ASTM D2029-97(2017)

Standard Test Methods for Water Vapor Content of Electrical Insulating Gases by Measurement of Dew Point

SIGNIFICANCE AND USE
5.1 Certain gases have excellent dielectric and electric arc interruption characteristics which make their use in electrical installations very desirable.  
5.2 Water content, as the test parameter, is of great importance in determining the dielectric effectiveness of the gas. Under certain conditions, water may condense and become a conducting liquid resulting in a catastrophic dielectric breakdown of the insulation. The water content of these insulating gases as expressed by dew point is listed in Specifications D1933, D2472, and D3283.  
5.3 Once the dew point is determined, a conversion to moisture content may be performed using Table 1. Once moisture content is known, the lowest temperature at which gas insulated equipment can be safely operated can usually be determined by reviewing manufacturers' specifications for the equipment. (A) Vapor pressures in atmospheres at various dew points can be obtained by dividing the values for “volume percent'' in this table by 100. Calculations for this table were made by using the International Critical Table values for the vapor pressure of ice and liquid water. The vapor pressure of liquid water was used for values from 50 to 0°C. The vapor pressure of ice was used from 0 to − 81°C.  
5.4 The dew point of the test gas is independent of the gas temperature but does depend on its pressure. Many moisture measurement test instruments are sensitive to pressure, and display moisture values at the instrument inlet pressure and not necessarily at the pressure of the system being sampled. It is therefore important to account for this condition to avoid serious measurement errors.
SCOPE
1.1 These test methods describe the determination of the water vapor content of electrical insulating gases by direct or indirect measurement of the dew point and the calculation of the water vapor content.  
1.2 The following four test methods are provided:  
1.2.1 Method A describes the automatic chilled mirror method for measurement of dew point as low as − 73°C (−99°F).  
1.2.2 Method B describes the manual chilled mirror or dew cup method for measurement of dew point as low as − 73°C (−99°F).  
1.2.3 Method C describes the adiabatic expansion method for measurement of dew point as low as − 62°C (−80°F).  
1.2.4 Method D describes the capacitance method for measurement of dew point as low as − 110°C (−166°F).  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific warnings, see 8.1.1, 9.2, 10.1.2 and 10.2.5.

General Information

• Status: Published
• Publication Date: 31-Dec-2016
• ICS: 29.040.20 - Insulating gases
• Technical Committee: D27 - Electrical Insulating Liquids and Gases
• Drafting Committee: D27.07 - Physical Test

Relations

• Replaces: ASTM D2029-97(2008) - Standard Test Methods for Water Vapor Content of Electrical Insulating Gases by Measurement of Dew Point
• Effective Date: 01-Jan-2017
• Refers: ASTM D2472-15(2023) - Standard Specification for Sulfur Hexafluoride
• Effective Date: 01-Dec-2023
• Refers: ASTM D1933-03(2017) - Standard Specification for Nitrogen Gas as an Electrical Insulating Material
• Effective Date: 01-Jan-2017
• Refers: ASTM D2472-15 - Standard Specification for Sulfur Hexafluoride
• Effective Date: 01-Oct-2015
• Refers: ASTM D2472-00(2014) - Standard Specification for Sulfur Hexafluoride
• Effective Date: 01-Nov-2014
• Refers: ASTM D1933-03(2008) - Standard Specification for Nitrogen Gas as an Electrical Insulating Material
• Effective Date: 01-May-2008
• Refers: ASTM D2472-00(2006) - Standard Specification for Sulfur Hexafluoride
• Effective Date: 01-Apr-2006
• Refers: ASTM D3283-98(2004) - Standard Specification for Air as an Electrical Insulating Material
• Effective Date: 01-Oct-2004
• Refers: ASTM D1933-03 - Standard Specification for Nitrogen Gas as an Electrical Insulating Material
• Effective Date: 01-Oct-2003
• Refers: ASTM D2472-00 - Standard Specification for Sulfur Hexafluoride
• Effective Date: 10-Apr-2000
• Refers: ASTM D3283-98 - Standard Specification for Air as an Electrical Insulating Material
• Effective Date: 10-Oct-1998
• Refers: ASTM D1933-97 - Standard Specification for Nitrogen Gas as an Electrical Insulating Material
• Effective Date: 10-Apr-1997
• Referred By: ASTM D3283-98(2020) - Standard Specification for Air as an Electrical Insulating Material
• Effective Date: 01-Jan-2017

Overview

ASTM D2029-97(2017), titled Standard Test Methods for Water Vapor Content of Electrical Insulating Gases by Measurement of Dew Point, establishes four recognized methods for determining the water vapor content in insulating gases. The core purpose is to evaluate dew point and, by extension, moisture levels, to support the safe and effective use of gases such as nitrogen, sulfur hexafluoride (SF6), and air in electrical installations. Maintaining appropriate water content is crucial, as excess moisture can lead to condensation, compromise dielectric properties, and increase risk of catastrophic electrical insulation failures.

These test methods apply to both laboratory and field settings and are relevant for quality assurance, commissioning, and ongoing maintenance of gas-insulated electrical equipment.

Key Topics

• Dew Point Measurement: Dew point is the temperature at which a gas becomes saturated with water vapor and condensation occurs. Accurate dew point measurement translates directly into precise control over water vapor content.
• Moisture Content Conversion: Once the dew point is established, it can be converted into specific moisture content metrics (such as mg/L or volume percent), supporting informed operational decisions.
• Pressure Considerations: The dew point reading depends on gas pressure. Many testing instruments display values at the sampling pressure, so pressure adjustments or awareness are necessary to avoid errors.
• Test Methods:
  • Automatic Chilled Mirror Method (Method A): Uses electronic chilling and optical detection for precise automated dew point readings.
  • Manual Chilled Mirror/Dew Cup Method (Method B): Involves manually cooling a mirror to identify dew formation.
  • Adiabatic Expansion Method (Method C): Cools gas rapidly via expansion, detecting condensation as fog.
  • Capacitance Method (Method D): Uses a moisture sensor that responds to varying water vapor levels.

Applications

These standardized test methods play a critical role in a variety of practical applications within the electrical and electronics industries:

• Quality Control of Insulating Gases: Ensures gases such as SF6, nitrogen, and air meet required purity and dryness levels before use in electrical apparatus.
• Commissioning of Electrical Equipment: Supports the validation of gas-insulated switchgear, circuit breakers, and transformers prior to operation, preventing dielectric breakdowns.
• Maintenance and Safety: Regular monitoring of gas moisture content helps identify leaks or contamination, thereby supporting preventive maintenance and extending equipment lifespan.
• Specification Compliance: Assists manufacturers and facility operators in meeting regulatory and industry standards regarding the moisture limits for insulating gases.
• Research and Development: Reliable moisture testing supports innovation in gas-insulated electrical technologies.

Accurate dew point and water content assessments help maintain high dielectric performance and minimize the risks associated with excessive condensation within sealed electrical systems.

Related Standards

• ASTM D1933 – Specification for Nitrogen Gas as an Electrical Insulating Material
• ASTM D2472 – Specification for Sulfur Hexafluoride (SF6) Gas as an Electrical Insulating Material
• ASTM D3283 – Specification for Air as an Electrical Insulating Material

These related ASTM standards provide additional reference criteria for acceptable moisture content and quality requirements for common insulating gases. Using ASTM D2029-97(2017) together with these specifications ensures holistic and compliant control over the water vapor content in vital electrical insulating applications.

---

Keywords: dew point, water vapor content, insulating gases, moisture measurement, chilled mirror method, adiabatic expansion, capacitance method, SF6, nitrogen, air, dielectric breakdown, ASTM D2029

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: D2029 − 97 (Reapproved 2017)
Standard Test Methods for
Water Vapor Content of Electrical Insulating Gases by
Measurement of Dew Point
This standard is issued under the fixed designation D2029; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 3. Terminology
1.1 These test methods describe the determination of the 3.1 Definitions:
water vapor content of electrical insulating gases by direct or 3.1.1 dew point, n—the temperature to which a gas must be
indirect measurement of the dew point and the calculation of cooledatconstantpressureandconstantwatervaporcontentin
the water vapor content. order for saturation to occur. Any further cooling usually
results in formation of the first drop of dew.
1.2 The following four test methods are provided:
3.1.2 hygroscopic, adj—readily taking up and retaining
1.2.1 Method A describes the automatic chilled mirror
moisture.
method for measurement of dew point as low as−73°C
(−99°F).
4. Summary of Test Methods
1.2.2 Method B describes the manual chilled mirror or dew
4.1 Method A—The automatic chilled mirror method uses
cup method for measurement of dew point as low as−73°C
the chilled mirror dew point condensation principle to deter-
(−99°F).
mine the water vapor content in gas mixtures. An internal
1.2.3 Method C describes the adiabatic expansion method
mirror, which is in the path of the test gas, is automatically
for measurement of dew point as low as−62°C (−80°F).
cooled. Internal electronics sense the presence of moisture on
1.2.4 Method D describes the capacitance method for mea-
the mirror. The device then automatically brings itself to
surement of dew point as low as−110°C (−166°F).
equilibrium and provides a direct reading of dew point tem-
1.3 This standard does not purport to address all of the
perature.
safety concerns, if any, associated with its use. It is the
4.2 Method B—This method uses the same basic condensa-
responsibility of the user of this standard to establish appro-
tion principle in 4.1; however, the manual chilled mirror
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific method uses a mixture of acetone and ice or other cooling
media to manually chill the dew cup polished surface which
warnings, see 8.1.1, 9.2, 10.1.2 and 10.2.5.
acts as the mirror.
2. Referenced Documents
4.3 MethodC—Adiabaticexpansionusesaprocessinwhich
2.1 ASTM Standards: the test gas is cooled rapidly to determine dew point tempera-
ture.Thisrapidexhaustingofthetestgastoatmosphereresults
D1933Specification for Nitrogen Gas as an Electrical Insu-
lating Material in an expansion and cooling of the gas. If the cooling is
sufficient to reduce the temperature of the gas to or below the
D2472Specification for Sulfur Hexafluoride
D3283Specification for Air as an Electrical Insulating Ma- dew point, water vapor will condense out in the form of a fine
mist or fog. Successive trials will determine the minimum
terial
initialpressurethatwillproduceafog.Fromthis,thedewpoint
temperature can be calculated.
4.3.1 The relationship between pressure and temperature
These test methods are under the jurisdiction of ASTM Committee D27 on
during adiabatic expansion is as follows:
Electrical Insulating Liquids and Gasesand are the direct responsibility of Subcom-
mittee D27.07 on Physical Test.
K21/K
@ #
T 5 T P /P
F I@ F I#
Current edition approved Jan. 1, 2017. Published February 2017. Originally
approved in 1964. Last previous edition approved in 2008 as D2029-97 (2008).
where:
DOI: 10.1520/D2029-97R17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
K = ratio of specific heats for a given gas,
contactASTM Customer Service at [e-mail]. ForAnnual Book ofASTM
T = final temperature,
F
Standards volume information, refer to the standard’s Document Summary page on
T = initial temperature,
I
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2029 − 97 (2017)
6.3.1 For MethodsA, B, and C, the relatively large volume
P = final pressure, and
F
of gas required to deposit sufficient water vapor to create the
P = initial pressure.
I
“dew”.
4.4 Method D—The capacitance method uses a moisture
6.3.2 For Methods A, B, and C, that under very dry
sensor, typically aluminum oxide or silicon oxide, which
condition,thepossibilityexiststocondensethetestgaspriorto
changes its electrical output with the amount of water vapor to
deposition of moisture on the mirror.
which it is exposed.
6.3.3 For Methods A, B, C, and D, that the measuring
system (instrument and tubing) must not entrain moisture. If
5. Significance and Use
any moisture is entrained, several hours may be required for
the gas being measured to come into equilibrium with the
5.1 Certain gases have excellent dielectric and electric arc
measuring system.
interruption characteristics which make their use in electrical
6.3.4 ForMethodsBandC,thesensitivityofthehumaneye
installations very desirable.
in determining exactly when the dew first forms.
5.2 Water content, as the test parameter, is of great impor-
tance in determining the dielectric effectiveness of the gas. 7. General Requirements
Under certain conditions, water may condense and become a
7.1 Methods A, B, and C—Any properly constructed dew
conducting liquid resulting in a catastrophic dielectric break-
point apparatus may be used that provides a means to satisfy
down of the insulation. The water content of these insulating
the following basic requirements:
gases as expressed by dew point is listed in Specifications
7.1.1 Control the flow of gas entering and leaving the
D1933, D2472, and D3283.
apparatus while the apparatus is at a temperature at least 2°C
(3.6°F) above the dew point of the gas.
5.3 Once the dew point is determined, a conversion to
7.1.2 Control the cooling rate of a chamber in the apparatus
moisture content may be performed using Table 1. Once
through which the flowing gas passes to a temperature low
moisturecontentisknown,thelowesttemperatureatwhichgas
enough to cause water vapor to condense from the gas.
insulated equipment can be safely operated can usually be
7.1.3 Detectthedepositionofdewonthecoldportionofthe
determined by reviewing manufacturers’ specifications for the
apparatus and measure the temperature at which dew is
equipment.
formed.
5.4 The dew point of the test gas is independent of the gas
7.1.4 Ensure that the test gas is at or near atmospheric
temperature but does depend on its pressure. Many moisture
pressure and is isolated from contamination from other gases.
measurement test instruments are sensitive to pressure, and
7.2 Method D—Any properly constructed capacitive type
displaymoisturevaluesattheinstrumentinletpressureandnot
moisture sensor may be used that provides a means to satisfy
necessarily at the pressure of the system being sampled. It is
the following basic requirements:
therefore important to account for this condition to avoid
7.2.1 Expose the sensor to a gas that is at a temperature at
serious measurement errors.
least 10°C (18°F) above the dew point of the gas.
7.2.2 Measurethepartialvaporpressureofwaterinagasby
6. Interferences
means of a capacitive type sensor.
6.1 Tubing:
7.3 These test methods provide for several techniques, each
6.1.1 Most new metal tubing contains oil deposits on the
utilizing different types of apparatus for measuring dew point.
interior walls due to the manufacturing process. This residue
The techniques in these test methods are provided for general
should be removed before using these lines for gas sampling.
information and are not intended as a substitute for manufac-
6.1.2 Tubing should be free of leaks, since even a pinhole
turer’s instructions. When using any instrument, the manufac-
leak will result in a false indication (higher dew point), due to
turer’s instructions should be followed to ensure proper and
the partial pressure of water vapor in the atmosphere.
safe operation.
6.1.3 Whenthegasbeingtestedisextremelydry[dewpoint
8. Apparatus
below approximately−40°C (−40°F)], results can be mislead-
ing until the moisture adsorbed in the system (tubing, 8.1 General:
regulators,etc.)hasbeenremovedbypurgingwiththetestgas.
8.1.1 Tubing—Although not true of all applications, stain-
At this point, all moisture present within the system should be less steel, glass, and nickel alloy tubing are the best possible
due to that contained in the test gas.
nonhygroscopic materials and should be used for low dew
point applications−18 to−73°C (0 to−100°F). Copper and
6.2 When testing gases that contain readily liquefiable
aluminum alloys, as well as stabilized polypropylene tubing,
impurities, it must be kept in mind that the dew point that is
are acceptable above−29°C (−20°F) dew point. (Warning—
measured by condensation type instruments may be due to
All materials will adsorb moisture to some extent; therefore,
these impurities rather than to water. Under these conditions,
the internal surface of apparatus, tubing, and fittings should be
the measured dew point is not an indication of the water
minimized to enable the system to dry out more quickly and
content of the gas.
achieve equilibrium sooner. However, it should be noted that
6.3 Measurement of water vapor in very dry gases is when one switches from measurement of a high dew point to
complicated by four considerations, as follows: a lower dew point [that is, 0 to−60°C (32 to−76°F)] copper
D2029 − 97 (2017)
TABLE 1 Relationship Between Dew Point and Moisture Content of Gases
NOTE 1—With a known dew point which is indicated by the dew point indicator or recorder, the moisture content can be read directly from the table.
The table shows the amount of water in air or other gas at various dew points at a pressure of 1 atm (14.7) psi.
Dew Point Moisture Content Dew Point Moisture Content
A A
lb/1000 lb/1000
volume volume
°C °F mg/L °C °F mg/L
3 3
percent percent
ft ft
50 122.0 5.16 82.7 12.2 −16 3.2 0.079 1.27 0.149
49 120.2 4.92 78.9 11.6 −17 1.4 0.072 1.16 0.136
48 118.4 4.69 75.1 11.0 −18 −0.4 0.066 1.06 0.123
47 116.6 4.48 71.9 10.5 −19 −2.2 0.060 0.965 0.112
46 114.8 4.26 68.4 9.95 −20 −4.0 0.055 0.882 0.102
45 113.0 4.06 65.0 9.45 −21 −5.8 0.050 0.809 0.093
44 111.2 3.88 62.1 8.99 −22 −7.6 0.046 0.733 0.084
43 109.4 3.69 59.1 8.52 −23 −9.4 0.042 0.666 0.076
42 107.6 3.52 56.4 8.10 –24 −11.2 0.038 0.608 0.069
41 105.8 3.34 53.5 7.67 −25 −13.0 0.035 0.556 0.063
40 104.0 3.18 50.9 7.27 −26 −14.8 0.031 0.506 0.057
39 102.2 3.02 48.4 6.89 −26 −16.6 0.028 0.454 0.057
38 100.4 2.87 46.0 6.54 −28 −18.4 0.025 0.411 0.046
37 98.6 2.74 43.8 6.20 −29 −20.2 0.023 0.377 0.042
36 96.8 2.60 41.6 5.87 −30 −22.0 0.021 0.343 0.038
35 95.0 2.46 39.4 5.55 −31 −23.8 0.019 0.307 0.034
34 93.2 2.34 37.4 5.25 −32 −25.6 0.017 0.273 0.030
33 91.4 2.22 35.6 4.96 −33 −27.4 0.015 0.246 0.027
32 89.6 2.11 33.8 4.70 −34 −29.2 0.014 0.229 0.025
31 87.8 2.00 32.0 4.44 −35 −31.0 0.013 0.202 0.022
30 86.0 1.89 30.3 4.19 −36 −32.8 0.012 0.185 0.020
29 84.2 1.84 29.2 4.01 −37 −34.6 0.010 0.167 0.018
28 82.4 1.69 27.1 3.7 −38 −36.4 0.0093 0.149 0.016
27 80.6 1.60 25.7 3.52 −39 −38.2 0.0082 0.131 0.014
26 78.8 1.52 24.4 3.33 −40 −40.0 0.0074 0.119 0.0127
25 77.0 1.44 23.0 3.12 −41 −41.8 0.0068 0.107 0.0113
24 75.2 1.35 21.7 2.94 −42 −43.6 0.0060 0.096 0.0102
23 73.4 1.28 20.6 2.78 −43 −45.4 0.0054 0.086 0.0090
22 71.6 1.21 19.4 2.61 −44 −47.2 0.0047 0.076 0.0080
21 69.8 1.14 18.3 2.46 −45 −49.0 0.0042 0.068 0.0071
20 68.0 1.08 17.3 2.31 −46 −50.8 0.0038 0.061 0.0063
19 66.2 1.02 16.3 2.17 −47 −52.6 0.0034 0.054 0.0056
18 64.4 0.961 15.4 2.04 −48 −54.4 0.0031 0.049 0.0050
17 62.6 0.899 14.4 1.91 −49 −56.2 0.0027 0.043 0.0044
16 60.8 0.855 13.7 1.80 −50 −58.0 0.0024 0.038 0.0039
15 59.0 0.799 12.8 1.68 −51 −59.8 0.0021 0.034 0.0034
14 57.2 0.749 12.0 1.57 −52 −61.6 0.0019 0.030 0.0030
13 55.4 0.706 11.3 1.48 −53 −63.4 0.0017 0.027 0.0027
12 53.6 0.668 10.7 1.39 −54 −65.2 0.0014 0.023 0.0023
11 51.8 0.620 9.94 1.29 −55 −67.0 0.0013 0.021 0.0021
10 50.0 0.584 9.37 1.21 −56 −68.8 0.0011 0.018 0.0018
9 48.2 0.547 8.76 1.13 −57 −70.6 0.0010 0.016 0.0016
8 46.4 0.516 8.27 1.06 −58 −72.4 0.00087 0.014 0.0014
7 44.6 0.482 7.73 0.988 −59 −74.2 0.00075 0.012 0.0012
6 42.8 0.452 7.25 0.924 −60 −76.0 0.00069 0.011 0.0011
5 41.0 0.424 6.79 0.861 −61 −77.8 0.00059 0.0095 0.00092
4 39.2 0.399 6.36 0.804 −62 −79.6 0.00052 0.0083 0.00080
3 37.4 0.370 5.94 0.748 −63 −81.4 0.00046 0.0073 0.00070
2 35.6 0.346 5.55 0.696 −64 −83.2 0.00040 0.0064 0.00061
1 33.8 0.323 5.18 0.649 −65 −85.0 0.00035 0.0056 0.00053
0 32.0 0.302 4.84 0.602 −66 −86.8 0.00030 0.0048 0.00045
−1 30.2 0.280 4.49 0.556 −67 −88.6 0.00027 0.0043 0.00040
−2 28.4 0.258 4.14 0.511 −68 −90.4 0.00022 0.0036 0.00034
−3 26.6 0.238 3.81 0.470 −69 −92.2 0.00019 0.0031 0.00029
−4 24.8 0.220 3.52 0.431 −70 −94.0 0.00017 0.0027 0.00025
−5 23.0 0.202 3.24 0.396 −71 −95.8 0.00015 0.0024 0.00022
−6 21.2 0.186 2.98 0.364 −72 −97.6 0.00013 0.0021 0.00019
−7 19.4 0.171 2.74 0.333 −73 −99.4 0.00011 0.0018 0.00016
−8 17.6 0.158 2.53 0.306 −74 −101.2 0.00009 0.0015 0.00014
−9 15.8 0.145 2.32 0.280 −75 −103.0 0.00008 0.0013 0.00012
−10 14.0 0.134 2.14 0.257 −76 −104.8 0.00007 0.0011 0.00010
−11 12.2 0.122 1.96 0.235 −77 −106.6 0.00006 0.0010 0.00009
−12 10.4 0.113 1.81 0.215 −78 −108.4 0.00005 0.0008 0.00007
−13 8.6 0.103 1.65 0.196 −79 −110.2 0.00004 0.0007 0.00006
−14 6.8 0.095 1.52 0.179 −80 −112.0 0.00004 0
...