ASTM E337-02(2007)
(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 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 to 80C, wet-bulb temperatures not lower than 1C, 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 50C, wet-bulb temperatures not lower than 1C 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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Designation: E337 − 02(Reapproved 2007)
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.Anumber 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 Thismethodisapplicablewithintheambienttempera-
turerange5to50°C,wet-bulbtemperaturesnotlowerthan1°C
1.1 General:
and restricted to ambient pressures not differing from standard
1.1.1 This test method covers the determination of the
atmospheric pressure by more than 30%.
humidity of atmospheric air by means of wet- and dry-bulb
1.4 This standard does not purport to address all of the
temperature readings.
safety concerns, if any, associated with its use. It is the
1.1.2 This test method is applicable for meteorological
responsibility of the user of this standard to establish appro-
measurements at the earth’s surface, for the purpose of the
priate safety and health practices and determine the applica-
testing of materials, and for the determination of the relative
bility of regulatory limitations prior to use. (For more specific
humidity of most standard atmospheres and test atmospheres.
safety precautionary statements, see 8.1 and 15.1.)
1.1.3 This test method is also applicable when the tempera-
ture of the wet bulb only is required. In this case, the
2. Referenced Documents
instrument comprises a wet-bulb thermometer only.
2.1 ASTM Standards:
1.1.4 Relative humidity (RH) does not denote a unit. Un-
D861PracticeforUseoftheTexSystemtoDesignateLinear
certainties in the relative humidity are expressed in the form
Density of Fibers, Yarn Intermediates, and Yarns
RH 6 rh%,whichmeansthattherelativehumidityisexpected
D1193Specification for Reagent Water
to lie in the range (RH − rh)% to (RH + rh )%, where RH is
D1356Terminology Relating to Sampling and Analysis of
theobservedrelativehumidity.Alluncertaintiesareatthe95%
Atmospheres
confidence level.
D1357Practice for Planning the Sampling of the Ambient
1.2 Method A—Psychrometer Ventilated by Aspiration:
Atmosphere
1.2.1 This method incorporates the psychrometer ventilated
D3631Test Methods for Measuring Surface Atmospheric
by aspiration. The aspirated psychrometer is more accurate
Pressure
than the sling (whirling) psychrometer (see Method B), and it
D4023Terminology Relating to Humidity Measurements
offers advantages in regard to the space which it requires, the
(Withdrawn 2002)
possibility of using alternative types of thermometers (for
D4230Test Method of Measuring Humidity with Cooled-
example, electrical), easier shielding of thermometer bulbs
Surface Condensation (Dew-Point) Hygrometer
from extraneous radiation, accidental breakage, and conve-
E1Specification for ASTM Liquid-in-Glass Thermometers
nience.
IEEE/ASTM SI-10Practice for Use of the International
1.2.2 Thismethodisapplicablewithintheambienttempera-
System of Units (SI) (the Modernized Metric System)
ture range 5 to 80°C, wet-bulb temperatures not lower than
1°C, and restricted to ambient pressures not differing from
3. Terminology
standard atmospheric pressure by more than 30%.
3.1 Definitions:
1.3 Method B—Psychrometer Ventilated by Whirling (Sling
3.1.1 For definitions of humidity terms used in this test
Psychrometer):
method, refer to Terminology D4023.
1.3.1 This method incorporates the psychrometer ventilated
3.1.2 Fordefinitionsofothertermsinthistestmethod,refer
by whirling (sling psychrometer).
to Terminology D1356.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method is under the jurisdiction of ASTM Committee D22 on Air contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Oct. 1, 2007. Published December 2007. Originally the ASTM website.
approved in 1931. Last previous edition approved in 2002 as E337-02. DOI: The last approved version of this historical standard is referenced on
10.1520/E0337-02R07. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E337 − 02 (2007)
3.2 Definitions of Terms Specific to This Standard: required for accurately measuring the humidity in the atmo-
sphere. Only the essential features of the psychrometer are
3.2.1 Method A—Aspirated Psychrometer:
specified.
3.2.1.1 aspiration—The wet and dry bulbs (and the psy-
chrometer) are described as aspirated because there is provi-
METHOD A—PSYCHROMETER VENTILATED
sionfortheforcedventilationbydrawingairoverthebulbsby
BY ASPIRATION
suction. The flow may be either transverse or parallel to the
axes of the bulbs.
6. Interferences
3.2.1.2 thermometer—for purposes of this standard, and
6.1 When an aspirated psychrometer is used for measure-
exceptwhereaspecifictypeisindicated,thetermthermometer
ments in a small enclosed space and steadily rising wet- and
means any temperature-measuring device.
dry-bulbtemperaturesareobserved,considerwhetherheatand
3.2.1.3 wet-bulb covering and wick—the wet bulb is pro-
moisture liberated by the instrument itself are affecting the
vided with a water-retaining covering of a woven-cotton
conditions.
material.Acottonwickwhichconnectsthecoveringtoawater
reservoir may be provided so that water is fed to the covering 6.2 Whilethethermometersarebeingread,keepallsurfaces
thatareattemperaturesotherthantheenvironment(suchasthe
continuously by capillarity.
hands, face, and other warmer or colder objects) as far as
3.2.2 Method B—Sling Psychrometer:
possible from the thermometer bulbs.
3.2.2.1 ventilation—the wet and dry bulbs (and the psy-
chrometer) are described as ventilated because there is provi-
6.3 This method should not be used where there is heavy
sion for a flow of the air over the bulbs.The flow is transverse
contamination of the air with gases, vapors, or dust.
to the axes of the bulbs.
7. Apparatus
3.2.2.2 wet-bulb covering—the wet bulb is provided with a
water-retaining covering of a woven-cotton material.
7.1 Thermometers for an Aspirated Psychrometer:
7.1.1 The range of the thermometers shall not exceed the
4. Summary of Methods
range 0 to 80°C. This range may be achieved by providing
more than a single pair of matched thermometers. When the
4.1 General:
uncertainty in the derived relative humidity is required to be
4.1.1 The wet-bulb temperature depression, the dry-bulb
not more than 63% RH, the thermometers shall be such that
temperature, and the ambient pressure provide the basis for
their readings give the temperature depression with an uncer-
deriving the relative humidity.
tainty of not more than 60.2°C. When the uncertainty in the
4.2 Method A—Aspirated Psychrometer:
relative humidity is required to be not more than 62% RH,
4.2.1 Establish the airflow (see 7.4) and maintain it until a
they shall be such that their readings give the temperature
minimum wet-bulb temperature is attained. (With mercury-in-
depression with an uncertainty of not more than 60.1°C. The
glass thermometers, about 2-min ventilation time is usually
uncertainty in the reading of the dry-bulb temperature shall be
necessary.)
not more than 60.2°C.
4.2.2 Read the thermometers with the necessary precision,
7.1.2 Electrical thermometers may be so connected that the
obtaining the dry-bulb temperature with an overall uncertainty
readings give the temperature depression and the dry-bulb
of 60.2°C or better, and the temperature depression with an
temperature directly.
overalluncertaintyof 60.2°Corbetterforanuncertaintyinthe
7.1.3 Each thermometer shall consist of a temperature
relative humidity of 63% RH. For an uncertainty in the
sensor of essentially cylindrical shape which is supported on a
relativehumidityof 62%RH,obtainthedry-bulbtemperature
single stem, the stem being coaxial with the sensor. The free
with an overall uncertainty of 60.2°C or better and the
end of each sensor shall be smoothly rounded. If the diameter
temperature depression with an overall uncertainty of 60.1°C
of the stems is small, compared with that of the sensors, then
or better. (Also see Section 12.)
bothendsofeachsensorshallbesmoothlyrounded.Thesensor
of a mercury-in-glass thermometer shall be that part of the
4.3 Method B—Sling Psychrometer:
thermometer extending from the bottom of the bulb to the top
4.3.1 Holding the instrument well away from the body, and
of the entrance flare of the capillary.
foroutdoormeasurementstowindwardandintheshade,whirl
7.1.4 With transverse ventilation, the diameters of the sen-
it at such a rate as to achieve the specified airspeed at the wet
sors (excluding wet covering) shall be not less than 1 mm and
and dry bulbs, see 14.4.
not greater than 4 mm.
4.3.2 Read the thermometers with the necessary precision,
7.1.5 With axial ventilation, the diameters of the sensors
obtaining the dry-bulb temperature with an overall uncertainty
(excluding wet covering) shall be not less than 2 mm and not
of 60.6°C or better, and the temperature depression with an
greater than 5 mm, and their length not less than 10 mm and
overalluncertaintyof 60.3°Corbetterforanuncertaintyinthe
not greater than 30 mm.
relative humidity of 65% RH, also see Section 19.
7.1.6 The connecting wires of electrical thermometers shall
be contained within the supporting stems and shall be isolated
5. Significance and Use
from the moisture of the wet covering.
5.1 The object of this test method is to provide guidelines 7.1.7 Mercury-in-glassshallbegraduatedto0.5°Corcloser
for the construction of a psychrometer and the techniques and be capable of being read to the nearest 0.1°C or better. (A
E337 − 02 (2007)
specification for mercury-in-glass thermometers suitable when 7.4.2 The sample air shall not pass over any obstruction or
the uncertainty in the derived relative humidity is required to through a fan before it passes over the wet and dry bulbs.
be not more than 63% RH is given in Annex A1.) 7.4.3 Withaxialflow,thedirectionoftheflowshallbefrom
the free end of each sensor towards the support end.
7.2 Wet-Bulb Covering, Wick, and Water Reservoir:
7.4.4 Noairwhichhasbeencooledbythewetbulborbythe
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 to 1.2 g/m, refer to Practice
7.5 Radiation Shields:
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 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
shallbeinphysicalcontactwiththebulboveritsentiresurface.
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 deter-
outside surfaces.The clearance between the wet and dry bulbs
mining the distance for which the covering must extend onto
and the shields shall be not less than half the overall diameter
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.)
theinside(vena-contractaeffect).Theshieldsmayformpartof
7.2.3 To maintain a snugly fit cover on the wet bulb, the
aductfortheairflow.Asecondshield,outside,isnotnecessary.
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
flaredtoformabell-mouthtopreventtheflowseparatingfrom
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.
7.2.6 During the test, the covering shall be completely
8. Precautions
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
7.2.7 The covering shall be washed in situ with distilled
serious physical impairment (1). If a mercury thermometer is
water from time to time and be renewed when it shows any
accidentally broken, carefully collect, place, and seal all of the
evidence of permanent change.
mercury in a strongly made nonmetallic container.Avoid skin
7.2.8 When a wick is provided, the free length of a wick
contact with mercury.
shall be at least twice the diameter of the wet bulb and at least
three times the wick diameter, ensuring that water arriving at
8.2 Technical Precautions—For reliabl
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
e1
Designation:E337 –84 (Reapproved 1996) Designation:E337 –02 (Reapproved 2007)
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.Anumber 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
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)(RH) does not denote a unit. Uncertainties in the relative humidity are expressed in the formURH
6 urh % rh, %, which means that the relative humidity is expected to lie in the range (URH − urh)% to (U RH + urh )%, where
U 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 Thismethodincorporatesthepsychrometerventilatedbyaspiration.Theaspiratedpsychrometerismoreaccuratethanthe
sling (whirling) psychrometer (see Method B), and it offers advantages in regard to the space which it requires, the possibility of
usingalternativetypesofthermometers(forexample,electrical),easiershieldingofthermometerbulbsfromextraneousradiation,
accidental breakage, and convenience.
1.2.2 Thismethodisapplicablewithintheambienttemperaturerange5to80°C,wet-bulbtemperaturesnotlowerthan1°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 Thismethodisapplicablewithintheambienttemperaturerange5to50°C,wet-bulbtemperaturesnotlowerthan1°Cand
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.)
2. Referenced Documents
2.1 ASTM Standards:
D861 Practice for Use of the Tex System to Designate Linear Density of Fibers, Yarn Intermediates, and Yarns
D1193 Specification for Reagent Water
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
D1357 Practice for Planning the Sampling of the Ambient Atmosphere
D3631 Test Methods for Measuring Surface Atmospheric Pressure
D4023 Terminology Relating to Humidity Measurements
D4230 Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer
E1 Specification for ASTM Liquid-in-Glass Thermometers
This test method is under the jurisdiction ofASTM Committee D-22 on Sampling andAnalysis ofAtmospheres and is the direct responsibility of Subcommittee D22.11
on Meteorology.
Current edition approved Nov. 30, 1984. Published June 1985. Originally published as E 337 – 31 T. Last previous edition E 337 – 62.
This test method is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved Oct. 1, 2007. Published December 2007. Originally approved in 1931. Last previous edition approved in 2002 as E337-02.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
, Vol 07.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E337 –02 (2007)
EI380EEE/ASTM SI-10 Practice for Use of the International System of Units (SI) (the Modernized Metric System)
3. Terminology
3.1 Definitions:
3.1.1 For definitions of humidity terms used in this test method, refer to Terminology D4023.
3.1.2 For definitions of other terms in this test method, refer to Terminology D1356.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 Method A—Aspirated Psychrometer:
3.2.1.1 aspiration—The wet and dry bulbs (and the psychrometer) are described as aspirated because there is provision for the
forced ventilation by drawing air over the bulbs by suction.The flow may be either transverse or parallel to the axes of the bulbs.
3.2.1.2 thermometer—forpurposesofthisstandard,andexceptwhereaspecifictypeisindicated,thetermthermometermeans
any temperature-measuring device.
3.2.1.3 wet-bulb covering and wick —the wet bulb is provided with a water-retaining covering of a woven-cotton material.A
cotton wick which connects the covering to a water reservoir may be provided so that water is fed to the covering continuously
by capillarity.
3.2.2 Method B—Sling Psychrometer:
3.2.2.1 ventilation— the wet and dry bulbs (and the psychrometer) are described as ventilated because there is provision for a
flow of the air over the bulbs. The flow is transverse to the axes of the bulbs.
3.2.2.2 wet-bulb covering—the wet bulb is provided with a water-retaining covering of a woven-cotton material.
4. Summary of Methods
4.1 General:
4.1.1 The wet-bulb temperature depression, the dry-bulb temperature, and the ambient pressure provide the basis for deriving
the relative humidity.
4.2 Method A—Aspirated Psychrometer:
4.2.1 Establish the airflow (see 7.4) and maintain it until a minimum wet-bulb temperature is attained. (With mercury-in-glass
thermometers, about 2-min ventilation time is usually necessary.)
4.2.2 Read the thermometers with the necessary precision, obtaining the dry-bulb temperature with an overall uncertainty of
60.2°C or better, and the temperature depression with an overall uncertainty of 60.2°C or better for an uncertainty in the relative
humidity of 63% rh.RH. For an uncertainty in the relative humidity of 62% rh,RH, obtain the dry-bulb temperature with an
overall uncertainty of 60.2°C or better and the temperature depression with an overall uncertainty of 60.1°C or better. (Also see
Section 12.)
4.3 Method B—Sling Psychrometer:
4.3.1 Holding the instrument well away from the body, and for outdoor measurements to windward and in the shade, whirl it
at such a rate as to achieve the specified airspeed at the wet and dry bulbs, see 14.4.
4.3.2 Read the thermometers with the necessary precision, obtaining the dry-bulb temperature with an overall uncertainty of
60.6°C or better, and the temperature depression with an overall uncertainty of 60.3°C or better for an uncertainty in the relative
humidity of 65% rh,RH, also see Section 19.
5. 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.
METHOD A—PSYCHROMETER VENTILATED
BYASPIRATION
6. Interferences
6.1 When an aspirated psychrometer is used for measurements in a small enclosed space and steadily rising wet- and dry-bulb
temperatures are observed, consider whether heat and moisture liberated by the instrument itself are affecting the conditions.
6.2 While the thermometers are being read, keep all surfaces that are at temperatures other than the environment (such as the
hands, face, and other warmer or colder objects) as far as possible from the thermometer bulbs.
6.3 This method should not be used where there is heavy contamination of the air with gases, vapors, or dust.
7. Apparatus
7.1 Thermometers for an Aspirated Psychrometer:
7.1.1 The range of the thermometers shall not exceed the range 0 to 80°C.This range may be achieved by providing more than
asinglepairofmatchedthermometers.Whentheuncertaintyinthederivedrelativehumidityisrequiredtobenotmorethan 63%
rh,RH, the thermometers shall be such that their readings give the temperature depression with an uncertainty of not more than
60.2°C. When the uncertainty in the relative humidity is required to be not more than 62% rh,RH, they shall be such that their
E337 –02 (2007)
readings give the temperature depression with an uncertainty of not more than 60.1°C. The uncertainty in the reading of the
dry-bulb temperature shall be 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
temperature directly.
7.1.3 Each thermometer shall consist of a temperature sensor of essentially cylindrical shape which is supported on a single
stem, the stem being coaxial with the sensor. The free end of each sensor shall be smoothly rounded. If the diameter of the stems
is small, compared with that of the sensors, then both ends of each sensor shall be smoothly rounded. The sensor of a
mercury-in-glassthermometershallbethatpartofthethermometerextendingfromthebottomofthebulbtothetopoftheentrance
flare of the capillary.
7.1.4 With transverse ventilation, the diameters of the sensors (excluding wet covering) shall be not less than 1 mm and not
greater than 4 mm.
7.1.5 With axial ventilation, the diameters of the sensors (excluding wet covering) shall be not less than 2 mm and not greater
than 5 mm, and their length not less than 10 mm and not greater than 30 mm.
7.1.6 Theconnectingwiresofelectricalthermometersshallbecontainedwithinthesupportingstemsandshallbeisolatedfrom
the moisture of the wet covering.
7.1.7 Mercury-in-glass shall be graduated to 0.5°C or closer and be capable of being read to the nearest 0.1°C or better. (A
specificationformercury-in-glassthermometerssuitablewhentheuncertaintyinthederivedrelativehumidityisrequiredtobenot
more than 63% rhRH is given in Annex A1.)
7.2 Wet-Bulb Covering, Wick, and Water Reservoir:
7.2.1 The covering shall be fabricated from white-cotton muslin of linear density from 1.0 to 1.2 g/m, refer to Practice D861.
Aseamlesssleeveispreferred,butaseamispermissible,providedthatitdoesnotaddappreciablytothegeneralroughnesswhich
the weave imparts to the surface.
7.2.2 The covering shall completely cover the sensor or bulb of the thermometer, fit snugly but not very tightly, and shall be
in physical contact with the bulb over its entire surface. It shall extend onto the stem for such a distance that the error in the
observed wet-bulb temperature due to heat conduction along the stem does not exceed 0.05°C. (A method of determining the
distance for which the covering must extend onto the stemis outlined inAnnexA2. For mercury-in-glass thermometers withsolid
stems, a distance of twice the stem diameter is usually adequate.)
7.2.3 Tomaintainasnuglyfitcoveronthewetbulb,thecoveringmaybesecuredwithacottonthreadattheendofthecovering
on the stem of the thermometer, at the top of the thermometer bulb, and at the bottom of the bulb. However, whenever a wicking
is used, the covering shall not be secured between the thermometer bulb and the cotton wicking which connects the covering to
a water reservoir.
7.2.4 After fabrication, the covering and wick shall have been washed in a dilute solution of sodium carbonate and thoroughly
rinsed with distilled water. They shall not subsequently be touched with the fingers.
7.2.5 The stem of each thermometer shall, for a length measured from the sensor and not less than 1.53 the length of the
extension of the covering required by 7.2.2, be clear of obstructions and freely exposed to the airstream.
7.2.6 Duringthetest,thecoveringshallbecompletelypermeatedwithwaterasevidencedbyaglisteningappearanceinabeam
of light.
7.2.7 The covering shall be washed in situ with distilled water from time to time and be renewed when it shows any evidence
of permanent change.
7.2.8 When a wick is provided, the free length of a wick shall be at least twice the diameter of the wet bulb and at least three
timesthewickdiameter,ensuringthatwaterarrivingatthecoveringisalreadyatpracticallythewet-bulbtemperature.Awickshall
be limp.
7.2.9 A water reservoir shall not obstruct the airflow, and its contents shall not affect the humidity of the sample air.
7.2.10 The level of the water in a water reservoir shall be between 5 and 25 mm below the level of the lowest part of the wet
bulb.
7.3 Water—Reagentwatershallbeproducedbydistillation,orbyionexchangeorreverseosmosisfollowedbydistillation,refer
to Specification D1193.
7.4 Airflow:
7.4.1 The flow of air over both the wet and dry bulbs shall be a forced draught of 3 to 10 m/s for thermometers with maximum
allowable diameter of the sensors.
7.4.2 The sample air shall not pass over any obstruction or through a fan before it passes over the wet and dry bulbs.
7.4.3 With axial flow, the direction of the flow shall be from the free end of each sensor towards the support end.
7.4.4 No air which has been cooled by the wet bulb or by the wick shall impinge on the dry bulb.
7.5 Radiation Shields:
7.5.1 Anyradiationshieldsshallbeofmetalwithathicknessof0.4to0.8mm.Surfacesrequiredtohaveapolishedfinishshall
be of a bare metal which will retain its brightness.
7.5.2 Withtransverseventilation,radiationshieldsmaybeprovidedtoshieldthewetanddrybulbsfromextraneousradiations.
The radiation shields, essentially in the form of parallel plates, can be either polished on the outside and blackened on the inside,
E337 –02 (2007)
or polished on both the inside and outside surfaces. The clearance between the wet and dry bulbs and the shields shall be not less
than half the overall diameter of the wet bulb. The shields shall be liberally flared outwards at the inlet to prevent the flow
separating from the shields on the inside (vena-contracta effect). The shields may form part of a duct for the airflow. A
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