ASTM D4230-83(1996)e1
(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 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.
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e1
Designation: D 4230 – 83 (Reapproved 1996)
Standard Test Method of
Measuring Humidity with Cooled-Surface Condensation
(Dew-Point) Hygrometer
This standard is issued under the fixed designation D 4230; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number 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 3.1.2 For definitions of other terms in this method, refer to
Terminology D 1356.
1.1 This test method covers the determination of the ther-
3.2 Definitions of Terms Specific to This Standard:
modynamic dew- or frost-point temperature of ambient air by
3.2.1 nonhygroscopic material—material that neither ab-
the condensation of water vapor on a cooled surface. For
sorbs nor retains water vapor.
brevity this is referred to in this method as the condensation
3.2.2 mirror (front surface)—a polished surface, usually a
temperature.
metallic surface, on which condensates are deposited.
1.2 This test method is applicable for the range of conden-
3.3 Symbols:
sation temperatures from 60°C to − 70°C.
1.3 This test method includes a general description of the
instrumentation and operational procedures, including site
e = vapor pressure of water vapor in moist air.
selection, to be used for obtaining the measurements and a
e = saturation pressure of water vapor in equilibrium
i
description of the procedures to be used for calculating the
with the plane surface of ice.
results.
e = saturation pressure of water vapor in equilibrium
w
1.4 This test method is applicable for the continuous mea-
with the plane surface of water.
surement of ambient humidity in the natural atmosphere on a
f = enhancement factor.
stationary platform.
p = ambient pressure.
a
1.5 This standard does not purport to address all of the
p = mirror chamber pressure.
c
r = mixing ratio.
safety concerns, if any, associated with its use. It is the
t = ambient air temperature.
responsibility of the user of this standard to establish appro-
T = thermodynamic dew- or frost-point temperature.
priate safety and health practices and determine the applica- d
U = relative humidity with respect to ice.
bility of regulatory limitations prior to use. For specific i
U = relative humidity with respect to water.
w
precautionary statements, see Section 8.
x = mole fraction of water vapor.
v
x = saturation mole fraction of water vapor with respect
2. Referenced Documents
vi
to ice.
2.1 ASTM Standards:
x = saturation mole fraction of water vapor with respect
vw
D 1356 Terminology Relating to Sampling and Analysis of
to water.
Atmospheres
D 3631 Test Methods for Measuring Surface Atmospheric
4. Summary of Test Method
Pressure
4.1 The ambient humidity is measured with a dew- and
D 4023 Terminology Relating to Humidity Measurements
frost-point hygrometer.
4.2 The mirror or some other surface on which the conden-
3. Terminology
sate is deposited is provided with the means for cooling and
3.1 Definitions:
heating, detection of condensate, and the measurement of the
3.1.1 For definitions of terms used in this test method, refer
temperature of the mirror surface.
to Terminology D 4023.
5. Significance and Use
5.1 Humidity data is important for the understanding and
This test method is under the jurisdiction of ASTM Committee D-22 on
Sampling and Analysis of Atmospheres and is the direct responsibility of Subcom-
interpretation of a number of phenomena. Atmospheric water
mittee D22.11 on Meteorology.
vapor affects precipitation; the formation of dew and fog; the
Current edition approved Feb. 25, 1983. Published October 1983.
2 prediction of frosts damaging to agriculture; the potential
Annual Book of ASTM Standards, Vol 11.03.
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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D 4230
danger of forest fires; and the propagation of electromagnetic 8.1.3 Appropriate voltage surge protection circuitry must be
energy. It affects evaporation from rivers, lakes, reservoirs, incorporated.
oceans, and snow and ice surfaces. It affects the transpiration of
8.2 Technical Precautions:
moisture from soils, growing crops, and forest.
8.2.1 The accuracy of a cooled-surface condensation hy-
grometer is degraded by the presence of water-soluble materi-
6. Interferences
als. A mirror-cleaning schedule, consistent with the contami-
6.1 This method is not applicable if other constituents in the
nation rate, is necessary to maintain the initial calibration
atmosphere condense before water vapor.
accuracy. The user must determine the required maintenance
schedule for the specific site, by comparison of calibrations
7. Apparatus
made before and after cleaning.
7.1 Dew-point hygrometers, specifically designed for me-
8.2.2 Caution in performing this method should be taken if
teorological observations are available commercially. A sche-
the indicated mirror temperature is between 0°C and − 30°C.
matic arrangement of a typical optical dew-point hygrometer is
Below freezing, the initial formation of the condensate on the
shown in Fig. 1.
surface of a mirror may be either dew or frost. In the case of
7.1.1 The sample air flows through a small chamber.
nonfiltered atmospheric air, the supercooled water usually does
7.1.2 Within the chamber is a mirror or surface on which the
not persist long on a mirror surface and quickly changes to
condensate can be deposited.
frost. The only positive method for determining the state of the
7.1.3 A beam of light from an incandescent lamp, light
condensate is by visual observation of the mirror surface.
emitting diode or other suitable light source shines on the
8.2.2.1 The following illustrates the magnitude of the error
mirror.
involved when dew or frost is not differentiated: The saturation
7.1.4 Dew or frost is detected with an electro-optic device.
vapor pressure of supercooled water at − 30°C corresponds to
7.1.5 The mirror is cooled by a Peltier thermoelectric
saturation vapor pressure of ice at − 27.2°C; dew point
element. Peltier cooling is a convenient method for unattended
of − 20°C corresponds to frost point of − 18.0°C; − 10°C dew
and automatic instruments.
point corresponds to frost point of − 8.9°C. (The frost point
7.1.6 Preferred methods of sensing mirror temperature are
temperature is approximately 90 % of the dew-point tempera-
resistance thermometers, thermistors, and thermocouples.
ture in degrees Celsius.)
7.1.6.1 The temperature sensors shall be attached to or
8.2.3 A positive method for identifying the state of the
embedded in the mirror to measure the temperature of the
condensate is to visually observe the condensate on the mirror
surface of the mirror.
with the aid of a microscope or other optical magnifier.
7.1.7 Suitable control circuitry shall be provided to maintain
8.2.4 A finite length of time is required for the condensate to
a constant quantity of condensate on the mirror.
deposit on the mirror and for the hygrometer to reach equilib-
7.1.8 Suitable provisions shall be provided to compensate
rium with the ambient humidity. The response of the hygrom-
for the contamination of the surface of the mirror.
eter depends on the humidity of the ambient air, and on such
7.2 Auxiliary Equipment:
factors as the ventilation rate of the ambient air past the mirror,
7.2.1 Provision shall be provided for assuring air flow past
the sensitivity of the condensate detector, and the maximum
the dew-point mirror.
cooling rate of the hygrometer. The worst case occurs during
7.2.2 Readout instrumentation is available with the dew-
the initial dew-point reading after clearing the mirror of all
point hygrometer.
condensates. The time it takes the hygrometer to reach equi-
librium after clearing the mirror will vary from instrument to
8. Precautions
instrument. As an illustration of the magnitude of this time, the
8.1 Safety Precautions:
following are approximate times required by a hygrometer to
8.1.1 The hygrometer shall be packaged in a suitable
reach equilibrium after clearing the mirror.
enclosure for application in industrial or outdoor environment.
8.2.4.1 For dew points warmer than 0°C: 5 min after clear.
8.1.2 Electrical connectors and cables shall be suitable for
8.2.4.2 For dew points 0°C to − 20°C: 5 to 20 min after
the outdoor environment.
clear.
8.2.4.3 For dew points − 20°C to − 40°C: 20 min to 1 h after
clear.
8.2.4.4 For dew points − 40°C to − 60°C:1hto2h after
clear.
8.2.4.5 For dew points − 60°C to − 70°C:2hto6h after
clear.
8.2.5 The pressure differential between the mirror chamber
and the ambient shall not be greater than 0.5 % that is, not
more than 500 Pa.
8.2.6 The thermometer must measure the temperature of the
mirror surface and not be influenced by the ambient air
temperature.
FIG. 1 Schematic of a Thermoelectric Cooled Condensation
Hygrometer 8.2.7 All materials, which come into contact with the
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D 4230
sample air before it reaches the dew-point mirror, shall be 11. Procedure
nonhygroscopic. Metal, glass, polytetrafluoroethylene, or sta-
11.1 Selection of Sampling Site—Select sampling site as
bilized polypropylene are examples of suitable materials.
indicated in 3.3 and also in 1.3.2 of the World Meteorological
Polyvinyl chloride tubing must be avoided.
Organization, Guide to Meteorological Instrument and Observ-
ing Practices (1).
9. Sampling
11.2 Consult the manufacturer’s operating manual for
9.1 Automatic dew-point hygrometers provide an output
start-up procedures.
which may be recorded continuously. Modern data loggers
11.3 Perform necessary calibration as indicated in Section
sample temperature-sensor output periodically, convert the
10. The dew-point thermometer will not undergo large shifts
analog sensor signal to a digital form, and store the data. The
(.05°C) in calibration unless it is subjected to physical shock.
proper sampling interval depends on the data application (see
If the thermometer read-out instrumentation is subjected to
13.2).
varying ambient temperatures, the read-out instrumentation
9.2 Locate a blower or pump, which can be used to move
checks must be over the expected range of ambient tempera-
the air sample through the mirror chamber, downstream of the
tures. The frequency with which which these checks are
dew-point mirror.
required will be determined by the stability of the readout
9.3 Select the site or location so that the measurement data
instrumentation.
represents the water vapor content of the ambient atmosphere.
11.4 Check and verify that all necessary variables are
9.3.1 Select the location so that it is normally not influenced
measured and recorded to compute the humidity in the desired
by a local water vapor source. (Of course, if the purpose is to
unit(s) see also 8.2.1.
measure the effects of a local source that is, water cooling
NOTE 1—In general, it is recommended that ambient temperature and
ponds, etc., then it is necessary to locate the site downwind
pressure (the pressure in the mirror chamber should not differ from the
from the source.)
ambient pressure by more than 0.5 %) and the dew-point temperature be
9.3.2 Place the automatic dew-point hygrometer away from
measured and recorded. The ambient pressure is to be measured according
any paved surfaces that may be wet, the immediate influence of
to Test Methods D 3631. This will enable other users of the data to
trees and buildings, and as far as practicable, not too close to
calculate in the different units of humidity.
steep slopes, ridges, cliffs, or hollows. Avoid dusty areas.
12. Calculations
9.3.3 Mount the instrument over a surface which is repre-
sentative of the area. 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 The materials which come in contact with the sample air less. See Appendix X1.
upstream of the dew-point mirror might be wetted by rain, dew,
12.2 Calculate the ambient relative humidity with respect to
or frost; for example, dew forming on a surface in the early water as follows.
morning. Design the sampling system to minimize these
P e~t !
a d
U 5 100 % (1)
~ !
deleterious effects. w t,p
a
P e ~t!
c w
10. Calibration
where:
10.1 Provide the calibration data for the thermometer, used (U ) ) = relative humidity with respect to water, %, at
w t,p
a
temperature t and pressure p ,
for measuring the condensation temperature with the hygrom-
a
p = average barometric pressure, Pascal (Pa)
eter. Consult the manufacturer’s operating manual for calibrat-
a
during the sampling period,
ing the thermometer readout instrumentation.
p = average absolute pressure, Pa, in the dew-
10.2 The cooled-surface condensation (dew-point) method c
point mirror chamber during the sampling
is considered to be an absolute or fundamental method for
period,
measuring humidity. This method requires an accurate mea-
e(t ) = saturation vapor pressure, Pa, at condensa-
d
surement of the temperature of the surface of the dew-point
tion temperature t , °C, where t is the
mirror. It is not uncommon for the dew-point temperature to be d d
average value during the sampling period,
more than 35 K colder than the ambient air temperature. To
see Note 2, and
measure this temperature accurately, without being influenced
e (t) = saturation vapor pressure, Pa, over water at
w
by the warmer ambient and the colder heat-sink temperature,
ambi- ent temperature t, °C, where t is the
requires careful placement of the dew-point thermometer.
average value during the sampling period.
10.3 Therefore, in addition to the temperature calibration of
See Appendix X3.
the thermometer, (see 10.1), a humidity calibration must also
be performed to verify the proper operation of the hygrometer
NOTE 2—If the condensate on the mirror is water (dew), use the
(see Annex A1). The following are additional examples of saturation vapor pressure over water, Appendix X3, corresponding to the
factors that can
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