 1

Installation

with Radiation Shield, 18

Phone, 1
Power Requirements, 14

Radiation Shield, 18
References, 21
RH Sensor, 8

Calibration, 11

Conditioning, 11

Stability, 10

SDI-12 Communication, 15
Seller’s Liability, 2
Specifications, 3

Theory, 8
Troubleshooting, 20

Warranty, 1
Wiring

3.5 mm, 12

Pigtail, 12

23