ASTM D4230-02(2012)
(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
SIGNIFICANCE AND USE
Humidity information is important for the understanding of atmospheric phenomena and industrial processes. Measurements of the dew-point and calculations of related vapor pressures are important to quantify the humidity information.
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 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.
General Information
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Standards Content (Sample)
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Designation: D4230 − 02 (Reapproved 2012)
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 D3631Test Methods for Measuring Surface Atmospheric
Pressure
1.1 This test method covers the determination of the ther-
modynamic dew- or frost-point temperature of ambient air by
3. Terminology
the condensation of water vapor on a cooled surface. For
brevity this is referred to in this method as the condensation 3.1 Definitions:
temperature.
3.1.1 For definitions of other terms in this method, refer to
Terminology D1356.
1.2 This test method is applicable for the range of conden-
3.2 Definitions of Terms Specific to This Standard:
sation temperatures from 60°C to−70°C.
3.2.1 nonhygroscopic material—material that neither ab-
1.3 This test method includes a general description of the
sorbs nor retains water vapor.
instrumentation and operational procedures, including site
3.2.2 mirror (front surface)—a polished surface, usually a
selection, to be used for obtaining the measurements and a
metallic surface, on which condensates are deposited.
description of the procedures to be used for calculating the
results. 3.3 Symbols:
1.4 This test method is applicable for the continuous mea-
e = vapor pressure of water vapor in moist air.
surement of ambient humidity in the natural atmosphere on a
e = saturation pressure of water vapor in equilibrium
i
stationary platform.
with the plane surface of ice.
1.5 This standard does not purport to address all of the e = saturation pressure of water vapor in equilibrium
w
safety concerns, if any, associated with its use. It is the with the plane surface of water.
P = ambient pressure.
responsibility of the user of this standard to establish appro-
r = mixing ratio.
priate safety and health practices and determine the applica-
T = ambient air temperature.
bility of regulatory limitations prior to use. For specific
T = thermodynamic dew- or frost-point temperature.
d
precautionary statements, see Section 8.
RH = relative humidity with respect to ice.
i
1.6 This international standard was developed in accor-
RH = relative humidity with respect to water.
w
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
4. Summary of Test Method
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
4.1 The ambient humidity is measured with a dew- and
Barriers to Trade (TBT) Committee.
frost-point hygrometer.
4.2 The mirror or some other surface on which the conden-
2. Referenced Documents
sate is deposited is provided with the means for cooling and
2.1 ASTM Standards:
heating, detection of condensate, and the measurement of the
D1356Terminology Relating to Sampling and Analysis of
temperature of the mirror surface.
Atmospheres
4.3 Calculationsofsaturationvaporpressureoverwaterand
1 ice as functions of temperature are provided.
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.
Current edition approved April 1, 2012. Published July 2012. Originally
5. Significance and Use
approved in 1983. Last previous edition approved in 2007 as D4230–02 (2007).
DOI: 10.1520/D4230-02R12.
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 (2012)
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-
7.1 Dew-point hygrometers, specifically designed for me-
als. A mirror-cleaning schedule, consistent with the contami-
teorological observations are available commercially. A sche-
nation rate, is necessary to maintain the initial calibration
maticarrangementofatypicalopticaldew-pointhygrometeris
accuracy. The user must determine the required maintenance
shown in Fig. 1.
schedule for the specific site, by comparison of calibrations
7.1.1 The sample air flows through a small chamber.
made before and after cleaning.
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
8.2.4 Afinitelengthoftimeisrequiredforthecondensateto
the dewpoint mirror without changing the pressure in the
deposit on the mirror and for the hygrometer to reach equilib-
mirror chamber more than 0.5% from the ambient pressure
rium with the ambient humidity. The response of the hygrom-
surrounding the sensor.An air flow of approximately 1.1 litres
eter depends on the humidity of the ambient air, and on such
per minute is recommended for typical chambers.
factorsastheventilationrateoftheambientairpastthemirror,
7.2.2 Readout instrumentation is available with the dew-
the sensitivity of the condensate detector, and the maximum
point hygrometer.
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 Hy-
grometer an ambient pressure of 1000 hPa.
D4230 − 02 (2012)
8.2.6 Thethermometermustmeasurethetemperatureofthe thermometer, poor thermal contact, temperature gradient
mirror surface and not be influenced by the ambient air across the mirror, etc.
temperature.
11. Procedure
8.2.7 All materials, which come into contact with the
sample air before it reaches the dew-point mirror, shall be
11.1 Selection of Sampling Site—Select sampling site as
nonhygroscopic. Metal, glass, polytetrafluoroethylene, or sta-
indicated in 9.3 and also in 1.3.2 of the World Meteorological
bilized polypropylene are examples of suitable materials.
Organization, Guide to Meteorological Instrument and Observ-
Polyvinyl chloride tubing must be avoided.
ing Practices (1).
11.2 Consult the manufacturer’s operating manual for
9. Sampling
start-up procedures.
9.1 Automatic dew-point hygrometers provide an output
11.3 Perform necessary calibration as indicated in Section
which may be recorded continuously. Modern data loggers
10. The dew-point thermometer will not undergo large shifts
sample temperature-sensor output periodically, convert the
(0.05°C) in calibration unless it is subjected to physical shock.
analog sensor signal to a digital form, and store the data. The
If the thermometer read-out instrumentation is subjected to
proper sampling interval depends on the data application (see
varying ambient temperatures, the read-out instrumentation
13.2).
checks must be over the expected range of ambient tempera-
9.2 Locate a blower or pump, which can be used to move
tures.The frequency with which these checks are required will
the air sample through the mirror chamber, downstream of the
be determined by the stability of the readout instrumentation.
dew-point mirror. The airflow rate also depends on the data
11.4 Check and verify that all necessary variables are
application and sampling environment.
measured and recorded to compute the humidity in the desired
9.3 Select the site or location so that the measurement data
unit(s) see also 12.
represents the water vapor content of the ambient atmosphere
NOTE 1—In general, it is recommended that ambient temperature and
or industrial environment being sampled. Local water vapor
pressure (the pressure in the mirror chamber should not differ from the
sources, including ponds, wet roads, and structures can influ-
ambient pressure by more than 0.5%) and the dew-point temperature be
encetheambienthumidity.Avoidsourcesofairbornecontami- measuredandrecorded.Theambientpressureistobemeasuredaccording
to Test Methods D3631. This will enable other users of the data to
nants that can influence to condensation process on the mirror.
calculate in the different units of humidity.
9.4 The successful application of this method requires that
all the materials which come in contact with the sample air 12. Calculations
upstream of the dew-point mirror be nonhygroscopic.
12.1 In the meteorological range of pressure and
9.5 Thematerialswhichcomeincontactwiththesampleair temperature, the saturation vapor pressure of the pure water
upstreamofthedew-pointmirrormightbewettedbyrain,dew, phase and of the moist air will be assumed to be equal. This
or frost; for example, dew forming on a surface in the early assumption will introduce an error of approximately 0.5% of
morning. Design the sampling system to minimize these reading or less.
deleterious effects.
12.2 Calculatetheambientrelativehumiditywithrespectto
water using the following approximation.
10. Calibration
e~T !
d
~RH ! 5 100% (1)
10.1 Provide the calibration data for the thermometer, used
w T
e ~T!
w
for measuring the condensation temperature with the hygrom-
where:
eter. Consult the manufacturer’s operating manual for calibrat-
ing the thermometer readout instrumentation. (RH ) = relative humidity with respect to water,%, at
w T
temperature T (°C),
10.2 The cooled-surface condensation (dew-point) method
e(T ) = saturation vapor pressure, Pa, at condensation
d
is considered to be an absolute or fundamental method for
temperatureT ,°C,whereT istheaveragevalue
d d
measuring humidity. This method requires an accurate mea-
during the sampling period, see Note 2, and
surement of the temperature of the surface of the dew-point
e (T) = saturation vapor pressure, Pa, over water at
w
mirror.Itisnotuncommonforthedew-pointtemperaturetobe
ambient temperature T, °C, where T is the aver-
more than 35°C colder than the ambient air temperature. To
age value during the sampling period.
measure this temperature accurately, without being influenced
NOTE 2—If the condensate on the mirror is water (dew), use the
by the warmer ambient and the colder heat-sink temperature,
saturation vapor pressure over water corresponding to the condensation
requires careful placement of the dew-point thermometer. temperature T
. If the condensate is ice (frost), use the saturation vapor
d
pressure over ice corresponding to the condensation temperature T .
d
10.3 Therefore, in addition to the temperature calibration of
Equations for saturation vapor pressure are provided in 12.5.
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
factors that can affect the accuracy of the measurement:
extraneous thermally-induced voltage (emf), heat leakage
The boldface numbers in parentheses refer to the references at the end of this
through the thermometer leads, self-heating of the method.
D4230 − 02 (2012)
e T 12.4.5.2
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