ASTM E337-15(2023)

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

• Status: Published
• Publication Date: 31-Dec-2022
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: D22 - Air Quality
• Drafting Committee: D22.11 - Meteorology

Relations

• Refers: ASTM D1356-20a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Sep-2020
• Refers: ASTM D1356-20 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Mar-2020
• Refers: ASTM D4230-20 - Standard Test Method for Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer
• Effective Date: 01-Mar-2020
• Refers: ASTM D1357-95(2019) - Standard Practice for Planning the Sampling of the Ambient Atmosphere
• Effective Date: 01-Aug-2019
• Refers: ASTM D861-07(2018) - Standard Practice for Use of the Tex System to Designate Linear Density of Fibers, Yarn Intermediates, and Yarns
• Effective Date: 01-Jul-2018
• Refers: ASTM D3631-99(2017) - Standard Test Methods for Measuring Surface Atmospheric Pressure
• Effective Date: 01-Mar-2017
• Refers: ASTM D1356-15a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Oct-2015
• Refers: ASTM D1356-15 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Jul-2015
• Refers: ASTM D1356-14b - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Dec-2014
• Refers: ASTM D1356-14a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-May-2014
• Refers: ASTM D1356-14 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Jan-2014
• Refers: ASTM D861-07(2013) - Standard Practice for Use of the Tex System to Designate Linear Density of Fibers, Yarn Intermediates, and Yarns
• Effective Date: 01-Jul-2013
• Refers: ASTM E1-13 - Standard Specification for ASTM Liquid-in-Glass Thermometers
• Effective Date: 01-May-2013
• Refers: ASTM D4230-02(2012) - Standard Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer
• Effective Date: 01-Apr-2012
• Refers: ASTM D1357-95(2011) - Standard Practice for Planning the Sampling of the Ambient Atmosphere
• Effective Date: 01-Oct-2011

Overview

ASTM E337-15(2023), Standard Test Method for Measuring Humidity with a Psychrometer, provides a definitive approach to determining atmospheric humidity using wet- and dry-bulb temperature readings. Published by ASTM International, this standard sets out essential guidelines for assembling and using psychrometers, ensuring reliable humidity measurements in a range of meteorological and laboratory settings. The standard specifies only the fundamental elements of psychrometers, focusing on accuracy, practical assembly, and procedural clarity.

Key Topics

• Measurement Methods: Covers two main psychrometer types:
  • Aspirated Psychrometer (Method A): Utilizes forced ventilation for higher precision and is applicable for a broader temperature range (5 °C to 80 °C). This method allows for easier shielding from external radiation and can accommodate various thermometer types, including electrical sensors.
  • Sling Psychrometer (Method B): Employs manual whirling to achieve required airflow over thermometer bulbs, suitable for use from 5 °C to 50 °C.
• Humidity Determination: The core technique involves comparing wet- and dry-bulb temperature readings to derive relative humidity using established charts or calculation methods.
• Applicability: The methods are suitable for meteorological field use, standard laboratory atmospheres, and materials testing, and even for cases when only wet-bulb temperature is needed.
• Uncertainty and Calibration: Provides guidance on acceptable measurement uncertainties and best practices for periodic calibration, emphasizing measurement confidence and reliability.
• Safety and Environmental Considerations: Addresses mercury thermometer handling and relevant precautions, alongside guidance to meet local safety and regulatory requirements.

Applications

• Meteorological Measurements: Widely used for weather observation and climate monitoring at the earth’s surface, ensuring accurate assessment of local humidity.
• Materials Testing & Environmental Chambers: Critical for maintaining and verifying standard atmospheric conditions in material testing labs, product development, and quality assurance.
• Industrial & Research Laboratories: Applied in diverse industries that require precise control of environmental parameters for processes sensitive to moisture and temperature.
• Field Surveys & Mobile Operations: The portable nature of sling and aspirated psychrometers makes this test method suitable for outdoor and remote applications.

Related Standards

Several ASTM and international standards complement ASTM E337-15(2023), ensuring harmonization across humidity and temperature measurements:

• ASTM D861 - Practice for Use of the Tex System to Designate Linear Density of Fibers, Yarn Intermediates, and Yarns.
• ASTM D1193 - Specification for Reagent Water, ensuring high-purity water for wet-bulb operation.
• ASTM D1356 - Terminology Relating to Sampling and Analysis of Atmospheres.
• ASTM D3631 - Test Methods for Measuring Surface Atmospheric Pressure.
• ASTM D4230 - Test Method for Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer.
• ASTM E1 and E2251 - Specifications for liquid-in-glass thermometers and thermometers with low-hazard precision liquids.
• IEEE/ASTM SI-10 - Practice for Use of the International System of Units (SI).

Practical Value

Adopting ASTM E337-15(2023) ensures:

• Consistent, accurate humidity data vital for process control, quality assurance, and meteorological research.
• Reliability across applications by specifying methods for calibration, maintenance, and use, thus reducing human error and ensuring legal and regulatory compliance.
• International compatibility with well-established measurement principles and referenced standards, supporting global practices in atmospheric humidity monitoring.

By following ASTM E337-15(2023), organizations can confidently measure and report humidity with traceable, repeatable procedures essential for scientific integrity, industry standards, and safety compliance.

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 [e-mail]. 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 the wet bulb, the
a duct for the airflow. A second shield, outside, is not necessary.
covering may be secured with a cotton thread at the end of the
7.5.3 With axial ventilation, concentric radiation shields
covering on the stem of the thermometer, at the top of the
shall be provided for the wet and dry bulbs, and shall be
thermometer bulb, and at the bottom of the bulb. However,
polished inside and out. (The shield around the wet bulb plays
whenever a wicking is used, the covering shall not be secured
a vital role in reducing the radiative heat transfer between that
between the thermometer bulb and the cotton wicking which
bulb and its surroundings by a factor of about three.) The
connects the covering to a water reservoir.
diameter of the shield shall be not less than 1.8 d and not
7.2.4 After fabrication, the covering and wick shall have
greater than 2.5 d, where d is the overall diameter of the wet
been washed in a dilute solution of sodium carbonate and
bulb. Its length and position shall be such that its projection
thoroughly rinsed with distilled water. They shall not subse-
beyond each end of the wet covering is not less than d and not
quently be touched with the fingers.
greater than 3 d. The entrance to the shield shall be liberally
7.2.5 The stem of each thermometer shall, for a length
flared to form a bell-mouth to prevent the flow separating from
measured from the sensor and not less than 1.5× the length of
the shield on the inside. The shield may serve also as the duct
the extension of the covering required by 7.2.2, be clear of
for the airflow. A second shield, outside, is not necessary.
obstructions and freely exposed to the airstream.
8. Precautions
7.2.6 During the test, the covering shall be completely
permeated with water as evidenced by a glistening appearance 8.1 Safety Precautions—Mercury vapor is poisonous, even
in a beam of light. in small quantities, and prolonged exposure can produce
E337 − 15 (2023)
serious physical impairment (1). If a mercury thermometer is series of measurements, deliver an excess of clean water
accidentally broken, carefully collect, place, and seal all of the directly to the wet-bulb covering from the reduced tip of a
mercury in a strongly made nonmetallic container. Avoid skin clean glass or plastic tube which is a part of a small wash
contact with mercury. bottle, for example, squeeze bottle, so that drops of water fall
from the covering. This flooding procedure will help to remove
8.2 Technical Precautions—For reliable measurement and
organic contamination which may be on the surface of the
control, strict adherence to the exacting technique is necessary.
wet-bulb covering.) Wetting should be repeated before each
Aside from the obvious mistake of not using a psychrometric
separate measurement. A small bottle or porous porcelain cup
chart or table prepared for the existing barometric pressure,
will serve as a convenient water container. If new or dry
most errors of psychrometry tend to restrict lowering of the
through disuse, several minutes may be required for complete
wet-bulb temperature and thus indicate a higher relative
saturation of the fabric. Avoid touching the fabric with the
humidity than actually exists.
fingers, which may deposit oil or dirt. Replace soiled covering
8.2.1 Conditions which Contribute to High Wet-Bulb Tem-
and wick. Maintain the dry bulb absolutely dry.
perature:
8.2.1.1 Improper installation of wet-bulb covering (loose 10.3 Aspirating the Psychrometer—Operate the fan motor,
fitting, too short, or too long). thus exposing the thermometers to the action of the air until the
8.2.1.2 Dirty or contaminated covering. minimum wet-bulb temperature is indicated. Continue operat-
8.2.1.3 Contamination of wetting water. ing the fan until the readings of the thermometers become
8.2.1.4 Insufficient air flow. constant.
8.2.1.5 Failure to reach or read the minimum point of the 10.3.1 Reading Psychrometer—While operating the fan,
wet-bath depression. read the thermometers quickly but carefully. Read the wet bulb
8.2.1.6 Moisture or heat generation, or both, from the first. Under ordinary conditions, an approximate 0.15 °C error
operator taking readings and from the wet-bulb water reservoir. in wet-bulb depression results in a 1 % error in relative
8.2.1.7 Radiant heating effects. humidity. While the thermometers are being read, keep all
8.2.2 Heat from the fan or motor shall not affect the surfaces that are at temperatures other than the environment
thermometer readings. (such as the hands, face, and either warmer or colder objects)
8.2.3 Instrument shall be used in the shade and not exposed as far as possible from the thermometer bulbs.
to direct sunlight. 10.3.2 For measurements in nominally constant conditions,
8.2.4 Prior to a measurement, the instrument shall have been for example, where fluctuation period is long compared with
exposed long enough to the test atmosphere to have attained the measurement time, repeat steps 10.3 and 10.3.1, rewetting
the ambient temperature. the covering if necessary, until in three successive readings the
8.2.5 The shield shall not be allowed to become wetted. greatest temperature depression differs from the least by not
more than 0.2 °C for an uncertainty of 63 % RH or not more
9. Calibration
than 0.1 °C for an uncertainty of 62 % RH.
10.3.3 Where measurements are being made under condi-
9.1 The thermometers used in a psychrometer should be
tions fluctuating rapidly, take a number of readings over at least
compared once a year at four or more temperatures with the
two complete cycles.
covering removed from the wet-bulb thermometer. Once every
10.3.4 Where measurements are being made while condi-
three months, the thermometers should be compared, with the
tions are changing or are being changed under control, the
covering removed from the wet-bulb thermometer, at the
readings might not be meaningful.
ambient dry-bulb temperature. The readings shall conform to
the requirements (see 7.1.1 and Section 12) when the instru-
10.4 Check Readings—For purposes of checking, make as
ments are totally immersed. For highest accuracy, the ther-
many readings as necessary until three successive readings
mometers should be calibrated over their range of use while
agree. If atmospheric conditions are fluctuating, it may be
totally immersed. The corrections thus determined should be
desirable to obtain several readings in order to secure an
applied to the readings when making a measurement.
average (that is, if there is a definite cycling in conditions, then
readings should be continued for at least two cycles). It will be
10. Procedure
necessary to rewet the covering of the wet bulb when the fabric
starts to dry, as indicated by a rising wet-bulb temperature.
10.1 Location—Avoid locations where proximity to
machinery, direct heat from the sun, or other sources of
11. Calculation
radiation would have undue influence. Stand preferably facing
the air current so that the instrument receives the air before the 11.1 Subtract the wet-bulb reading from the dry-bulb read-
air has passed near you. The site or location is selected so that
ing (T - T ). The difference is the wet-bulb depression.
w
the air is a representative sample. (Also see Practice D1357.) Knowing the dry-bulb temperature and the wet-bulb
depression, the relative humidity could be calculated by using
10.2 Preparing Psychrometer—Moisten the covering of the
the basic psychrometric equation shown in the following
wet bulb thoroughly with distilled water. (Before the start of a
section. In practice, calculations directly involving the basic
equation are seldom needed. Instead, tables, charts, curves, and
other calculating devices developed from the basic equation are
The boldface numbers in parentheses refer to the list of references at the end of
this test method. used, such as the table in Appendix X3.
E337 − 15 (2023)
11.2 When calculating relative humidity from the psychro-
e = the saturation vapor pressure of water at the dry-bulb
s
metric equation, use the following equation or one that for the
temperature, see Appendix X2.
prevailing conditions is equivalent:
12. Precision and Bias
e 5 e T 2 AP T 2 T (1)
~ ! ~ !
w w w
12.1 The uncertainty in the derived relative humidity is
where:
estimated not to exceed the values shown in Table 1 if the
e = the partial pressure of water vapor in the
temperature depression and the dry-bulb temperature measure-
atmosphere, Pa,
ment do not exceed the uncertainty values shown in Table 1.
e (T ) = the saturation pressure of water vapor at the
w w
wet-bulb temperature t , Pa, see Appendix X2,
METHOD B—PSYCHROMETER VENTILATED BY
w
T = the dry-bulb temperature in °C,
WHIRLING (SLING PSYCHROMETER)
T = the wet-bulb temperature in °C,
w
P = the total (atmospheric) pressure, Pa, see Test
13. Interferences
Methods D3631,
13.1 (See 6.2 and 6.3.)
−1
A = the psychrometer coefficient in K , and
14. Apparatus
where e, e (T ) and P are expressed in the same units, see
w w
Practice IEEE/ASTM SI-10.
14.1 Thermometers for Sling Psychrometers:
11.2.1 The value of A shall be chosen in the range
14.1.1 The thermometers shall comply with Specifications
−4 −4 −1
6.2 × 10 to 6.9 × 10 K . The psychrometer coefficient E1 or E2251.
−4
developed by Ferrel, A = 6.6 × 10 (1 + 0.00115 T ), falls
14.1.2 The range of the thermometers shall not exceed the
w
within this range. If a value of A has been determined for the range 0 °C to 50 °C; however, this range may be achieved by
particular design of psychrometer and lies in this range, then it providing more than a single pair of matched thermometers.
shall be used. If a value has been determined, but lies outside When the uncertainty in the derived relative humidity is
this range, then the closer extreme value of the range shall be required to be not more than 65 % RH, the thermometers shall
used. If no value of A has been determined, then use the value be such that their readings give the temperature depression
developed by Ferrel. (It may be noted that if, for example, at with an uncertainty of not more than 60.3 °C and the uncer-
20 °C and standard atmospheric pressure use of the value tainty in the reading of the dry-bulb temperature shall be not
−4 −1
more than 60.6 °C. When the uncertainty in the derived
6.5 × 10 K led to a derived relative humidity of 50.0 %,
−4 −1
relative humidity is required to be not more than 63 % RH, the
then use of the value 6.9 × 10 K would lead to a derived
thermometers shall be such that their readings given the
relative humidity of 48.9 %.)
temperature depression and the dry-bulb temperature with an
11.3 Saturation Vapor Pressure—Saturation vapor pressure
uncertainty of not more than 60.2 °C.
of pure water vapor over a plane surface of water, ew in hPa at
14.1.3 The diameters of the thermometer bulbs (excluding
the wet-bulb temperature, T in Celsius can be calculated by
w
wet covering) shall be not greater than 4 mm.
this exponential expression (2):
14.1.4 (See 7.1.7.)
@17.625T /~243.041T !#
w w
e ~T ! 5 6.1094·e (2)
w w
14.2 Wet-Bulb Covering:
14.2.1 (See 7.2.1.)
The saturation vapor pressure (e ) for water in the tempera-
s
14.2.2 The covering shall completely cover the sensor or
ture range between 0 °C and 100 °C can also be calculated by
bulb of the thermometer, fit snugly but not very tightly, and
the following polynomial expression (3, 4):
shall be in physical contact with the bulb over its entire surface.
25 5 24 4
e 5 2.70102980826 × 10 T 12.92123923916 x 10 T
s
It shall extend onto the stem for such a distance that the error
22 3 2
12.53760036868 × 10 T 11.48376504190 T
in the observed wet-bulb temperature due to heat conduction
1 2
14.37196700302 × 10 T16.13141885322 × 10 (3) along the stem does not exceed 0.05 °C. For mercury-in-glass
thermometers with solid stems, a distance of twice the stem
11.4 Relative Humidity—The psychrometric equation gives
diameter is usually adequate.
the partial pressure of the water vapor. In the meterological
14.2.3 To maintain a snugly fit cover on the wet bulb, the
range of pressure and temperature, the saturation vapor pres-
covering may be secured with a cotton thread at the end of the
sure of the pure water phase and of the moist air will be
assumed to be equal. (Water vapor and air mixture is assumed
to behave as ideal gas.) This assumption will introduce an error
TABLE 1
of approximately 0.5 % or less in the calculated partial pressure
Uncertainty in Uncertainty in Uncertainty in
of water vapor and an error of less than 0.5 % in the calculated
Derived Relative Temperature Dry-bulb
Humidity, %, RH Depression, °C Temperature, °C
relative humidity. (See Test Method D4230.) If water vapor
and air are assumed to behave as ideal gases, then ±4 ±0.3 ±0.2
±3 ±0.2 ±0.2
RH 5 e/e ·100 % (4)
±2 ±0.1 ±0.2
s
±5 ±0.3 ±0.6
where:
±4 ±0.2 ±0.6
±3 ±0.1 ±0.6
e = the partial pressure of the water vapor, and
E337 − 15 (2023)
covering on the stem of the thermometer, at the top of the 17.3 Ventilating the Psychrometer—Holding the instrument
thermometer bulb, and at the bottom of the bulb. well away from the body, and for outdoor measurements to
14.2.4 (See 7.2.4.) windward and in the shade, whirl it at such a rate as to achieve
14.2.5 The stem of each thermometer shall, for a length the specified airspeed at the wet and dry bulbs (see 14.4) and
measured from the sensor and not less than 1.5 × the length of then stop the motion after 30 s to read the thermometers.
the extension of the covering required by 14.2.2, be clear of Resume whirling for an additional 10 s before stopping the
obstructions and freely exposed to the airstream. motion to read the thermometers. Continue this procedure of
14.2.6 (See 7.2.6 and 7.2.7.) whirling for 10 s, rewetting the covering if necessary (see
10.4), until a minimum wet-bulb temperature has been at-
14.3 Water—(See 7.3.)
tained. (About 2-min ventilation time is usually necessary.)
14.4 Airflow:
17.3.1 Reading Psychrometer—After whirling the
14.4.1 The psychrometer shall be whirled so that the flow of
psychrometer, read the thermometers quickly but carefully.
air over both the wet and dry bulbs is equivalent to 3 m ⁄s to
Read the wet bulb first. Under ordinary conditions, an approxi-
10 m ⁄s for thermometers with maximum allowable diameter of
mate 0.3 °C error in wet-bulb depression results in a 2 % error
the sensors.
in relative humidity. While the thermometers are being read,
14.4.2 The sample air shall not pass over any obstruction
keep all surfaces that are at temperatures other than the
before it passes over the wet and dry bulbs.
environment (such as the hands, face, and either warmer or
14.4.3 (See 7.4.4.)
colder objects) as far as possible from the thermometer bulbs.
14.5 Radiation Shields—Radiation shields are not neces-
17.3.2 For measurements in nominally constant conditions,
sary.
for example, where a fluctuation period is long compared with
the measurement time, repeat Steps 17.3 and 17.3.1, rewetting
15. Precautions
the covering if necessary, until in three successive readings the
15.1 Safety Precautions:
greatest temperature depression differs from the least by not
15.1.1 Mercury vapor is poisonous, even in small quantities,
more than 0.3 °C for an uncertainty of 65 % RH.
and prolonged exposure can produce serious physical impair-
17.3.3 (See 10.3.3 and 10.3.4.)
ment (1), see 8.1.
17.4 Check Readings—(See 10.4.)
15.1.2 Before using a sling psychrometer, check for ad-
equate clearance to freely sling or whirl the thermometers
18. Calculation
without hitting any solid surfaces; for example, the knee.
15.1.3 If a mercury thermometer is accidentally broken,
18.1 (See Section 11.)
follow the handling procedure in 8.1.
19. Precision and Bias
15.2 Technical Precautions—(See 8.2.)
15.2.1 Conditions which Contribute to High Wet-Bulb
19.1 The uncertainty in the derived relative humidity is
Temperature—(See 8.2.1.1 – 8.2.1.7.)
estimated not to exceed the values shown in Table 2 if the
15.2.2 (See 8.2.3 and 8.2.4.)
temperature depression and the reading of the dry-bulb tem-
perature do not exceed the uncertainty values shown in Table 2.
16. Calibration
16.1 The thermometers used in a psychrometer should be
20. Keywords
compared once a year at four or more temperatures with the
20.1 aspiration; humidity; psychrometer; psychrometric
covering removed from the wet-bulb thermometer. Once every
table; temperature; vapor pressure; ventilation; wet-bulb tem-
three months, the thermometers should be compared, with the
perature
covering removed from the wet-bulb thermometer, at the
ambient dry-bulb temperature. The readings shall conform to
the requirements, (see 14.1.2 and Section 19) when the
instruments are totally immersed. For highest accuracy, the
TABLE 2
thermometers should be calibrated over their range of use
Uncertainty in Uncertainty in Uncertainty in
while totally immersed. The corrections thus determined
Derived Relative Temperature Dry-bulb
should be applied to the readings when making a measurement.
Humidity, %, RH Depression, °C Temperature, °C
±4 ±0.3 ±0.2
17. Procedure
±3 ±0.2 ±0.2
±5 ±0.3 ±0.6
17.1 Location—(See 10.1.)
±4 ±0.2 ±0.6
17.2 Preparing Psychrometer—(See 10.2.)
E337 − 15 (2023)
ANNEXES
(Mandatory Information)
A1. MERCURY-IN-GLASS THERMOMETERS SUITABLE WHEN THE UNCERTAINTY IN THE
MEASURED RELATIVE HUMIDITY IS REQUIRED NOT TO EXCEED 63 % RH
A1.1 Mercury-in-glass thermometers conforming to the fol- A1.1.7 Dimensions:
lowing specification are suitable when the uncertainty in the
mm
Length from bottom of bulb to underside of button or ring 240
measured relative humidity is required not to exceed 63 %
top (maximum)
RH.
Scale length corresponding to the nominal range 130
(minimum)
A
A1.1.1 Type—The thermometers shall be of the solid-stem
Bulb length 10 to 30
Bulb diameter 3 to 4
type, and the stem may have a slight neck near the bulb to
Stem diameter 4 to 5
allow the wet-bulb covering to be secured more easily by a
A
Distance of neck (if any) from top of bulb 8 to 12
A
cotton thread. Distance of lowest scale line from top of bulb (min) 30
Distance of expansion chamber from highest scale line 10
A1.1.2 Temperature Scale—The thermometers shall be (min)
graduated for total immersion and in accordance with the
A
The top of the entrance flare of the capillary is taken to be the top of the bulb.
Celsius scale which corresponds with the International Practi-
A1.1.8 Graduation and Figuring—The thermometers shall
cal Temperature Scale of 1968.
be graduated at each 0.5 °C, with a spacing of approximately
A1.1.3 Range—The nominal temperature range of the ther-
2 mm and with a longer line at each 1 °C. The graduations shall
mometers shall be 0 °C to 80 °C for an aspirated psychrometer
be numbered at each 5 °C.
and 0 °C to 50 °C for a sling psychrometer.
A1.1.9 Accuracy—Readings of each thermometer made by
a knowledgeable and experienced observer with the thermom-
A1.1.4 Materials—The stem shall be made of suitable
eters totally immersed shall not be in error by more than 0.2 °C
thermometer glass with an enamel back. The bulb shall be
for any temperature in the nominal range. For any two
made of glass meeting the Specification E1.
temperatures in the nominal range, readings of the two
A1.1.5 Annealing and Stabilization—The glass shall be
thermometers, so made, shall give the difference of the
suitably annealed, and the thermometers shall be stabilized by
temperatures with an error not exceeding 0.2 °C.
a suitable heat treatment before they are filled with mercury.
A1.1.10 Spare Thermometer—If a third thermometer is
associated with the psychrometer, then A1.1.9 shall apply to
A1.1.6 Expansion Chamber—Each thermometer shall in-
each of the three possible combinations of two thermometers.
clude an expansion chamber above the highest scale line so that
a temperature of at least 100 °C can be sustained without the A1.1.11 Thermometers with Low-Hazard Precision
Liquids—Thermometers meeting the requirements of Specifi-
likelihood of damage.
cation E2251 may be used if performance equivalent to
mercury-in-glass thermometers is demonstrated.
A2. DETERMINATION OF THE DISTANCE FOR WHICH THE WET-BULB COVERING MUST EXTEND ONTO THE THER-
MOMETER STEM TO LIMIT THE HEAT-CONDUCTION ERROR TO 0.05 °C
A2.1 Temporarily fit the bulbs (sensors) of both thermom- A2.3 Progressively reduce the extension of one of the
eters with coverings similar to that to be used on the wet bulb,
coverings onto the stem until the difference of the readings of
but allow the coverings to extend onto the stems considerably
the thermometers is estimated to have changed by 0.05 °C. The
further than usual, say 1.5 ×, the usual distance.
extension existing at that stage is the minimum permissible.
A2.2 Operate the instrument in the usual manner but with
A2.4 A more accurate determination can be made if the
both coverings wet, choosing a location where the conditions
difference of the readings is plotted against the extension for a
are steady. Observe the difference of the thermometer readings
number of extensions both greater than and less than that
as accurately as possible. (This difference is due mainly to the
errors of the thermometers themselves.)
E337 − 15 (2023)
corresponding to a change of 0.05 °C. The minimum extension under conditions which differ substantially from those under
which corresponds to a change of this amount may then easily which the psychrometer is normally used, then allowance
be read from the plot.
should be made for the fact that for a given extension of the
covering the temperature error due to the heat conduction is
A2.5 During the procedure, as in normal operation of the
roughly proportional to the temperature depression. For
psychrometer, care must be taken to preserve the cleanliness of
example, if the present procedure is carried out under atmo-
the coverings, and in particular to avoid touching them with the
spheric conditions such that the temperature depression is
fingers.
twice the value which occurs in the normal use of the
instrument, then the extension which results in a change of
A2.6 The procedure determines the extension necessary for
the conditions which prevail at the time. If it is carried out 0.1 °C in the temperature difference is the required minimum.
APPENDIXES
(Nonmandatory Information)
X1. SKELETON TABLE OF RELATIVE HUMIDITIES
TABLE X1.1 Comparison of the Calculated Relative Humidity Using Three Different Values of the Psychrometer Coefficient A
Dry-Bulb Temperature in °C Dry-Bulb Temperature in °C
t − t 10 20 30 40 50 60 70 80 t − t 10 20 30 40 50 60 70 80
w w
100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 . . . 8.5 19.0 26.0 31.0 35.5
0.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 22.0 . . . 7.5 18.5 26.0 31.0 35.0
100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 . . . 7.0 18.0 26.5 31.0 35.0
76.5 82.5 86.0 88.0 89.5 90.5 91.0 92.0 . . . 3.0 14.5 22.0 27.5 31.5
2.0 76.5 82.5 86.0 88.0 89.5 90.5 91.0 92.0 24.0 . . . 2.5 14.0 21.5 27.0 31.5
76.0 82.5 86.0 88.0 89.5 90.5 91.0 92.0 . . . 2.0 13.5 21.5 27.0 31.5
54.5 66.5 73.0 77.0 79.5 81.5 83.0 84.5 . . . . 10.5 18.0 23.5 28.0
4.0 54.0 66.0 73.0 77.0 79.5 81.5 83.0 84.5 26.0 . . . . 10.0 18.0 23.5 28.0
53.5 66.0 72.5 77.0 79.5 81.5 83.0 84.5 . . . . 9.5 17.5 23.5 28.0
34.0 51.5 61.0 67.0 70.5 73.5 75.5 77.0 . . . . 6.5 14.5 20.5 25.0
6.0 33.0 51.0 60.5 66.5 70.5 73.5 75.5 77.0 28.0 . . . . 6.0 14.5 20.0 24.5
32.0 50.5 60.5 66.5 70.5 73.0 75.5 77.0 . . . . 5.5 14.0 20.0 24.5
14.5 37.5 50.0 57.5 62.0 65.5 63.5 70.5 . . . . 3.0 11.5 17.5 22.0
8.0 13.5 36.5 49.5 57.0 62.0 65.5 63.5 70.5 30.0 . . . . 2.5 11.0 17.0 22.0
12.0 36.0 49.0 57.0 62.0 65.5 63.5 70.5 . . . . 2.0 11.0 17.0 21.5
... 24.5 39.5 48.5 54.5 58.5 62.0 64.5 ... ... ... ... ... 8.5 14.5 19.0
10.0 . 23.5 39.0 48.5 54.5 58.5 62.0 64.5 32.0 . . . . . 8.0 14.5 19.0
... 22.5 38.5 48.0 54.0 58.5 61.5 64.5 ... ... ... ... ... 7.5 14.0 19.0
... 12.0 30.0 40.5 47.5 52.0 55.5 58.5 ... ... ... ... ... 5.5 12.0 16.5
12.0 . 11.0 29.5 40.0 47.0 52.0 55.5 58.5 34.0 . . . . . 5.5 11.5 16.5
... 10.0 29.0 40.0 47.0 52.0 55.5 58.5 ... ... ... ... ... 5.0 11.5 16.5
... 0.5 21.0 33.0 40.5 46.0 50.0 53.5 ... ... ... ... ... 3.0 9.5 14.0
14.0 . . 20.5 32.5 40.5 46.0 50.0 53.0 36.0 . . . . . 2.5 9.0 14.0
... ... 20.0 32.5 40.0 45.5 50.0 53.0 ... ... ... ... ... 2.5 9.0 14.0
... ... 13.0 26.0 34.5 40.5 45.0 48.5 ... ... ... ... ... 0.5 7.0 12.0
16.0 . . 12.0 25.5 34.5 40.0 44.5 48.0 38.0 . . . . . 0.5 7.0 12.0
... ... 11.5 25.5 34.0 40.0 44.5 48.0 ... ... ... ... ... ... 6.5 11.5
... ... 5.0 20.0 29.0 35.0 40.0 43.5 ... ... ... ... ... ... 5.0 10.0
18.0 . . 4.5 19.5 28.5 35.0 40.0 43.5 40.0 . . . . . . 5.0 10.0
... ... 3.5 19.0 28.5 35.0 39.5 43.5 ... ... ... ... ... ... 4.5 9.5
... ... ... 14.0 23.5 30.5 35.5 39.5
20.0 . . . 13.5 23.5 30.0 35.0 39.0
... ... ... 12.5 23.0 30.0 35.0 39.0
X1.1 Relative humidities rounded to the nearest 0.5 % RH the psychrometer coefficient. Table X1.1 is given so that other
are tabulated for various temperatures and temperature depres- more detailed tables may be compared with it. The dry-bulb
sions and for standard atmospheric pressure and three values of temperature interval of 10 °C and the temperature depression
E337 − 15 (2023)
interval of 2 °C are too wide to allow the table to be used for
Values
−4 −1
Upper A = 6.5 × 10 K
routine humidity measurement. The saturation vapor pressure
−4
Intermediate A = 6.7 × 10
of water has been taken from A. Wexler, Appendix X2.
−4
Lower A = 6.9 × 10
Standard atmospheric pressure is 1.01325 × 10 Pa.
X2. SATURATION VAPOR PRESSURE OVER WATER
X2.1 The saturation vapor pressure of the pure phase over where:
plane surface of pure water for temperatures 0 °C to 100 °C
g = −0.63536311 × 10 ,
was obtained from Wexler’s 1976 formulation (5). Other
g = 0.3404926034 × 10 ,
−1
suitable saturation vapor pressure tables are given in the g = −0.19509874 × 10 ,
−4
Smithsonian Meteorological Tables (6), International Meteoro- g = 0.12811805 × 10 ,
e = in Pascal, and
logical Tables (7), and ASHRAE Handbook and Product
s
T = 273.15 + t , and
Directory (8). The following simplified equation (5) yields
68 68
t = −degree Celsius (International Practical Temperature
values of the saturation vapor pressure over water which differ 68
Scale of 1968).
from those given in Table X2.1 by 20 ppm or less:
i22
lne 5 g ~T ! (X2.1)
s ( i 68
i51
A
TABLE X2.1 Saturation Vapor Pressure Over Water (IPTS—68)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Temp
°C
Pa Pa Pa Pa Pa Pa Pa Pa Pa Pa
0 611.213 615.667 620.150 624.662 629.203 633.774 638.373 643.003 647.662 652.350
1 657.069 661.819 666.598 671.408 676.249 681.121 686.024 690.958 695.923 700.920
2 705.949 711.010 716.103 721.228 726.386 731.576 736.799 742.055 747.344 752.667
3 758.023 763.412 768.836 774.294 779.786 785.312 790.873 796.469 802.100 807.766
4 813.467 819.204 824.977 830.786 836.631 842.512 848.429 854.384 860.375 866.403
5 872.469 878.572 884.713 890.892 897.109 903.364 909.658 915.991 922.362 928.773
6 935.223 941.712 948.241 954.810 961.419 968.069 974.759 981.490 988.262 995.075
7 1001.93 1008.83 1015.76 1022.74 1029.77 1036.83 1043.94 1051.09 1058.29 1065.52
8 1072.80 1080.13 1087.50 1094.91 1102.37 1109.87 1117.42 1125.01 1132.65 1140.33
9 1148.06 1155.84 1163.66 1171.53 1179.45 1187.41 1195.42 1203.48 1211.58 1219.74
10 1227.94 1236.19 1244.49 1252.84 1261.24 1269.68 1278.18 1286.73 1295.33 1303.97
11 1312.67 1321.42 1330.22 1339.08 1347.98 1356.94 1365.95 1375.01 1384.12 1393.29
12 1402.51 1411.79 1421.11 1430.50 1439.93 1449.43 1458.97 1468.58 1478.23 1487.95
13 1497.72 1507.54 1517.43 1527.36 1537.36 1547.42 1557.53 1567.70 1577.93 1588.21
14 1598.56 1608.96 1619.43 1629.95 1640.54 1651.18 1661.89 1672.65 1683.48 1694.37
15 1705.32 1716.33 1727.41 1738.54 1749.75 1761.01 1772.34 1783.73 1795.18 1806.70
16 1818.29 1829.94 1841.66 1853.44 1865.29 1877.20 1889.18 1901.23 1913.34 1925.53
17 1937.78 1950.10 1962.48 1974.94 1987.47 2000.06 2012.73 2025.46 2038.27 2051.14
18 2064.09 2077.11 2090.20 2103.37 2116.61 2129.92 2143.30 2156.75 2170.29 2183.89
19 2197.57 2211.32 2225.15 2239.06 2253.04 2267.10 2281.23 2295.44 2309.73 2324.10
20 2338.54 2353.07 2367.67 2382.35 2397.11 2411.95 2426.88 2441.88 2456.94 2472.13
21 2487.37 2502.70 2518.11 2533.61 2549.18 2564.85 2580.59 2596.42 2612.33 2628.33
22 2644.42 2660.59 2676.85 2693.19 2709.62 2726.14 2742.75 2759.45 2776.23 2793.10
23 2810.06 2827.12 2844.26 2861.49 2878.82 2896.23 2913.74 2931.34 2949.04 2966.82
24 2984.70 3002.68 3020.74 3038.91 3057.17 3075.52 3093.97 3112.52 3131.16 3149.90
25 3168.74 3187.68 3206.71 3225.85 3245.08 3264.41 3283.85 3303.38 3323.02 3342.76
26 3362.60 3382.54 3402.59 3422.73 3442.99 3463.34 3483.81 3504.37 3525.05 3545.83
27 3566.71 3857.71 3608.81 3630.02 3651.33 3672.76 3694.29 3715.94 3737.69 3759.56
28 3781.54 3803.63 3825.83 3848.14 3870.57 3893.11 3915.77 3938.54 3961.42 3984.42
29 4007.54 4030.77 4054.12 4077.59 4101.18 4124.88 4148.71 4172.65 4196.71 4220.90
30 4245.20 4269.63 4294.18 4318.85 4343.64 4368.56 4393.60 4418.77 4444.06 4469.48
31 4495.02 4520.69 4546.49 4572.42 4598.47 4624.65 4650.96 4677.41 4703.98 4730.68
32 4757.52 4784.48 4811.58 4838.81 4866.18 4893.68 4921.32 4949.09 4976.99 5005.04
33 5033.22 5061.53 5089.99 5118.58 5147.32 5176.19 5205.20 5234.36 5263.65 5293.09
34 5322.67 5352.39 5382.26 5412.27 5442.43 5472.73 5503.18 5533.78 5564.52 5595.41
35 5626.45 5657.64 5688.97 5720.46 5752.10 5783.89 5815.83 584
...