ASTM D4230-02(2007)
(Test Method)Standard Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer
Standard Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer
SCOPE
1.1 This test method covers the determination of the thermodynamic dew- or frost-point temperature of ambient air by the condensation of water vapor on a cooled surface. For brevity this is referred to in this method as the condensation temperature.
1.2 This test method is applicable for the range of condensation temperatures from 60C to 70C.
1.3 This test method includes a general description of the instrumentation and operational procedures, including site selection, to be used for obtaining the measurements and a description of the procedures to be used for calculating the results.
1.4 This test method is applicable for the continuous measurement of ambient humidity in the natural atmosphere on a stationary platform.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precautionary statements, see Section 8 .
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Designation:D4230–02 (Reapproved 2007)
Standard Test Method of
Measuring Humidity with Cooled-Surface Condensation
(Dew-Point) Hygrometer
This standard is issued under the fixed designation D4230; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This test method covers the determination of the ther- 3.1 Definitions:
modynamic dew- or frost-point temperature of ambient air by 3.1.1 For definitions of other terms in this method, refer to
the condensation of water vapor on a cooled surface. For Terminology D1356.
brevity this is referred to in this method as the condensation 3.2 Definitions of Terms Specific to This Standard:
temperature. 3.2.1 nonhygroscopic material—material that neither ab-
1.2 This test method is applicable for the range of conden- sorbs nor retains water vapor.
sation temperatures from 60°C to−70°C. 3.2.2 mirror (front surface)—a polished surface, usually a
1.3 This test method includes a general description of the metallic surface, on which condensates are deposited.
instrumentation and operational procedures, including site 3.3 Symbols:
selection, to be used for obtaining the measurements and a
description of the procedures to be used for calculating the
e = vapor pressure of water vapor in moist air.
results.
e = saturation pressure of water vapor in equilibrium
i
1.4 This test method is applicable for the continuous mea-
with the plane surface of ice.
surement of ambient humidity in the natural atmosphere on a
e = saturation pressure of water vapor in equilibrium
w
stationary platform.
with the plane surface of water.
1.5 This standard does not purport to address all of the
P = ambient pressure.
safety concerns, if any, associated with its use. It is the
r = mixing ratio.
responsibility of the user of this standard to establish appro-
T = ambient air temperature.
priate safety and health practices and determine the applica-
T = thermodynamic dew- or frost-point temperature.
d
bility of regulatory limitations prior to use. For specific
RH = relative humidity with respect to ice.
i
precautionary statements, see Section 8.
RH = relative humidity with respect to water.
w
2. Referenced Documents
4. Summary of Test Method
2.1 ASTM Standards:
4.1 The ambient humidity is measured with a dew- and
D1356 Terminology Relating to Sampling and Analysis of
frost-point hygrometer.
Atmospheres
4.2 The mirror or some other surface on which the conden-
D3631 Test Methods for Measuring Surface Atmospheric
sate is deposited is provided with the means for cooling and
Pressure
heating, detection of condensate, and the measurement of the
temperature of the mirror surface.
4.3 Calculationsofsaturationvaporpressureoverwaterand
This test method is under the jurisdiction of ASTM Committee D22 on Air
ice as functions of temperature are provided.
Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved Oct. 1, 2007. Published December 2007. Originally
5. Significance and Use
´1
approved in 1983. Last previous edition approved in 2002 as D4230-02 . DOI:
10.1520/D4230-02R07.
5.1 Humidityinformationisimportantfortheunderstanding
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
of atmospheric phenomena and industrial processes. Measure-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ments of the dew-point and calculations of related vapor
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. pressures are important to quantify the humidity information.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D4230–02 (2007)
6. Interferences 8.1.3 Appropriatevoltagesurgeprotectioncircuitrymustbe
incorporated.
6.1 Thismethodisnotapplicableifotherconstituentsinthe
atmosphere condense before water vapor. 8.2 Technical Precautions:
8.2.1 The accuracy of a cooled-surface condensation hy-
7. Apparatus
grometer is degraded by the presence of water-soluble materi-
als. A mirror-cleaning schedule, consistent with the contami-
7.1 Dew-point hygrometers, specifically designed for me-
nation rate, is necessary to maintain the initial calibration
teorological observations are available commercially. A sche-
accuracy. The user must determine the required maintenance
maticarrangementofatypicalopticaldew-pointhygrometeris
shown in Fig. 1. schedule for the specific site, by comparison of calibrations
made before and after cleaning.
7.1.1 The sample air flows through a small chamber.
7.1.2 Withinthechamberisamirrororsurfaceonwhichthe
8.2.2 Caution in performing this method should be taken if
condensate can be deposited.
the indicated mirror temperature is between 0°C and−30°C.
7.1.3 A beam of light from an incandescent lamp, light
Below freezing, the initial formation of the condensate on the
emitting diode or other suitable light source shines on the
surface of a mirror may be either dew or frost. In the case of
mirror.
nonfilteredatmosphericair,thesupercooledwaterusuallydoes
7.1.4 Dew or frost is detected with an electro-optic device.
not persist long on a mirror surface and quickly changes to
7.1.5 The mirror is cooled by a Peltier thermoelectric
frost.Theonlypositivemethodfordeterminingthestateofthe
element. Peltier cooling is a convenient method for unattended
condensate is by visual observation of the mirror surface.
and automatic instruments.
8.2.2.1 The following illustrates the magnitude of the error
7.1.6 Preferred devices of sensing mirror temperature are
involvedwhendeworfrostisnotdifferentiated:Thesaturation
resistance thermometers, thermistors, and thermocouples.
vapor pressure of supercooled water at−30°C corresponds to
7.1.6.1 The temperature sensors shall be attached to or
saturation vapor pressure of ice at−27.2°C; dew point
embedded in the mirror to measure the temperature of the
of−20°C corresponds to frost point of−18.0°C;−10°C dew
surface of the mirror.
point corresponds to frost point of−8.9°C. (The frost point
7.1.7 Suitablecontrolcircuitryshallbeprovidedtomaintain
temperature is approximately 90% of the dew-point tempera-
a constant quantity of condensate on the mirror.
ture in degrees Celsius.)
7.1.8 Suitable provisions shall be provided to compensate
8.2.3 A positive method for identifying the state of the
for the contamination of the surface of the mirror.
condensate is to visually observe the condensate on the mirror
7.2 Auxiliary Equipment:
with the aid of a microscope or other optical magnifier.
7.2.1 Provision shall be provided for assuring air flow past
the dewpoint mirror without changing the pressure in the 8.2.4 Afinitelengthoftimeisrequiredforthecondensateto
mirror chamber more than 0.5% from the ambient pressure deposit on the mirror and for the hygrometer to reach equilib-
surrounding the sensor.An air flow of approximately 1.1 litres
rium with the ambient humidity. The response of the hygrom-
per minute is recommended for typical chambers.
eter depends on the humidity of the ambient air, and on such
7.2.2 Readout instrumentation is available with the dew-
factorsastheventilationrateoftheambientairpastthemirror,
point hygrometer.
the sensitivity of the condensate detector, and the maximum
cooling rate of the hygrometer. The worst case occurs during
8. Precautions
the initial dew-point reading after clearing the mirror of all
condensates. The time it takes the hygrometer to reach equi-
8.1 Safety Precautions:
librium after clearing the mirror will vary from instrument to
8.1.1 The hygrometer shall be packaged in a suitable
instrument.Asanillustrationofthemagnitudeofthistime,the
enclosure for application in industrial or outdoor environment.
following are approximate times required by a hygrometer to
8.1.2 Electrical connectors and cables shall be suitable for
reach equilibrium after clearing the mirror.
the industrial or outdoor environment.
8.2.4.1 For dew points warmer than 0°C: 5 min after
clearing.
8.2.4.2 For dew points 0°C to−20°C: 5 to 20 min after
clearing.
8.2.4.3 Fordewpoints−20°Cto−40°C:20minto1hafter
clearing.
8.2.4.4 For dew points−40°C to−60°C:1hto2h after
clearing.
8.2.4.5 For dew points−60°C to−70°C:2hto6h after
clearing.
8.2.5 The pressure differential between the mirror chamber
and the ambient shall not be greater than 0.5% of the ambient
pressure. For example, the difference shall not exceed 5 hPa at
FIG. 1 Schematic of a Thermoelectric Cooled Condensation
Hygrometer an ambient pressure of 1000 hPa.
D4230–02 (2007)
8.2.6 Thethermometermustmeasurethetemperatureofthe 11. Procedure
mirror surface and not be influenced by the ambient air
11.1 Selection of Sampling Site—Select sampling site as
temperature.
indicated in 9.3 and also in 1.3.2 of the World Meteorological
8.2.7 All materials, which come into contact with the
Organization, Guide to Meteorological Instrument and Observ-
sample air before it reaches the dew-point mirror, shall be
ing Practices (1).
nonhygroscopic. Metal, glass, polytetrafluoroethylene, or sta-
11.2 Consult the manufacturer’s operating manual for
bilized polypropylene are examples of suitable materials.
start-up procedures.
Polyvinyl chloride tubing must be avoided.
11.3 Perform necessary calibration as indicated in Section
10. The dew-point thermometer will not undergo large shifts
9. Sampling
(0.05°C) in calibration unless it is subjected to physical shock.
If the thermometer read-out instrumentation is subjected to
9.1 Automatic dew-point hygrometers provide an output
varying ambient temperatures, the read-out instrumentation
which may be recorded continuously. Modern data loggers
checks must be over the expected range of ambient tempera-
sample temperature-sensor output periodically, convert the
tures.The frequency with which these checks are required will
analog sensor signal to a digital form, and store the data. The
be determined by the stability of the readout instrumentation.
proper sampling interval depends on the data application (see
11.4 Check and verify that all necessary variables are
13.2).
measured and recorded to compute the humidity in the desired
9.2 Locate a blower or pump, which can be used to move
unit(s) see also 12.
the air sample through the mirror chamber, downstream of the
dew-point mirror. The airflow rate also depends on the data
NOTE 1—In general, it is recommended that ambient temperature and
application and sampling environment.
pressure (the pressure in the mirror chamber should not differ from the
9.3 Select the site or location so that the measurement data
ambient pressure by more than 0.5%) and the dew-point temperature be
measuredandrecorded.Theambientpressureistobemeasuredaccording
represents the water vapor content of the ambient atmosphere
to Test Methods D3631. This will enable other users of the data to
or industrial environment being sampled. Local water vapor
calculate in the different units of humidity.
sources, including ponds, wet roads, and structures can influ-
encetheambienthumidity.Avoidsourcesofairbornecontami-
12. Calculations
nants that can influence to condensation process on the mirror.
12.1 In the meteorological range of pressure and tempera-
9.4 The successful application of this method requires that
ture, the saturation vapor pressure of the pure water phase and
all the materials which come in contact with the sample air
of the moist air will be assumed to be equal. This assumption
upstream of the dew-point mirror be nonhygroscopic.
will introduce an error of approximately 0.5% of reading or
9.5 Thematerialswhichcomeincontactwiththesampleair
less.
upstreamofthedew-pointmirrormightbewettedbyrain,dew,
12.2 Calculatetheambientrelativehumiditywithrespectto
or frost; for example, dew forming on a surface in the early
water using the following approximation.
morning. Design the sampling system to minimize these
e~T !
deleterious effects. d
~RH ! 5 100% (1)
w T
e ~T!
w
10. Calibration
where:
10.1 Provide the calibration data for the thermometer, used
(RH ) = relative humidity with respect to water,%, at
w T
for measuring the condensation temperature with the hygrom- temperature T (°C),
eter. Consult the manufacturer’s operating manual for calibrat- e(T ) = saturation vapor pressure, Pa, at condensation
d
temperature T , °C, where T is the average
ing the thermometer readout instrumentation.
d d
value during the sampling period, see Note 2,
10.2 The cooled-surface condensation (dew-point) method
and
is considered to be an absolute or fundamental method for
e (T) = saturation vapor pressure, Pa, over water at
measuring humidity. This method requires an accurate mea- w
ambient temperature T, °C, where T is the
surement of the temperature of the surface of the dew-point
average value during the sampling period.
mirror.Itisnotuncommonforthedew-pointtemperaturetobe
more than 35°C colder than the ambient air temperature. To
NOTE 2—If the condensate on the mirror is water (dew), use the
measure this temperature accurately, without being influenced
saturation vapor pressure over water corresponding to the condensation
by the warmer ambient and the colder heat-sink temperature,
temperature T . If the condensate is ice (frost), use the saturation vapor
d
requires careful placement of the dew-point thermometer. pressure over ice corresponding to the condensation temperature T .
d
Equations for saturation vapor pressure are provided in 12.5.
10.3 Therefore, in addition to the temperature calibration of
the thermometer, (see 10.1), a humidity calibration must also
12.3 Calculate the relative humidity with respect to ice as
be performed to verify the proper operation of the hygrometer
follows:
(see Annex A1). The following are additional examples of
e~T !
d
~RH ! 5 100% (2)
factors that can affect the accuracy of the measurement:
i T
e ~T!
i
extraneous thermally-induced voltage (emf), heat leakage
through the thermometer leads, self-heating of the thermom-
eter, poor thermal contact, temperature gradient across the
The boldface numbers in parentheses refer to the references at the end of this
mirror, etc. method.
D4230–02 (2007)
where: u = (T + 273.15) / 273.16
(RH) = relative humidity with respect to ice,%, at tem- a = -13.9281690
i T 1
a = 34.7078238
perature T (°C),
e (T) = saturationvaporpressure,Pa,overiceatambient
12.5 Calculate the mixing ratio as follows:
i
temperature T, °C, where T is the average value
r 50.6220 e~T
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
e1
Designation:D4230–83(Reapproved1996) Designation:D4230–02 (Reapproved 2007)
Standard Test Method of
Measuring Humidity with Cooled-Surface Condensation
(Dew-Point) Hygrometer
This standard is issued under the fixed designation D4230; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval. e NOTE—Section 14 was added editorially in
April 1996.
1. Scope
1.1 This test method covers the determination of the thermodynamic dew- or frost-point temperature of ambient air by the
condensation of water vapor on a cooled surface. For brevity this is referred to in this method as the condensation temperature.
1.2 This test method is applicable for the range of condensation temperatures from 60°C to−70°C.
1.3 This test method includes a general description of the instrumentation and operational procedures, including site selection,
to be used for obtaining the measurements and a description of the procedures to be used for calculating the results.
1.4 Thistestmethodisapplicableforthecontinuousmeasurementofambienthumidityinthenaturalatmosphereonastationary
platform.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use. For specific precautionary statements, see Section 8.
2. Referenced Documents
2.1 ASTM Standards:
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
D3631Test Methods for Measuring Surface Atmospheric Pressure
D4023Terminology Relating to Humidity Measurements Test Methods for Measuring Surface Atmospheric Pressure
3. Terminology
3.1 Definitions:
3.1.1 For definitions of other terms used in this test method, refer to Terminology D4023D1356.
3.1.2For definitions of other terms in this method, refer to Terminology D1356.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 nonhygroscopic material—material that neither absorbs nor retains water vapor.
3.2.2 mirror (front surface)—a polished surface, usually a metallic surface, on which condensates are deposited.
3.3 Symbols:
e
e = vapor pressure of water vapor in moist air.
e = saturation pressure of water vapor in equilibrium with the plane surface of ice.
i
e = saturation pressure of water vapor in equilibrium with the plane surface of water.
w
fP = enhancement factor.
p = ambient pressure.
a
p = mirror chamber pressure.
c
r = mixing ratio.
tT = ambient air temperature.
This test method is under the jurisdiction ofASTM Committee D-22 on Sampling andAnalysis ofAtmospheres and is the direct responsibility of Subcommittee D22.11
on Meteorology.
Current edition approved Feb. 25, 1983. Published October 1983.
This test method is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
e1
Current edition approved Oct. 1, 2007. Published December 2007. Originally approved in 1983. Last previous edition approved in 2002 as D4230-02 .
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
, Vol 11.03.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D4230–02 (2007)
T = thermodynamic dew- or frost-point temperature.
d
U = relative humidity with respect to ice.
RH
i
U = relative humidity with respect to water.x =mole fractio
RH v
w
x = saturation mole fraction of water vapor with respect to ice.
vi
x = saturation mole fraction of water vapor with respect to water.
vw
4. Summary of Test Method
4.1 The ambient humidity is measured with a dew- and frost-point hygrometer.
4.2 The mirror or some other surface on which the condensate is deposited is provided with the means for cooling and heating,
detection of condensate, and the measurement of the temperature of the mirror surface.
4.3 Calculations of saturation vapor pressure over water and ice as functions of temperature are provided.
5. Significance and Use
5.1Humidity data is important for the understanding and interpretation of a number of phenomena. Atmospheric water vapor
affects precipitation; the formation of dew and fog; the prediction of frosts damaging to agriculture; the potential danger of forest
fires;andthepropagationofelectromagneticenergy.Itaffectsevaporationfromrivers,lakes,reservoirs,oceans,andsnowandice
surfaces. It affects the transpiration of moisture from soils, growing crops, and forest.
5.1 Humidityinformationisimportantfortheunderstandingofatmosphericphenomenaandindustrialprocesses.Measurements
of the dew-point and calculations of related vapor pressures are important to quantify the humidity information.
6. Interferences
6.1 This method is not applicable if other constituents in the atmosphere condense before water vapor.
7. Apparatus
7.1 Dew-point hygrometers, specifically designed for meteorological observations are available commercially. A schematic
arrangement of a typical optical dew-point hygrometer is shown in Fig. 1.
7.1.1 The sample air flows through a small chamber.
7.1.2 Within the chamber is a mirror or surface on which the condensate can be deposited.
7.1.3 A beam of light from an incandescent lamp, light emitting diode or other suitable light source shines on the mirror.
7.1.4 Dew or frost is detected with an electro-optic device.
7.1.5 The mirror is cooled by a Peltier thermoelectric element. Peltier cooling is a convenient method for unattended and
automatic instruments.
7.1.6 Preferred methodsdevices of sensing mirror temperature are resistance thermometers, thermistors, and thermocouples.
7.1.6.1 Thetemperaturesensorsshallbeattachedtoorembeddedinthemirrortomeasurethetemperatureofthesurfaceofthe
mirror.
7.1.7 Suitable control circuitry shall be provided to maintain a constant quantity of condensate on the mirror.
7.1.8 Suitable provisions shall be provided to compensate for the contamination of the surface of the mirror.
7.2 Auxiliary Equipment:
7.2.1Provision shall be provided for assuring air flow past the dew-point mirror.
7.2.1 Provision shall be provided for assuring air flow past the dewpoint mirror without changing the pressure in the mirror
chamber more than 0.5% from the ambient pressure surrounding the sensor. An air flow of approximately 1.1 litres per minute
is recommended for typical chambers.
7.2.2 Readout instrumentation is available with the dew-point hygrometer.
8. Precautions
8.1 Safety Precautions:
FIG. 1 Schematic of a Thermoelectric Cooled Condensation
Hygrometer
D4230–02 (2007)
8.1.1 The hygrometer shall be packaged in a suitable enclosure for application in industrial or outdoor environment.
8.1.2 Electrical connectors and cables shall be suitable for the industrial or outdoor environment.
8.1.3 Appropriate voltage surge protection circuitry must be incorporated.
8.2 Technical Precautions:
8.2.1The8.2.1 The accuracy of a cooled-surface condensation hygrometer is degraded by the presence of water-soluble
materials. A mirror-cleaning schedule, consistent with the contamination rate, is necessary to maintain the initial calibration
accuracy. The user must determine the required maintenance schedule for the specific site, by comparison of calibrations made
before and after cleaning.
8.2.2 Cautioninperformingthismethodshouldbetakeniftheindicatedmirrortemperatureisbetween0°Cand−30°C.Below
freezing, the initial formation of the condensate on the surface of a mirror may be either dew or frost. In the case of nonfiltered
atmospheric air, the supercooled water usually does not persist long on a mirror surface and quickly changes to frost. The only
positive method for determining the state of the condensate is by visual observation of the mirror surface.
8.2.2.1 The following illustrates the magnitude of the error involved when dew or frost is not differentiated: The saturation
vapor pressure of supercooled water at−30°C corresponds to saturation vapor pressure of ice at−27.2°C; dew point of−20°C
corresponds to frost point of−18.0°C;−10°C dew point corresponds to frost point of−8.9°C. (The frost point temperature is
approximately 90% of the dew-point temperature in degrees Celsius.)
8.2.3 Apositive method for identifying the state of the condensate is to visually observe the condensate on the mirror with the
aid of a microscope or other optical magnifier.
8.2.4 Afinite length of time is required for the condensate to deposit on the mirror and for the hygrometer to reach equilibrium
withtheambienthumidity.Theresponseofthehygrometerdependsonthehumidityoftheambientair,andonsuchfactorsasthe
ventilation rate of the ambient air past the mirror, the sensitivity of the condensate detector, and the maximum cooling rate of the
hygrometer.Theworstcaseoccursduringtheinitialdew-pointreadingafterclearingthemirrorofallcondensates.Thetimeittakes
the hygrometer to reach equilibrium after clearing the mirror will vary from instrument to instrument. As an illustration of the
magnitude of this time, the following are approximate times required by a hygrometer to reach equilibrium after clearing the
mirror.
8.2.4.1 For dew points warmer than 0°C: 5 min after clearing.
8.2.4.2 For dew points 0°C to−20°C: 5 to 20 min after clearing.
8.2.4.3 For dew points−20°C to−40°C: 20 min to 1 h after clearing.
8.2.4.4 For dew points−40°C to−60°C:1hto2h after clearing.
8.2.4.5 For dew points−60°C to−70°C:2hto6h after clearing.
8.2.5 Thepressuredifferentialbetweenthemirrorchamberandtheambientshallnotbegreaterthan0.5%thatis,oftheambient
pressure. For example, the difference shall not more than 500 Pa. exceed 5 hPa at an ambient pressure of 1000 hPa.
8.2.6 Thethermometermustmeasurethetemperatureofthemirrorsurfaceandnotbeinfluencedbytheambientairtemperature.
8.2.7 Allmaterials,whichcomeintocontactwiththesampleairbeforeitreachesthedew-pointmirror,shallbenonhygroscopic.
Metal, glass, polytetrafluoroethylene, or stabilized polypropylene are examples of suitable materials. Polyvinyl chloride tubing
must be avoided.
9. Sampling
9.1 Automatic dew-point hygrometers provide an output which may be recorded continuously. Modern data loggers sample
temperature-sensor output periodically, convert the analog sensor signal to a digital form, and store the data.The proper sampling
interval depends on the data application (see 13.2).
9.2Locate a blower or pump, which can be used to move the air sample through the mirror chamber, downstream of the
dew-point mirror.
9.3Select the site or location so that the measurement data represents the water vapor content of the ambient atmosphere.
9.3.1Select the location so that it is normally not influenced by a local water vapor source. (Of course, if the purpose is to
measure the effects of a local source that is, water cooling ponds, etc., then it is necessary to locate the site downwind from the
source.)
9.3.2Placetheautomaticdew-pointhygrometerawayfromanypavedsurfacesthatmaybewet,theimmediateinfluenceoftrees
and buildings, and as far as practicable, not too close to steep slopes, ridges, cliffs, or hollows. Avoid dusty areas.
9.3.3Mount the instrument over a surface which is representative of the area.
9.2 Locate a blower or pump, which can be used to move the air sample through the mirror chamber, downstream of the
dew-point mirror. The airflow rate also depends on the data application and sampling environment.
9.3 Select the site or location so that the measurement data represents the water vapor content of the ambient atmosphere or
industrial environment being sampled. Local water vapor sources, including ponds, wet roads, and structures can influence the
ambient humidity. Avoid sources of airborne contaminants that can influence to condensation process on the mirror.
9.4 Thesuccessfulapplicationofthismethodrequiresthatallthematerialswhichcomeincontactwiththesampleairupstream
of the dew-point mirror be nonhygroscopic.
9.5 The materials which come in contact with the sample air upstream of the dew-point mirror might be wetted by rain, dew,
D4230–02 (2007)
or frost; for example, dew forming on a surface in the early morning. Design the sampling system to minimize these deleterious
effects.
10. Calibration
10.1 Provide the calibration data for the thermometer, used for measuring the condensation temperature with the hygrometer.
Consult the manufacturer’s operating manual for calibrating the thermometer readout instrumentation.
10.2 Thecooled-surfacecondensation(dew-point)methodisconsideredtobeanabsoluteorfundamentalmethodformeasuring
humidity. This method requires an accurate measurement of the temperature of the surface of the dew-point mirror. It is not
uncommon for the dew-point temperature to be more than 35K35°C colder than the ambient air temperature. To measure this
temperature accurately, without being influenced by the warmer ambient and the colder heat-sink temperature, requires careful
placement of the dew-point thermometer.
10.3 Therefore, in addition to the temperature calibration of the thermometer, (see 10.1), a humidity calibration must also be
performed to verify the proper operation of the hygrometer (seeAnnexA1).The following are additional examples of factors that
can affect the accuracy of the measurement: extraneous thermally-induced voltage (emfs), heat leakage through the thermometer
leads, self-heating of the thermometer, poor thermal contact, temperature gradient across the mirror, etc.
11. Procedure
11.1 Selection of Sampling Site—Select sampling site as indicated in 3.39.3 and also in 1.3.2 of the World Meteorological
Organization, Guide to Meteorological Instrument and Observing Practices (1) .
11.2 Consult the manufacturer’s operating manual for start-up procedures.
11.3 PerformnecessarycalibrationasindicatedinSection10.Thedew-pointthermometerwillnotundergolargeshifts(0.05°C)
in calibration unless it is subjected to physical shock. If the thermometer read-out instrumentation is subjected to varying ambient
temperatures, the read-out instrumentation check
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