ASTM E337-84(1996)e1
(Test Method)Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
SCOPE
1.1 General:
1.1.1 This test method covers the determination of the humidity of atmospheric air by means of wet- and dry-bulb temperature readings.
1.1.2 This test method is applicable for meteorological measurements at the earth's surface, for the purpose of the testing of materials, and for the determination of the relative humidity of most standard atmospheres and test atmospheres.
1.1.3 This test method is also applicable when the temperature of the wet bulb only is required. In this case, the instrument comprises a wet-bulb thermometer only.
1.1.4 Relative humidity (rh) does not denote a unit. Uncertainties in the relative humidity are expressed in the form U + u % rh, which means that the relative humidity is expected to lie in the range ( U - )% to ( U + )%, where U is the observed relative humidity. All uncertainties are at the 95% confidence level.
1.2 Method A - Psychrometer Ventilated by Aspiration:
1.2.1 This method incorporates the psychrometer ventilated by aspiration. The aspirated psychrometer is more accurate than the sling (whirling) psychrometer (see Method B), and it offers advantages in regard to the space which it requires, the possibility of using alternative types of thermometers (for example, electrical), easier shielding of thermometer bulbs from extraneous radiation, accidental breakage, and convenience.
1.2.2 This method is applicable within the ambient temperature range 5 to 80°C, wet-bulb temperatures not lower than 1°C, and restricted to ambient pressures not differing from standard atmospheric pressure by more than 30%.
1.3 Method B - Psychrometer Ventilated by Whirling (Sling Psychrometer):
1.3.1 This method incorporates the psychrometer ventilated by whirling (sling psychrometer).
1.3.2 This method is applicable within the ambient temperature range 5 to 50°C, wet-bulb temperatures not lower than 1°C and restricted to ambient pressures not differing from standard atmospheric pressure by more than 30%.
1.4 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 more specific safety precautionary statements, see 8.1 and 15.1.)
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e1
Designation: E 337 – 84 (Reapproved 1996)
Standard Test Method for
Measuring Humidity with a Psychrometer (the Measurement
of Wet- and Dry-Bulb Temperatures)
This standard is issued under the fixed designation E 337; 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 20 was added editorially in April 1996.
1. Scope ture range 5 to 50°C, wet-bulb temperatures not lower than 1°C
and restricted to ambient pressures not differing from standard
1.1 General:
atmospheric pressure by more than 30 %.
1.1.1 This test method covers the determination of the
1.4 This standard does not purport to address all of the
humidity of atmospheric air by means of wet- and dry-bulb
safety concerns, if any, associated with its use. It is the
temperature readings.
responsibility of the user of this standard to establish appro-
1.1.2 This test method is applicable for meteorological
priate safety and health practices and determine the applica-
measurements at the earth’s surface, for the purpose of the
bility of regulatory limitations prior to use. (For more specific
testing of materials, and for the determination of the relative
safety precautionary statements, see 8.1 and 15.1.)
humidity of most standard atmospheres and test atmospheres.
1.1.3 This test method is also applicable when the tempera-
2. Referenced Documents
ture of the wet bulb only is required. In this case, the
2.1 ASTM Standards:
instrument comprises a wet-bulb thermometer only.
D 861 Practice for Use of the Tex System to Designate
1.1.4 Relative humidity (rh) does not denote a unit. Uncer-
Linear Density of Fibers, Yarn Intermediates, and Yarns
tainties in the relative humidity are expressed in the form U 6
D 1193 Specification for Reagent Water
u % rh, which means that the relative humidity is expected to
D 1356 Terminology Relating to Sampling and Analysis of
lie in the range (U − u) % to (U + u) %, where U is the
Atmospheres
observed relative humidity. All uncertainties are at the 95 %
D 1357 Practice for Planning the Sampling of the Ambient
confidence level.
Atmosphere
1.2 Method A—Psychrometer Ventilated by Aspiration:
D 3631 Test Methods for Measuring Surface Atmospheric
1.2.1 This method incorporates the psychrometer ventilated
Pressure
by aspiration. The aspirated psychrometer is more accurate
D 4023 Terminology Relating to Humidity Measurements
than the sling (whirling) psychrometer (see Method B), and it
D 4230 Test Method of Measuring Humidity with Cooled-
offers advantages in regard to the space which it requires, the
Surface Condensation (Dew-Point) Hygrometer
possibility of using alternative types of thermometers (for
E 1 Specification for ASTM Thermometers
example, electrical), easier shielding of thermometer bulbs
E 380 Practice for Use of the International System of Units
from extraneous radiation, accidental breakage, and conve-
(SI) (the Modernized Metric System)
nience.
1.2.2 This method is applicable within the ambient tempera-
3. Terminology
ture range 5 to 80°C, wet-bulb temperatures not lower than
3.1 Definitions:
1°C, and restricted to ambient pressures not differing from
3.1.1 For definitions of humidity terms used in this test
standard atmospheric pressure by more than 30 %.
method, refer to Terminology D 4023.
1.3 Method B—Psychrometer Ventilated by Whirling (Sling
3.1.2 For definitions of other terms in this test method, refer
Psychrometer):
to Terminology D 1356.
1.3.1 This method incorporates the psychrometer ventilated
3.2 Definitions of Terms Specific to This Standard:
by whirling (sling psychrometer).
3.2.1 Method A—Aspirated Psychrometer:
1.3.2 This method is applicable within the ambient tempera-
1 2
This test method is under the jurisdiction of ASTM Committee D-22 on Annual Book of ASTM Standards, Vol 07.01.
Sampling and Analysis of Atmospheres and is the direct responsibility of Subcom- Annual Book of ASTM Standards, Vol 11.01.
mittee D22.11 on Meteorology. Annual Book of ASTM Standards, Vol 11.03.
Current edition approved Nov. 30, 1984. Published June 1985. Originally Annual Book of ASTM Standards, Vol 14.03.
published as E 337 – 31 T. Last previous edition E 337 – 62. Annual Book of ASTM Standards, Vol 14.02. (Excerpts in all other volumes.)
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 337
3.2.1.1 aspiration—The wet and dry bulbs (and the psy- METHOD A—PSYCHROMETER VENTILATED
chrometer) are described as aspirated because there is provi- BY ASPIRATION
sion for the forced ventilation by drawing air over the bulbs by
6. Interferences
suction. The flow may be either transverse or parallel to the
6.1 When an aspirated psychrometer is used for measure-
axes of the bulbs.
ments in a small enclosed space and steadily rising wet- and
3.2.1.2 thermometer—for purposes of this standard, and
dry-bulb temperatures are observed, consider whether heat and
except where a specific type is indicated, the term thermometer
moisture liberated by the instrument itself are affecting the
means any temperature-measuring device.
conditions.
3.2.1.3 wet-bulb covering and wick—the wet bulb is pro-
6.2 While the thermometers are being read, keep all surfaces
vided with a water-retaining covering of a woven-cotton
that are at temperatures other than the environment (such as the
material. A cotton wick which connects the covering to a water
hands, face, and other warmer or colder objects) as far as
reservoir may be provided so that water is fed to the covering
possible from the thermometer bulbs.
continuously by capillarity.
6.3 This method should not be used where there is heavy
3.2.2 Method B—Sling Psychrometer:
contamination of the air with gases, vapors, or dust.
3.2.2.1 ventilation—the wet and dry bulbs (and the psy-
chrometer) are described as ventilated because there is provi-
7. Apparatus
sion for a flow of the air over the bulbs. The flow is transverse
7.1 Thermometers for an Aspirated Psychrometer:
to the axes of the bulbs.
7.1.1 The range of the thermometers shall not exceed the
3.2.2.2 wet-bulb covering—the wet bulb is provided with a
range 0 to 80°C. This range may be achieved by providing
water-retaining covering of a woven-cotton material.
more than a single pair of matched thermometers. When the
uncertainty in the derived relative humidity is required to be
4. Summary of Methods
not more than 63 % rh, the thermometers shall be such that
4.1 General: their readings give the temperature depression with an uncer-
4.1.1 The wet-bulb temperature depression, the dry-bulb tainty of not more than 60.2°C. When the uncertainty in the
relative humidity is required to be not more than 62 % rh, they
temperature, and the ambient pressure provide the basis for
deriving the relative humidity. shall be such that their readings give the temperature depres-
sion with an uncertainty of not more than 60.1°C. The
4.2 Method A—Aspirated Psychrometer:
uncertainty in the reading of the dry-bulb temperature shall be
4.2.1 Establish the airflow (see 7.4) and maintain it until a
not more than 60.2°C.
minimum wet-bulb temperature is attained. (With mercury-in-
7.1.2 Electrical thermometers may be so connected that the
glass thermometers, about 2-min ventilation time is usually
readings give the temperature depression and the dry-bulb
necessary.)
temperature directly.
4.2.2 Read the thermometers with the necessary precision,
7.1.3 Each thermometer shall consist of a temperature
obtaining the dry-bulb temperature with an overall uncertainty
sensor of essentially cylindrical shape which is supported on a
of 60.2°C or better, and the temperature depression with an
single stem, the stem being coaxial with the sensor. The free
overall uncertainty of 60.2°C or better for an uncertainty in the
end of each sensor shall be smoothly rounded. If the diameter
relative humidity of 63 % rh. For an uncertainty in the relative
of the stems is small, compared with that of the sensors, then
humidity of 62 % rh, obtain the dry-bulb temperature with an
both ends of each sensor shall be smoothly rounded. The sensor
overall uncertainty of 60.2°C or better and the temperature
of a mercury-in-glass thermometer shall be that part of the
depression with an overall uncertainty of 60.1°C or better.
thermometer extending from the bottom of the bulb to the top
(Also see Section 12.)
of the entrance flare of the capillary.
4.3 Method B—Sling Psychrometer:
7.1.4 With transverse ventilation, the diameters of the sen-
4.3.1 Holding the instrument well away from the body, and
sors (excluding wet covering) shall be not less than 1 mm and
for outdoor measurements to windward and in the shade, whirl
not greater than 4 mm.
it at such a rate as to achieve the specified airspeed at the wet
7.1.5 With axial ventilation, the diameters of the sensors
and dry bulbs, see 14.4.
(excluding wet covering) shall be not less than 2 mm and not
4.3.2 Read the thermometers with the necessary precision,
greater than 5 mm, and their length not less than 10 mm and
obtaining the dry-bulb temperature with an overall uncertainty
not greater than 30 mm.
of 60.6°C or better, and the temperature depression with an
7.1.6 The connecting wires of electrical thermometers shall
overall uncertainty of 60.3°C or better for an uncertainty in the
be contained within the supporting stems and shall be isolated
relative humidity of 65 % rh, also see Section 19.
from the moisture of the wet covering.
7.1.7 Mercury-in-glass shall be graduated to 0.5°C or closer
5. Significance and Use
and be capable of being read to the nearest 0.1°C or better. (A
5.1 The object of this test method is to provide guidelines specification for mercury-in-glass thermometers suitable when
for the construction of a psychrometer and the techniques the uncertainty in the derived relative humidity is required to
required for accurately measuring the humidity in the atmo- be not more than 63 % rh is given in Annex A1.)
sphere. Only the essential features of the psychrometer are 7.2 Wet-Bulb Covering, Wick, and Water Reservoir:
specified. 7.2.1 The covering shall be fabricated from white-cotton
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 337
muslin of linear density from 1.0 to 1.2 g/m, refer to Practice 7.5 Radiation Shields:
D 861. A seamless sleeve is preferred, but a seam is permis- 7.5.1 Any radiation shields shall be of metal with a thick-
sible, provided that it does not add appreciably to the general ness of 0.4 to 0.8 mm. Surfaces required to have a polished
roughness which the weave imparts to the surface. finish shall be of a bare metal which will retain its brightness.
7.2.2 The covering shall completely cover the sensor or 7.5.2 With transverse ventilation, radiation shields may be
provided to shield the wet and dry bulbs from extraneous
bulb of the thermometer, fit snugly but not very tightly, and
shall be in physical contact with the bulb over its entire surface. radiations. The radiation shields, essentially in the form of
parallel plates, can be either polished on the outside and
It shall extend onto the stem for such a distance that the error
in the observed wet-bulb temperature due to heat conduction blackened on the inside, or polished on both the inside and
outside surfaces. The clearance between the wet and dry bulbs
along the stem does not exceed 0.05°C. (A method of deter-
mining the distance for which the covering must extend onto and the shields shall be not less than half the overall diameter
of the wet bulb. The shields shall be liberally flared outwards
the stem is outlined in Annex A2. For mercury-in-glass
thermometers with solid stems, a distance of twice the stem at the inlet to prevent the flow separating from the shields on
the inside (vena-contracta effect). The shields may form part of
diameter is usually adequate.)
a duct for the airflow. A second shield, outside, is not necessary.
7.2.3 To maintain a snugly fit cover on the wet bulb, the
7.5.3 With axial ventilation, concentric radiation shields
covering may be secured with a cotton thread at the end of the
shall be provided for the wet and dry bulbs, and shall be
covering on the stem of the thermometer, at the top of the
polished inside and out. (The shield around the wet bulb plays
thermometer bulb, and at the bottom of the bulb. However,
a vital role in reducing the radiative heat transfer between that
whenever a wicking is used, the covering shall not be secured
bulb and its surroundings by a factor of about three.) The
between the thermometer bulb and the cotton wicking which
diameter of the shield shall be not less than 1.8 d and not
connects the covering to a water reservoir.
greater than 2.5 d, where d is the overall diameter of the wet
7.2.4 After fabrication, the covering and wick shall have
bulb. Its length and position shall be such that its projection
been washed in a dilute solution of sodium carbonate and
beyond each end of the wet covering is not less than d and not
thoroughly rinsed with distilled water. They shall not subse-
greater than 3 d. The entrance to the shield shall be liberally
quently be touched with the fingers.
flared to form a bell-mouth to prevent the flow separating from
7.2.5 The stem of each thermometer shall, for a length
the shield on the inside. The shield may serve also as the duct
measured from the sensor and not less than 1.53 the length of
for the airflow. A second shield, outside, is not necessary.
the extension of the covering required by 7.2.2, be clear of
obstructions and freely exposed to the airstream.
8. Precautions
7.2.6 During the test, the covering shall be completely
8.1 Safety Precautions—Mercury vapor is poisonous, even
permeated with water as evidenced by a glistening appearance
in small quantities, and prolonged exposure can produce
in a beam of light.
serious physical impairment (1). If a mercury thermometer is
7.2.7 The covering shall be washed in situ with distilled
accidentally broken, carefully collect, place, and seal all of the
water from time to time and be renewed when it shows any
mercury in a strongly made nonmetallic container. Avoid skin
evidence of permanent change.
contact with mercury.
7.2.8 When a wick is prov
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