ASTM E337-15(2023)
(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)
SIGNIFICANCE AND USE
5.1 The object of this test method is to provide guidelines for the construction of a psychrometer and the techniques required for accurately measuring the humidity in the atmosphere. Only the essential features of the psychrometer are specified.
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 RH ± rh %, which means that the relative humidity is expected to lie in the range (RH − rh) % to (RH + rh) %, where RH 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 °C 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 °C 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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 Warning—Mercury has been designated by many regulatory agencies as a hazardous material that can cause serious medical issues. Mercury, or its vapor, has been demonstrated to be hazardous to health and corrosive to materials. Caution should be taken when handling mercury and mercury containing products. See the applicable product Safety Data Sheet (SDS) for additional information. Users should be aware that selling mercury and/or mercury containing products into your state or country may be prohibited by law.
1.6 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. (For more specific safety precautionary statements, see 8.1 and 15.1.)
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E337 − 15 (Reapproved 2023)
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 E337; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.3.2 This method is applicable within the ambient tempera-
ture range 5 °C to 50 °C, wet-bulb temperatures not lower than
1.1 General:
1 °C and restricted to ambient pressures not differing from
1.1.1 This test method covers the determination of the
standard atmospheric pressure by more than 30 %.
humidity of atmospheric air by means of wet- and dry-bulb
1.4 The values stated in SI units are to be regarded as
temperature readings.
standard. No other units of measurement are included in this
1.1.2 This test method is applicable for meteorological
standard.
measurements at the earth’s surface, for the purpose of the
testing of materials, and for the determination of the relative
1.5 Warning—Mercury has been designated by many regu-
humidity of most standard atmospheres and test atmospheres.
latory agencies as a hazardous material that can cause serious
1.1.3 This test method is also applicable when the tempera-
medical issues. Mercury, or its vapor, has been demonstrated to
ture of the wet bulb only is required. In this case, the
be hazardous to health and corrosive to materials. Caution
instrument comprises a wet-bulb thermometer only.
should be taken when handling mercury and mercury contain-
1.1.4 Relative humidity (RH) does not denote a unit. Un-
ing products. See the applicable product Safety Data Sheet
certainties in the relative humidity are expressed in the form
(SDS) for additional information. Users should be aware that
RH 6 rh %, which means that the relative humidity is expected
selling mercury and/or mercury containing products into your
to lie in the range (RH − rh) % to (RH + rh) %, where RH is
state or country may be prohibited by law.
the observed relative humidity. All uncertainties are at the 95 %
1.6 This standard does not purport to address all of the
confidence level.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
1.2 Method A—Psychrometer Ventilated by Aspiration:
priate safety, health, and environmental practices and deter-
1.2.1 This method incorporates the psychrometer ventilated
mine the applicability of regulatory limitations prior to use.
by aspiration. The aspirated psychrometer is more accurate
(For more specific safety precautionary statements, see 8.1 and
than the sling (whirling) psychrometer (see Method B), and it
15.1.)
offers advantages in regard to the space which it requires, the
1.7 This international standard was developed in accor-
possibility of using alternative types of thermometers (for
dance with internationally recognized principles on standard-
example, electrical), easier shielding of thermometer bulbs
ization established in the Decision on Principles for the
from extraneous radiation, accidental breakage, and conve-
Development of International Standards, Guides and Recom-
nience.
mendations issued by the World Trade Organization Technical
1.2.2 This method is applicable within the ambient tempera-
Barriers to Trade (TBT) Committee.
ture range 5 °C to 80 °C, wet-bulb temperatures not lower than
1 °C, and restricted to ambient pressures not differing from
2. Referenced Documents
standard atmospheric pressure by more than 30 %.
2.1 ASTM Standards:
1.3 Method B—Psychrometer Ventilated by Whirling (Sling
D861 Practice for Use of the Tex System to Designate Linear
Psychrometer):
Density of Fibers, Yarn Intermediates, and Yarns
1.3.1 This method incorporates the psychrometer ventilated
D1193 Specification for Reagent Water
by whirling (sling psychrometer).
D1356 Terminology Relating to Sampling and Analysis of
Atmospheres
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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Jan. 1, 2023. Published February 2023. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1931. Last previous edition approved in 2015 as E337 – 15. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0337-15R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E337 − 15 (2023)
D1357 Practice for Planning the Sampling of the Ambient of 60.2 °C or better, and the temperature depression with an
Atmosphere overall uncertainty of 60.2 °C or better for an uncertainty in
D3631 Test Methods for Measuring Surface Atmospheric the relative humidity of 63 % RH. For an uncertainty in the
Pressure relative humidity of 62 % RH, obtain the dry-bulb temperature
D4023 Terminology Relating to Humidity Measurements with an overall uncertainty of 60.2 °C or better and the
(Withdrawn 2002) temperature depression with an overall uncertainty of 60.1 °C
D4230 Test Method for Measuring Humidity with Cooled- or better. (Also see Section 12.)
Surface Condensation (Dew-Point) Hygrometer
4.3 Method B—Sling Psychrometer:
E1 Specification for ASTM Liquid-in-Glass Thermometers
4.3.1 Holding the instrument well away from the body, and
E2251 Specification for Liquid-in-Glass ASTM Thermom-
for outdoor measurements to windward and in the shade, whirl
eters with Low-Hazard Precision Liquids
it at such a rate as to achieve the specified airspeed at the wet
IEEE/ASTM SI-10 Practice for Use of the International
and dry bulbs, see 14.4.
System of Units (SI) (the Modernized Metric System)
4.3.2 Read the thermometers with the necessary precision,
obtaining the dry-bulb temperature with an overall uncertainty
3. Terminology
of 60.6 °C or better, and the temperature depression with an
3.1 Definitions:
overall uncertainty of 60.3 °C or better for an uncertainty in
3.1.1 For definitions of humidity terms used in this test
the relative humidity of 65 % RH, also see Section 19.
method, refer to Terminology D4023.
5. Significance and Use
3.1.2 For definitions of other terms in this test method, refer
to Terminology D1356.
5.1 The object of this test method is to provide guidelines
3.2 Definitions of Terms Specific to This Standard:
for the construction of a psychrometer and the techniques
3.2.1 Method A—Aspirated Psychrometer:
required for accurately measuring the humidity in the atmo-
3.2.1.1 aspiration—the wet and dry bulbs (and the psy-
sphere. Only the essential features of the psychrometer are
chrometer) are described as aspirated because there is provi-
specified.
sion for the forced ventilation by drawing air over the bulbs by
suction. The flow may be either transverse or parallel to the METHOD A—PSYCHROMETER VENTILATED
axes of the bulbs. BY ASPIRATION
3.2.1.2 thermometer—for purposes of this standard, and
6. Interferences
except where a specific type is indicated, the term thermometer
means any temperature-measuring device.
6.1 When an aspirated psychrometer is used for measure-
3.2.1.3 wet-bulb covering and wick—the wet bulb is pro- ments in a small enclosed space and steadily rising wet- and
vided with a water-retaining covering of a woven-cotton dry-bulb temperatures are observed, consider whether heat and
material. A cotton wick which connects the covering to a water moisture liberated by the instrument itself are affecting the
reservoir may be provided so that water is fed to the covering conditions.
continuously by capillarity.
6.2 While the thermometers are being read, keep all surfaces
3.2.2 Method B—Sling Psychrometer:
that are at temperatures other than the environment (such as the
3.2.2.1 ventilation—the wet and dry bulbs (and the psy-
hands, face, and other warmer or colder objects) as far as
chrometer) are described as ventilated because there is provi-
possible from the thermometer bulbs.
sion for a flow of the air over the bulbs. The flow is transverse
6.3 This method should not be used where there is heavy
to the axes of the bulbs.
contamination of the air with gases, vapors, or dust.
3.2.2.2 wet-bulb covering—the wet bulb is provided with a
water-retaining covering of a woven-cotton material.
7. Apparatus
4. Summary of Methods 7.1 Thermometers for an Aspirated Psychrometer:
7.1.1 The range of the thermometers shall not exceed the
4.1 General:
range 0 °C to 80 °C. This range may be achieved by providing
4.1.1 The wet-bulb temperature depression, the dry-bulb
more than a single pair of matched thermometers. When the
temperature, and the ambient pressure provide the basis for
uncertainty in the derived relative humidity is required to be
deriving the relative humidity.
not more than 63 % RH, the thermometers shall be such that
4.2 Method A—Aspirated Psychrometer:
their readings give the temperature depression with an uncer-
4.2.1 Establish the airflow (see 7.4) and maintain it until a
tainty of not more than 60.2 °C. When the uncertainty in the
minimum wet-bulb temperature is attained. (With mercury-in-
relative humidity is required to be not more than 62 % RH,
glass thermometers, about 2-min ventilation time is usually
they shall be such that their readings give the temperature
necessary.)
depression with an uncertainty of not more than 60.1 °C. The
4.2.2 Read the thermometers with the necessary precision,
uncertainty in the reading of the dry-bulb temperature shall be
obtaining the dry-bulb temperature with an overall uncertainty
not more than 60.2 °C.
7.1.2 Electrical thermometers may be so connected that the
readings give the temperature depression and the dry-bulb
The last approved version of this historical standard is referenced on www.ast-
m.org. temperature directly.
E337 − 15 (2023)
7.1.3 Each thermometer shall consist of a temperature 7.2.7 The covering shall be washed in situ with distilled
sensor of essentially cylindrical shape which is supported on a water from time to time and be renewed when it shows any
single stem, the stem being coaxial with the sensor. The free evidence of permanent change.
end of each sensor shall be smoothly rounded. If the diameter 7.2.8 When a wick is provided, the free length of a wick
of the stems is small, compared with that of the sensors, then shall be at least twice the diameter of the wet bulb and at least
both ends of each sensor shall be smoothly rounded. The sensor three times the wick diameter, ensuring that water arriving at
of a thermometer shall be that part of the thermometer the covering is already at practically the wet-bulb temperature.
extending from the bottom of the bulb to the top of the entrance A wick shall be limp.
flare of the capillary. 7.2.9 A water reservoir shall not obstruct the airflow, and its
7.1.4 With transverse ventilation, the diameters of the sen- contents shall not affect the humidity of the sample air.
7.2.10 The level of the water in a water reservoir shall be
sors (excluding wet covering) shall be not less than 1 mm and
not greater than 4 mm. between 5 mm and 25 mm below the level of the lowest part of
the wet bulb.
7.1.5 With axial ventilation, the diameters of the sensors
(excluding wet covering) shall be not less than 2 mm and not
7.3 Water—Reagent water shall be produced by distillation,
greater than 5 mm, and their length not less than 10 mm and
or by ion exchange or reverse osmosis followed by distillation,
not greater than 30 mm.
refer to Specification D1193.
7.1.6 The connecting wires of electrical thermometers shall
7.4 Airflow:
be contained within the supporting stems and shall be isolated
7.4.1 The flow of air over both the wet and dry bulbs shall
from the moisture of the wet covering.
be a forced draught of 3 m ⁄s to 10 m ⁄s for thermometers with
7.1.7 Thermometers shall be graduated to 0.5 °C or closer
maximum allowable diameter of the sensors.
and be capable of being read to the nearest 0.1 °C or better. (A
7.4.2 The sample air shall not pass over any obstruction or
specification for mercury-in-glass thermometers suitable when
through a fan before it passes over the wet and dry bulbs.
the uncertainty in the derived relative humidity is required to
7.4.3 With axial flow, the direction of the flow shall be from
be not more than 63 % RH is given in Annex A1.)
the free end of each sensor towards the support end.
7.2 Wet-Bulb Covering, Wick, and Water Reservoir:
7.4.4 No air which has been cooled by the wet bulb or by the
7.2.1 The covering shall be fabricated from white-cotton
wick shall impinge on the dry bulb.
muslin of linear density from 1.0 g ⁄m to 1.2 g ⁄m, refer to
7.5 Radiation Shields:
Practice D861. A seamless sleeve is preferred, but a seam is
7.5.1 Any radiation shields shall be of metal with a thick-
permissible, provided that it does not add appreciably to the
ness of 0.4 mm to 0.8 mm. Surfaces required to have a polished
general 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
bulb of the thermometer, fit snugly but not very tightly, and
provided to shield the wet and dry bulbs from extraneous
shall be in physical contact with the bulb over its entire surface.
radiations. The radiation shields, essentially in the form of
It shall extend onto the stem for such a distance that the error
parallel plates, can be either polished on the outside and
in the observed wet-bulb temperature due to heat conduction
blackened on the inside, or polished on both the inside and
along the stem does not exceed 0.05 °C. (A method of
outside surfaces. The clearance between the wet and dry bulbs
determining the distance for which the covering must extend
and the shields shall be not less than half the overall diameter
onto the stem is outlined in Annex A2. For mercury-in-glass
of the wet bulb. The shields shall be liberally flared outwards
thermometers with solid stems, a distance of twice the stem
at the inlet to prevent the flow separating from the shields on
diameter is usually adequate.)
the inside (vena-contracta effect). The shields may form part of
7.2.3 To maintain a snugly fit cover on
...
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