Page 1
S503
15Vdc
GND
INPUT
VO LT
INPUT
4-20mA
OFF
ON
8 6 3
% RH
SETPOINT
TEMP
RH
10V
5V
1V
mA
INPUT
SELECT
S-503
HUMIDITY GENERATOR
Humidity Calibrator
User’s Manual
97210 Issue 01, April 2011 Michell Instruments
Page 2
Inside front cover (blank)
Page 3
S503 User’s Manual
15Vdc
GND
INPUT
VO LT
INPUT
4-20mA
OFF
ON
8 6 3
% RH
SETPOINT
TEMP
RH
10V
5V
1V
mA
INPUT
SELECT
S-503
HUMIDITY GENERATOR
S503-DIG
© 2011 Michell Instruments
This document is the property of Michell Instruments Ltd. and may not be copied or otherwise reproduced,
communicated in any way to third parties, nor stored in any Data Processing System without the express
written authorization of Michell Instruments Ltd.
97210 Issue 01, April 2011 iii Michell Instruments
Page 4
S503 User’s Manual
97210 Issue 01, April 2011 iv Michell Instruments
Contents
Safety .............................................................................................................................................. v
Electrical Safety ......................................................................................................................... v
Toxic Materials .......................................................................................................................... v
Repair and Maintenance............................................................................................................. v
Calibration ................................................................................................................................ v
Safety Conformity ...................................................................................................................... v
Abbreviations ....................................................................................................................................vi
Recycling Policy ................................................................................................................................vii
WEEE and RoHS Compliance .............................................................................................................vii
Calibration Facilities .......................................................................................................................... vii
Manufacturing Quality ...................................................................................................................... viii
Warranty ........................................................................................................................................ viii
Return Policy ................................................................................................................................... viii
1 INTRODUCTION ........................................................................................................................ 1
1.2 Applications .................................................................................................................... 1
1.3 Dimensions .................................................................................................................... 1
1.4 S503 Combinations ......................................................................................................... 2
1.4.1 S503-DIG ................................................................................................................... 2
1.4.2 S503-DIG-OPT ............................................................................................................ 2
1.4.3 S503-DIG-SET ............................................................................................................ 3
1.4.4 S-503-DIG-LC ............................................................................................................. 3
2 OPERATION .............................................................................................................................. 4
2.1 Front Panel Functions ...................................................................................................... 5
2.2 Back Panel Functions ...................................................................................................... 6
2.3 Getting Started ............................................................................................................... 7
2.4 Operating Instructions ..................................................................................................... 8
2.5 Connecting a Transmitter................................................................................................. 8
3 MAINTENANCE .......................................................................................................................... 9
4 TROUBLESHOOTING ................................................................................................................10
5 CALIBRATION ..........................................................................................................................11
Figures
Figure 1.1 S503 dimensions ......................................................................................................1
Figure 1.2 S503-DIG Digital Humidity Generator ........................................................................2
Figure 1.3 S503-DIG-OPT Digital Humidity Generator & Optidew Vision ......................................2
Figure 1.4 S503-DIG-SET Calibration Kit ....................................................................................3
Figure 1.5 S503-DIG-LC S503 with extended chamber ................................................................3
Figure 2.1 Inside the S503 ........................................................................................................4
Figure 2.2 S503 front panel ......................................................................................................5
Figure 2.3 S503 back panel ......................................................................................................6
Figure 2.4 Maximum water level................................................................................................7
Figure 2.5 Sponge inside chamber ............................................................................................7
Appendices
Appendix A Technical Specications ...........................................................................................13
Appendix B Recommended Practices in Humidity Measurements ..................................................15
Appendix C List of Worldwide Michell Instruments’ Ofces ..........................................................20
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S503 User’s Manual
Michell Instruments v 97210 Issue 01, April 2011
Safety
The manufacturer has designed this equipment to be safe when operated using the procedures detailed in this
manual. The user must not use this equipment for any other purpose than that stated. Do not apply values
greater than the maximum value stated.
This manual contains operating and safety instructions, which must be followed to ensure the safe operation
and to maintain the equipment in a safe condition. The safety instructions are either warnings or cautions
issued to protect the user and the equipment from injury or damage. Use qualied personnel and good
engineering practice for all procedures in this Manual.
Electrical Safety
The instrument is designed to be completely safe when used with options and accessories supplied by the
manufacturer for use with the instrument.
Toxic Materials
The use of hazardous materials in the construction of this instrument has been minimized. During normal
operation it is not possible for the user to come into contact with any hazardous substance which might be
employed in the construction of the instrument. Care should, however, be exercised during maintenance and
the disposal of certain parts.
Repair and Maintenance
The instrument must be maintained either by the manufacturer or an accredited service agent. Refer to
Appendix B for details of Michell Instruments’ worldwide ofces contact information.
Calibration
Under normal ambient conditions (0 to +50ºC (+32 to +122ºF) 0-70% RH) and for accuracy of ±2% RH, an
annual calibration is recommended. For an accuracy ±5% RH a calibration is recommended every ve years.
For environments with airborne chemicals or for high humidity and high temperature conditions more frequent
calibration is recommended.
Safety Conformity
This product meets the essential protection requirements of the relevant EU directives. Further details of
applied standards may be found in Section 5.
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S503 User’s Manual
97210 Issue 01, April 2011 vi Michell Instruments
Abbreviations
The following abbreviations are used in this manual.
AC alternating current
DC direct current
ºC degrees Celsius
ºF degrees Fahrenheit
g grams
in inches
lbs pounds
kg kilograms
mA milliampere
mm millimetres
oz ounces
% percentage
RH relative humidity
RS232 serial data transmission standard
temp temperature
T temperature
V Volts
Ω Ohms
Warnings
The following general warning listed below is applicable to this instrument. It is repeated in the text in the
appropriate locations.
Where this hazard warning symbol appears in the following
sections, it is used to indicate areas where potentially
hazardous operations need to be carried out.
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S503 User’s Manual
Michell Instruments vii 97210 Issue 01, April 2011
Recycling Policy
Michell Instruments is concerned with the protection of the environment. It is our commitment to reduce
and eliminate from our operations, wherever possible, the use of substances which may be harmful to the
environment. Similarly, we are increasingly using recyclable and/or recycled material in our business and
products wherever it is practical to do so.
The product that you have purchased may contain recyclable and/or recycled parts and we will be happy to
provide you with information on these components if required.
WEEE and RoHS Compliance
The Waste Electronic and Electrical Equipment (WEEE) Directive, and the Restriction of Hazardous Substances
(RoHS) Directive place rules upon European manufacturers of electrical and electronic equipment. The
directives’ aim is to reduce the impact that electronic devices have on the environment.
Michell products are currently exempt from the RoHS directive, however all future products will be developed
entirely using compliant materials. Furthermore, Michell is taking active steps to remove non-compliant
materials and components from existing products wherever possible.
Michell is in full compliance with the WEEE Directive (Registration No. WEE/JB0235YW). Customers may be
required to return certain instruments for treatment at the end of their working life.
June 2010
Calibration Facilities
Each unit has an internal reference which is calibrated against our working factory standard which is traceable
to ‘VSL’ in The Netherlands and National Physical Laboratory (NPL) in the UK.
Calibration Certicates:
In addition to the normal calibration procedure, each internal reference transmitter can be supplied with its
own VSL or NPL traceable calibration certicate.
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S503 User’s Manual
97210 Issue 01, April 2011 viii Michell Instruments
Manufacturing Quality
Michell Instruments UK is registered with the British Standards Institute for Quality Assurance to:
BS EN ISO 9001: 2008
Rigorous procedures are performed at every stage of production to ensure that the materials of construction,
manufacturing, calibration and nal test procedures meet the requirements laid down by our BSI approved
Quality System.
Please contact Michell Instruments if the product does not arrive in perfect working order.
Warranty
Unless otherwise agreed, the Supplier warrants that, as from the date of delivery for a period of 12 months
the goods and all their component parts, where applicable, are free from any defects in design, workmanship,
construction or materials.
The Supplier warrants that the services undertaken shall be performed using reasonable skill and care, and of
a quality conforming to generally accepted industry standards and practices.
Except as expressly stated, all warranties, whether express or implied, by operation of law or otherwise, are
hereby excluded in relation to the goods and services to be provided by the Supplier.
All warranty services are provided on a return to base basis. Any transportation costs for the return of a
warranty claim shall reside with the Customer.
Return Policy
If a Michell Instruments’ product malfunctions within the warranty period, the following procedure must be
completed:
Notify a Michell Instruments’ distributor, giving full details of the problem, the model 1.
variant and the serial number of the product.
If the nature of the problem indicates the need for factory service then the instrument 2.
should be returned to Michell Instruments, carriage prepaid, preferably in the original
packaging, with a full description of the fault and the customer contact
information.
Upon receipt, Michell Instruments will evaluate the product to determine the cause of 3.
the malfunction. Then, one of the following courses of action will be taken:
If the fault is covered under the terms of the warranty, the instrument will •
be repaired at no cost to the owner and returned.
If Michell Instruments determines that the fault is not covered under the •
terms of the warranty, or if the warranty has expired, an estimate for the
cost of the repairs, at standard rates, will be provided. Upon receipt of the
owner’s approval to proceed, the product will be repaired and returned.
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Michell Instruments 1 97210 Issue 01, April 2011
A
Notes:
Hole A = Power Hole
for adapter
Back view
Top view
Side view
Bottom view
1 INTRODUCTION
The S503 Humidity Generator/Calibrator enables users of humidity sensors, transmitters and read-out devices
to quickly and accurately generate reference humidity conditions at ambient temperatures.
1.1 Features
Generates humidity levels between 10-90% RH•
Highly portable•
Fast stabilization of humidity chamber•
In combination with Michell Instruments’ Optidew Vision, which works on a •
proven, fundamental optical dew-point measurement principle, calibrations
can be done using the most accurate reference (optional).
Multiple sensors can be calibrated at the same time•
1.2 Applications
Calibration and validation of RH transmitters, sensors and data loggers.
1.3 Dimensions
Figure 1.1 S503 dimensions
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1.4 S503 Combinations
1.4.1 S503-DIG
The S503-DIG consists of - S503, including 7 user-specied port adapters, power adapter, desiccant and
adapter tool.
1.4.2 S503-DIG-OPT
Figure 1.3 S503-DIG-OPT Digital Humidity Generator & Optidew Vision
Figure 1.2 S503-DIG Digital Humidity Generator
In combination with Michell Instruments’ Optidew Vision, calibrations can be done using the most accurate
reference. The Optidew Vision is traceable to national standards.
The Optidew Vision is a high performance Optical Dew-point Transmitter working on a proven, fundamental
optical dew-point measurement principle and giving unmatched and drift-free long-term performance. It offers
a wide measurement range from -60 to +90°Cdp (-76 to +194°Fdp) (or 0.5 to 100% RH) at temperatures from
-40 to +90°C (-40 to +194°F), and -20 to +130°C (-4 to +266°F) at temperatures from -20 to +130°C (-4 to
+266°F) for the high temperature version.
For more information on the Optidew Vision please contact Michell Instruments or your local distributor (see
contact details in Appendix B).
The S503-DIG-OPT consists of - S503, with 5 user-specied port adapters, 2 Optidew adapters (A000272
(stainless steel port adapter) and A000273 (polymer housing port adapter ø18.5mm (0.73”) to ø3mm (0.19”)),
power adapter, desiccant and adapter tool.
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Michell Instruments 3 97210 Issue 01, April 2011
1.4.3 S503-DIG-SET
Figure 1.4 S503-DIG-SET Calibration Kit
The S503-DIG-SET consists of - S503, including hand meter and probe (with calibration certicate),
7 user-specied port adapters, power adapter, desiccant, distilled water bottle (water not included), adapter
tool and carrying case.
1.4.4 S-503-DIG-LC
Figure 1.5 S503-DIG-LC S503 with extended chamber
The S503-DIG-LC has an extended chamber (0.54 liter extra)
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S503 User’s Manual
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2 OPERATION
Desiccant
chamber
Water
chamber
Power supply
12 V DC
Calibration chamber
Figure 2.1 Inside the S503
A vapor saturator and desiccant chamber are coupled to the calibration chamber. Saturated or dry air is
pumped into the chamber to achieve the set point. The electronics assure fast ramping to the actual calibration
point. An LCD displays the actual chamber humidity, as well as the actual chamber temperature.
The S503 utilizes a high accuracy reference sensor which has been calibrated to a traceable reference. The
overall accuracy between 20 and 80% RH is ±2% RH. Although the internal reference is accurate and
stable, an external reference probe, either a traceable RH indicator or a chilled mirror dew-point probe, is
recommended as the calibrated reference.
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Michell Instruments 5 97210 Issue 01, April 2011
2.1 Front Panel Functions
15Vdc
GND
INPUT
VO LT
INPUT
4-20mA
OFF
ON
8 6 3
% RH
SETPOINT
TEMP
RH
10V
5V
1V
mA
INPUT
SELECT
S-503
HUMIDITY GENERATOR
2 3 4 5 6 7
1
11
12
13
14
108 9
Figure 2.2 S503 front panel
Display Screen1.
Input Select
Temp 2. Shows the actual calibration chamber temperature
RH 3. Shows the actual calibration chamber relative humidity
10 V 4. Shows the actual output of a transmitter in the calibration
chamber with an output of 0-10 V.
Display reads from 0-100%RH / 0 to +100ºC
5 V5. Shows the actual output of a transmitter in the calibration
chamber with an output of 0-5 V.
Display reads from 0-100%RH / 0 to +100ºC
1 V 6. Shows the actual output of a transmitter in the calibration
chamber with an output of 0-1 V.
Display reads from 0-100%RH / 0 to +100ºC
mA 7. Shows the actual output of a transmitter in the calibration
chamber with an output of 4-20 mA.
Display reads from 0-100%RH / 0 to +100ºC
Set point 8. Selects the desired relative humidity: 10-90% RH in steps of
0.1% RH
Humidify 9. When lit, indicates that the vapor saturator is activated
De-Humidify 10. When lit, indicates that the desiccant chamber is activated
15 V DC 11. +15 V DC supply for a transmitter in the calibration
chamber
Input Volt12. Connect to v+ transmitter signal in the calibration chamber
with voltage output (0-1, 0-5 or 0-10 V)
Input mA13. Connect to + signal of transmitter in the calibration chamber
Gnd 14. Common ground for v- transmitter signal and power supply
with current output (2-wire 4-20 mA)
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97210 Issue 01, April 2011 6 Michell Instruments
Power Supply
Socket
2.2 Back Panel Functions
Figure 2.3 S503 back panel
The power supply cable comes with a country-specic plug attached.
The input is 12 V (850 mA).
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S503 User’s Manual
Michell Instruments 7 97210 Issue 01, April 2011
2.3 Getting Started
Stabilize the unit to room temperature before use.
Always close the calibration chamber ports before use.
Remove the cover from the desiccant chamber (see •
of new desiccant.
Remove the cover and the blue sponge (see •
(see
Figure 2.1)
Maximum water level
(shown with cover removed)
and ll with water to BELOW the maximum water level.
Do not overll as the water can ow directly to the calibration
Figure 2.1)
Figure 2.5)
from the water chamber
and ll with 25g
Figure 2.4 Maximum water level
chamber, causing the generator to malfunction.
IMPORTANT: THE CALIBRATOR SHOULD NOT BE TRANSPORTED
WHEN FILLED WITH WATER. ALL WATER MUST BE DRAINED
BEFORE MOVING THE UNIT (SEE SECTION 3 FOR DETAILS)
Place the blue sponge back into the water chamber - this sponge prevents •
water from entering the grey tube and water droplets getting into the humidity
chamber.
Figure 2.5 Sponge inside chamber
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2.4 Operating Instructions
The unit must be used at normal room temperature (approximately +20ºC 1.
(+68ºF)) and must be stabilized for a minimum of 1 hour, if used for different
temperature environments.
Power the unit with 12 V DC (connection at the rear of the instrument). 2.
A 100 - 240 V AC adapter is included.
Make sure the calibration ports are closed.3.
Switch the unit to ‘RH’ and choose the desired humidity.4.
Install the humidity transmitter to be calibrated (without lter) into the 5.
calibration chamber.
If applicable, connect the transmitter to the built-in power supply and read out. 6.
See Section 2.5 for further information.
Press ‘ON’. The unit will now start to dry or humidify, depending on the humidity 7.
set, until it reaches its set point.
If the humidity probe is connected to the built-in power supply and read out, 8.
switch to the corresponding input sensitivity.
If the transmitter has an output:
of 0-10 V, switch to ‘10 V’•
of 0-5 V, switch to ‘5 V’•
of 0-1 V, switch to ‘1 V’ •
of 4-20mA switch to ‘mA’ •
The display shows the actual output of the transmitter under calibration.
2.5 Connecting a Transmitter
The S503 Humidity Generator/Calibrator accepts any probe with a diameter varying from 5mm to a maximum
of 25mm.
The S503 has a built-in power supply of 15 V DC for transmitters under calibration.
For example, if the transmitter under calibration has a 0-10 V output, the output signal should be connected
to the voltage input and the input select switch should be set to 10 V. The actual reading of the transmitter
is shown on the display from 0-100% RH. To see the actual relative humidity in the chamber the input select
switch should be set to ‘RH’.
Before placing the transmitter in the calibration chamber,
remove the dust lters. Use slotted caps only - this will reduce
the stabilization time of the transmitter under calibration.
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Michell Instruments 9 97210 Issue 01, April 2011
3 MAINTENANCE
WATER
Only add distilled water to the water compartment.
Do not shake or turn over the instrument as water may ow
into the calibration chamber or internal electronics.
Do not overll as the water can ow directly to the calibration
chamber, causing the generator to malfunction.
IMPORTANT: THE CALIBRATOR SHOULD NOT BE
TRANSPORTED WHEN FILLED WITH WATER. ALL WATER
To remove water (either when exchanging the water or removing before transportation):
Remove the cover and sponge from the water chamber.•
Use a syringe to remove the water from the chamber.•
Replace the sponge and the cover. •
MUST BE DRAINED BEFORE MOVING THE UNIT.
DESICCANT
Replace desiccant when colorless. Use only dark blue or orange, ultra dry desiccant.
The desiccant can be regenerated by heating it to +150ºC for 1-2 hours.
NOTE: Remove water before removing desiccant.
To remove desiccant:
Firstly, remove water by following the above instructions.•
Remove the cap from the desiccant chamber.•
Turn the S503 upside-down to remove the desiccant.•
Use only genuine desiccant to rell.•
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4 TROUBLESHOOTING
UNIT DOES NOT DRY
Desiccant may be saturated (transparent color) •
Replace with fresh ultra dry desiccant following the instructions in Section 3.
Condensation or water may be on the wall of the calibration •
chamber
Remove the ports and open the chamber. Wipe carefully with a dry cloth and
allow to dry for approximately 15 minutes.
Calibration chamber may contain standing water •
Remove the water and allow the unit to dry for 2 days with all the ports open
or with the top plate removed.
UNIT IS TOO MOIST OR TOO DRY
Chamber temperature could be extremely cold or hot•
Stabilize to room temperature.
Cover is open or there is leakage through the cover•
Unscrew and remove the top plate of the instrument and check the seals around
the chambers (see
Figure 2.1)
.
LARGE DEVIATIONS IN CALIBRATION
Large calibration deviations when starting up are usually a result of a •
difference in temperature between the transmitter under calibration
and the actual chamber temperature.
Allow to stabilize for at least 1 hour.
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Michell Instruments 11 97210 Issue 01, April 2011
5 CALIBRATION
Calibration Interval Time:
Under normal ambient conditions (0 to +50ºC (+32 to +122ºF) 0-70% RH) and for
an accuracy of ±2% RH, we recommend an annual calibration.
For an accuracy of ±5% RH we recommend calibration every ve years.
For environments with airborne chemicals or for high humidity and high temperature
conditions we recommend more frequent calibration.
EMC Compatibility:
The series S503 Humidity Generators/Calibrators are designed to meet the following
European standards:
EN 61326 (1997) + A1 (1998) + A2 (2001)
Emission: Class B
Immunity: Industrial
EN 61000-3-2 (1995) + A1 (1998) + A2 (1998)
EN 61000-3-3 (1995)
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97210 Issue 01, April 2011 12 Michell Instruments
Appendix A
Technical Specications
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Michell Instruments 13 97210 Issue 01, April 2011
Appendix A Technical Specications
Humidity
Generation range 10-90% RH in steps of 0.1% RH
Accuracy ±2% RH or better (5-95% RH)
Stability chamber Better than ±0.5%
Stabilization time to set point <10 minutes
Temperature
Temperature accuracy ±0.3ºC (±0.54ºF) - internal reference
Operating temperature (ambient) +15 to +35ºC (+59 to +95ºF)
Sensor Under Calibration
Voltage read out 0-1, 0-5, 0-10 V
Current read out 4-20 mA
Voltage supply 15 V DC, ±10% at 30 mA max
General
Probe ports 7 off sensor body diameters 5 to 25mm (0.2 to 0.98”)
accommodated by port adapters
Desiccant chamber 25g (0.88oz) capacity
Saturation chamber 25ml capacity, distilled water
Display 3½ digit LCD, 13mm (0.5”) characters
Supply 12 V DC (100 - 240 V AC adapter included)
Weight 1.85kg (4.08lb)
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Appendix B
Recommended Practices
in Humidity Measurements
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Michell Instruments 15 97210 Issue 01, April 2011
Appendix B Recommended Practices in Humidity Measurements
The following text is reproduced with kind permission from the National Physical Laboratory. It is originally
published in the booklet,
Denition of Relative Humidity
Relative Humidity – The ratio of the actual vapor pressure to the saturation vapor pressure over a plane
liquid water surface at the same temperature, expressed as a percentage. This is commonly understood when
the term ‘X percent relative humidity’ is used.
A Guide to the Measurement of Humidity.
For actual vapor pressure, e, and saturation vapor pressure, e
s
e
relative humidity (in %) = ––– x 100
e
s
USAGE: The phrase ‘relative humidity’ is commonly abbreviated RH although this is not a recognized
abbreviation. Values of relative humidity are commonly expressed in units of percent relative humidity (%
RH).
Recommended practices in humidity measurements
General practical recommendations
Where relative humidity is of interest, a direct measurement of relative humidity is usually best. Where an •
absolute measure of humidity is needed, choose dew point, vapor pressure or similar measurements.
Establish the measurement requirements at the purchasing stage in order to have the right instrument for •
the job.
Allow hygrometers to equilibrate in any new environment. This is particularly necessary after changes in •
temperature due to transportation or storage. Depending on the instrument and on how great the change
in conditions, this may require from only a few minutes to many hours.
Follow Michell Instruments’ care instructions for the instrument. Some instruments need routine cleaning •
or other maintenance. Before using any solvent cleaner, check with Michell Instruments that this will not
harm the sensor or other materials of construction.
Wherever possible, ensure that hygrometers are calibrated under the conditions of use, i.e. at similar values •
of humidity and temperature, and (if relevant) in similar conditions of pressure, airow, etc.
Keep a record of calibrations and any adjustments to the hygrometer. This will show the long-term stability •
of the instrument and allow the associated uncertainty to be assessed.
Check instruments, if possible, at intervals between calibrations, by comparison with another (stable) •
instrument, to monitor for long-term drift. Routine checks are also useful before and after subjecting an
instrument to transportation or other stress, which might lead to a shift in its performance. Where the
check is against two (or more) instruments this is even better: not only does this add condence, but in the
event of one instrument drifting among a set of three, it can be seen which reading is most suspect.
Cleanliness of the environment will affect different hygrometers in different ways. Dust and airborne droplets •
should be avoided or ltered out if possible. Contaminants can come from the most surprising sources,
ordinary urban pollution, for example.
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The readings given by some types of hygrometer are sensitive to gas type. For any Instrument which •
reads in terms of mass per unit volume, e.g. in grams per cubic metre, it must be conrmed whether the
calibration is valid for the gas in use.
Avoid using instruments in direct sunlight or near any other source of heat, unless they are suitably shielded •
to prevent measurement errors.
Sampling in general
Relative humidity measurements should be carried out at a representative temperature. •
Failure to allow temperature equilibration will lead to a false indication of the relative humidity.
Variations in vapor pressure from place to place can occur where an environment is subject to any •
addition or removal of water. If so, care must be taken over where to make a measurement in order to
obtain a representative result.
Sources and sinks of water vapor should be avoided in any sampling system. Invasion of stray water can be •
minimised by attention to leaks, hygroscopic materials, droplets and condensation. The lower the humidity,
the more critical these precautions are.
Hygroscopic materials should be avoided. Many materials contain moisture as part of their structure, •
particularly organic materials (whether natural or synthetic), salts (or anything which contains them), and
anything which has small pores. Temperature changes can increase the tendency of these materials to
affect the humidity of the surrounding air.
Condensation in a sampling process can invalidate humidity measurements by reducing the water content •
of the gas being measured. What is more, condensed liquid may alter the humidity elsewhere by dripping
or running to other locations and evaporating there. In these circumstances, measurement results may be
misleading if hygrometer location is not considered carefully.
Water droplets or mist must be avoided. These can result in overestimates of the humidity of the air between •
the droplets. Such results may exceed 100% RH, or may be impossible to interpret meaningfully. Droplets of
liquid also damage some electrical types of humidity sensor. Filtering the air sample can eliminate droplets.
If pumps are used for sampling gas, these should be located after the hygrometer, to avoid contaminating •
the measurement environment. Where possible, oil free pumps should be used, or lters employed.
Oscillations in pressure due to pumping can sometimes be reduced or buffered using a needle valve or a
reservoir of large volume.
Special treatments such as ltration can change the amount of moisture in a gas. Some drying agents take •
out other gases, too
When sealing any sensor or probe into a port or manifold in a duct or chamber, leaks through the probe •
or electrical cable should be considered. These are not always sealed against passage of ambient air.
Where sampling involves a step change in temperature, pressure or gas ow rate, relative to the process •
being sampled, results may need to be converted or interpreted. For example ‘pressure dew point’ will differ
from the value found after expanding the gas sample to atmospheric pressure. Care should be taken to
distinguish between ‘gauge’ and absolute values of pressure.
Dew point in general
The measuring environment and all parts of the sampling pathway must be kept above the dew point •
if condensation is to be avoided. Electrical trace heating or other heating methods should be used if
necessary. An excess temperature of 10°C above the dew point is usually a safe margin.
For measurements in the region below 0°C it must be clear whether the condensate is dew or frost. Failure •
to distinguish between these can result in errors of about 1°C for every 10°C below zero.
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Michell Instruments 17 97210 Issue 01, April 2011
Relative humidity in general
Due care must be taken of temperature. The effect of temperature on humidity is highly signicant. Failure •
to take this into account can sometimes lead to errors so large that the measurement is meaningless.
In many situations, the largest single source of uncertainty in a humidity measurement is the effect of
temperature differences from place to place in the process, room or chamber. The importance of considering
the temperature effects carefully cannot be overstated when relative humidity is the parameter of interest.
Care must be taken when expressing uncertainties, changes or fractional differences in relative humidity. •
For example, the difference between 50% RH and 52% RH is 2% RH. This can also be expressed as a
difference of 4% of value. It is important to distinguish clearly between these two kinds of statement.
Recommendations specic to ranges of measurements
Ambient humidity• - Avoid using hygrometers near the body, which is a source of heat and moisture. Do not
breathe close to the measurement.
High humidity, above the ambient range -• Ample lines should be maintained above the dew point of the gas
being measured, to avoid condensation. Electrical trace heating is often the most practical method.
Low humidity, and very dry gases -• If possible, prepare for measurements by ushing sample lines and
hygrometers with dry gas, or by evacuating to low pressure. Drive off stray residual water by baking
assemblies if possible (but not instruments – unless designed for this!). The lower the moisture content to
be measured, the more dramatically the required drying time multiplies.
Avoid hygroscopic materials. At low humidity (anything much below a dew point of 0°C) the amounts of •
water given off by organic and porous materials can dramatically affect the value of humidity. The lower
the level of moisture, the more signicant the effects.
Choose impermeable materials, to avoid inward diffusion of moisture through sampling tubes and enclosures. •
Steel and other metals are practically impermeable. PTFE (‘Teon’) is only slightly permeable and will
usually be satisfactory for dew points above -20°C, and sometimes below this level. Materials such as PVC
and rubber are relatively permeable and so totally unsuitable at low humidity, and not really satisfactory in
any humidity range.
Surface nish of pipework is important for very dry gases. Even the tiny quantities of water adsorbed on •
the surfaces of non-hygroscopic materials can have signicant effect. Polished or electropolished steel is
recommended for the best results.
Clean environments are always best for humidity measurements, but this is especially critical at very •
low humidity. Even ngerprints harbour water. High purity cleaning agents are recommended: Analytical
Reagent (AR) quality solvents for oil-based contaminants, and puried water (distilled or de-ionised) for
salts. Cleaning should be followed by thorough drying by a clean method.
Sample tubing should be as short in length as possible. The surface area should be minimised by using the •
narrowest tubing that the ow conditions will permit.
Avoid leaks. Minimising the number of connections (elbows, tees, valves, etc.) helps with this.•
Adequate ow of the gas sample should be ensured, to minimise the inuence of sources of stray water in •
the ow path.
‘Dead ends’ should be avoided, as they cannot easily be ushed.•
Back-diffusion of moisture should be minimised, e.g. by fast ow rates of gas, long exhaust tubes after the •
sensor, or by valves which isolate the low-humidity region from ambient air.
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Practical recommendations for specic types of hygrometer
Relative humidity capacitive sensor
Care should be taken to avoid mechanical shock (impact) or thermal shock (sudden temperature changes). •
Sensors should be protected from steam or water sprays, and from direct sunlight.
Where a sensor is at risk of exposure to dust, droplets, or the occasional knock during handling, the •
appropriate guard or lters for the sensor head should be used.
Any temptation to breathe on the sensor, or to wave it over cups of tea, etc. should be resisted. Filters and •
saturation guarding may protect the sensor, but these actions carry a risk of damage by condensation or
other contamination.
Protective lters can slow the response time of sensors. This can be avoided by removing any lter, but the •
benet must be weighed against the risk of damage to the sensor.
Sensors should not normally be submerged in liquids. In the case of a resistive (electrolytic) sensor, water •
or other liquids would certainly damage the sensor beyond repair.
Salt solutions are especially commonly used for calibration of electrical sensors, and should be provided •
with traceability directly or via a calibrated hygrometer. Protection of sensors from direct contact with salt
or solution is most important as contamination would destroy or seriously impair the sensing element.
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Michell Instruments 19 97210 Issue 01, April 2011
Appendix C
List of Worldwide
Michell Instruments’ Ofces
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Appendix C List of Worldwide Michell Instruments’ Ofces
Asia
Michell Asia
PO Box 3149
Joondalup
WA 6027
Australia
Tel: +61 893 046587
E-mail: au.info@michell.com
Web: www.michell.com/au
China
Michell Instruments (Shanghai) Ltd
Room 505, Qilai Building
889 Yishan Road
Shanghai, 200233
P R China
Tel: +86 21 5401 2255
Fax: +86 21 5401 2085
E-mail:
cn.info@michell.com
Web: www.michell.com/cn
Germany, Austria, Switzerland
Michell Instruments GmbH
Industriestrasse 27
D-61381 Friedrichsdorf
Germany
Tel: +49 6172 591700
Fax: +49 6172 591799
E-mail:
de.info@michell.com
Web: www.michell.com/de
Benelux
Michell Instruments Benelux BV
Krombraak 11
4906 CR Oosterhout
The Netherlands
Tel: +31 162 680 471
Fax: +31 162 437 566
E-mail: nl.info@michell.com
Web:
www.michell.com/nl
France
Michell Instruments SAS
2-4, rue Jean Desparmet
69008 Lyon
France
Tel: +33 437 53 88 20
Fax: +33 437 53 88 21
E-mail: fr.info@michell.com
Web: www.michell.com/fr
Italy
Michell Italia Srl
Via Capecelatro, 10
20148 Milano
Italy
Tel: +39 02 4047194
Fax: + 39 02 40010565
E-mail: it.info@michell.com
Web: www.michell.com/it
Japan
Michell Japan KK
Musashino Center Building
1-19-18 Nakacho, Musashino
Tokyo 180-0006
Japan
Tel: +81 422 502600
Fax: +81 422 521700
E-mail: info@michell-japan.co.jp
Web: www.michell-japan.co.jp
North America
Michell Instruments Inc
319 Newburyport Turnpike, Suite 207
Rowley, MA 01969
USA
Tel: +01 978 484 0005
Fax: +01 978 843 7669
E-mail:
us.info@michell.com
Web: www.michell.com/us
Middle East
Michell Instruments Middle East
P-06, #097
Sharjah Airport Int’l free zone
Sharjah,
United Arab Emirates
Tel: +971 6 5575028
+971 6 5575029
Fax:
E-mail: me.info@michell.com
United Kingdom
Michell Instruments Ltd
48 Lancaster Way Business Park
Ely, CB6 3NW
Cambridgeshire
England
Tel: +44 1353 658000
Fax: +44 1353 658199
E-mail: info@michell.com
Web: www.michell.com/uk
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NOTES:
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NOTES:
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NOTES:
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http://www.michell.com
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