ASTM E104-20a

Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions

SIGNIFICANCE AND USE
5.1 Standard value relative humidity environments are important for conditioning materials in shelf-life studies or in the testing of mechanical properties such as dimensional stability and strength. Relative humidity is also an important operating variable for the calibration of many species of measuring instruments.
SCOPE
1.1 This practice describes one method for generating constant relative humidity (rh) environments in relatively small containers.  
1.2 This practice is applicable for obtaining constant relative humidities ranging from dryness to near saturation at temperatures spanning from 0 °C to 50 °C.  
1.3 This practice is applicable for closed systems such as environmental conditioning containers and for the calibration of hygrometers.  
1.4 This practice is not recommended for the generation of continuous (flowing) streams of constant humidity unless precautionary criteria are followed to ensure source stability. (See Section 9.)  
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.6 Warning—Saturated salt solutions are extremely corrosive, and care should be taken in their preparation and handling. There is also the possibility of corrosive vapors in the atmospheres over the saturated salt solutions.2  
1.7 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 information see 1.6 and 10.1.  
1.8 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-Oct-2020
• ICS: 19.040 - Environmental testing
• 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 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 D1356-05(2010) - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Apr-2010
• Refers: ASTM D1193-06 - Standard Specification for Reagent Water
• Effective Date: 01-Mar-2006
• Refers: ASTM D1356-05 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-May-2005
• Refers: ASTM D1356-00a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 10-Nov-2000
• Refers: ASTM D1193-99e1 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999
• Refers: ASTM D1193-99 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999

Overview

ASTM E104-20a, titled Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions, provides a systematic approach for generating and maintaining stable relative humidity (RH) environments within closed systems using aqueous saturated salt solutions. Developed by ASTM International, this standard is widely used for conditioning materials, shelf-life studies, calibration of hygrometers, and accurate testing of material properties under controlled humidity.

The practice is applicable for achieving constant relative humidity values from near dryness to near saturation, across a temperature range of 0 °C to 50 °C. This method supports precise and reproducible environmental control, critical in both research and industrial quality assurance.

Key Topics

• Relative Humidity Generation
  This standard outlines the use of selected saturated aqueous salt solutions to create environments with specific RH values.
• Application Range
  Designed for small, closed containers, such as environmental test chambers for materials conditioning and instrument calibration.
• Temperature Stability
  Tight temperature control (to within ±0.1 °C) is emphasized for accurate relative humidity maintenance, as small fluctuations can impact RH significantly.
• Container Selection
  Requires non-hygroscopic, corrosion-resistant containers (e.g., glass or specific plastics/metals) to avoid undesired reactions or absorption.
• Chemical Safety
  Saturated salt solutions can be corrosive, requiring careful handling, appropriate safety measures, and suitable container materials to mitigate hazards.
• Preparation of Solutions
  Guidance includes using reagent-grade salts and water, the preferred use of hydrated over amorphous salts, and minimizing excess liquid in the container.

Applications

ASTM E104-20a is vital for professionals and laboratories that rely on stable humidity control for:

• Materials Conditioning:
  Pre-conditioning samples (plastics, fibers, electronics, papers) before mechanical or physical testing to reduce variability caused by ambient moisture.
• Shelf-Life and Aging Studies:
  Simulating real-world or accelerated storage conditions for pharmaceuticals, food products, adhesives, and coatings.
• Calibration of Hygrometers:
  Ensuring accurate humidity measurement for process control in industries such as HVAC, manufacturing, and environmental monitoring.
• Quality Assurance Laboratories:
  Maintaining consistent testing conditions for comparative studies and regulatory compliance.
• Research and Development:
  Studying moisture effects on novel and sensitive materials under highly controlled atmospheric conditions.

Related Standards

The ASTM E104-20a standard references and complements several other key documents:

• ASTM D1193 - Specification for Reagent Water, critical for preparing salt solutions with minimal impurities.
• ASTM D1356 - Terminology Relating to Sampling and Analysis of Atmospheres, providing essential definitions for humidity control.
• Best Practices:
  Users must adhere to local regulations and laboratory safety protocols when preparing corrosive solutions, handling chemicals, and operating environmental chambers.

Practical Value

By following ASTM E104-20a, organizations can:

• Achieve highly reproducible control of RH for sensitive tests and calibrations.
• Ensure product reliability and regulatory compliance concerning environmental exposure.
• Minimize errors attributable to fluctuating humidity and temperature in testing setups.

Implementing this standard is fundamental for any workflow where precise relative humidity control directly impacts data validity, product performance, or equipment accuracy. It is a recognized international method, facilitating consistency across different facilities and industries worldwide.

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: E104 − 20a
Standard Practice for
Maintaining Constant Relative Humidity by Means of
Aqueous Solutions
This standard is issued under the fixed designation E104; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 This practice describes one method for generating con-
mendations issued by the World Trade Organization Technical
stant relative humidity (rh) environments in relatively small
Barriers to Trade (TBT) Committee.
containers.
1.2 This practice is applicable for obtaining constant rela-
2. Referenced Documents
tive humidities ranging from dryness to near saturation at
2.1 ASTM Standards:
temperatures spanning from 0 °C to 50 °C.
D1193 Specification for Reagent Water
1.3 This practice is applicable for closed systems such as
D1356 Terminology Relating to Sampling and Analysis of
environmental conditioning containers and for the calibration
Atmospheres
of hygrometers.
3. Terminology
1.4 This practice is not recommended for the generation of
continuous (flowing) streams of constant humidity unless 3.1 Definitions—For definitions of other terms used in this
precautionary criteria are followed to ensure source stability. practice, refer to Terminology D1356.
(See Section 9.) 3.1.1 non-hygroscopic material, n—material which neither
absorbs nor retains water vapor.
1.5 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
4. Summary of Practice
standard.
4.1 Standard value relative humidity environments are gen-
1.6 Warning—Saturated salt solutions are extremely
erated using selected aqueous saturated salt solutions.
corrosive, and care should be taken in their preparation and
handling.Thereisalsothepossibilityofcorrosivevaporsinthe
5. Significance and Use
atmospheres over the saturated salt solutions.
5.1 Standard value relative humidity environments are im-
1.7 This standard does not purport to address all of the
portant for conditioning materials in shelf-life studies or in the
safety concerns, if any, associated with its use. It is the
testing of mechanical properties such as dimensional stability
responsibility of the user of this standard to establish appro-
and strength. Relative humidity is also an important operating
priate safety, health, and environmental practices and deter-
variable for the calibration of many species of measuring
mine the applicability of regulatory limitations prior to use.
instruments.
For more specific safety precautionary information see 1.6 and
10.1.
6. Interferences
1.8 This international standard was developed in accor-
6.1 Temperature regulation of any solution-head space en-
dance with internationally recognized principles on standard-
vironment to 60.1 °C is essential for realizing generated
relative humidity values within 60.5 % (expected).
This practice is under the jurisdiction ofASTM Committee D22 on Air Quality
6.2 Some aqueous saturated salt solutions change composi-
and is the direct responsibility of Subcommittee D22.11 on Meteorology.
tion following preparation by hydrolysis or by reaction with
Current edition approved Nov. 1, 2020. Published December 2020. Originally
approved in 1951. Last previous edition approved in 2020 as E104 – 20. DOI:
10.1520/E0104-20A.
2 3
Opila, R., Jr., Weschler, C. J., and Schubert, R., “Acidic Vapors Above For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Saturated Salt Solutions Commonly Used for Control of Humidity,” IEEE Trans. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Components, Hybrids and Manufacturing Technology, Vol 12, No. 1, March 1989, Standards volume information, refer to the standard’s Document Summary page on
pp. 114–120. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E104 − 20a
TABLE 1 Humidity Fixed Point (HFP) Salt Solutions
HFP Designation Salt Name Chemical Symbol Temperature Range (°C)
HFP4 Cesium fluoride CsF 15 to 80
HFP7 Lithium bromide LiBr 5 to 80
HFP12 Lithium chloride LiCl 5 to 80
HFP23 Potassium acetate CH COOK 10 to 30
HFP33 Magnesium chloride MgCl 5to80
HFP43 Potassium carbonate K CO 5to30
2 3
HFP53 Magnesium nitrate Mg(NO ) 5to50
3 2
HFP59 Sodium bromide NaBr 5 to 80
HFP70 Potassium iodide KI 5 to 80
HFP75 Sodium chloride NaCl 5 to 80
HFP85 Potassium chloride KCl 5 to 80
HFP94 Potassium nitrate KNO 5to50
HFP98 Potassium sulfate K SO 5to50
2 4
environmental components (for example, carbon dioxide ab- 9. Technical Precautions
sorption by alkaline materials). These solutions should be
9.1 Although a container capable of airtight closure is
freshly prepared on each occasion of use.
described in Section 7, it may be desirable to have a vent under
certain conditions of test or with some kinds of containers
7. Apparatus
(changes in pressure may produce undesirable cracks in some
7.1 Container—The container, including a cover or lid
types of containers).The vent should be as small as practical to
which can be secured airtight, should be made of corrosion
minimize loss of desired equilibrium conditions when in use.
resistant, non-hygroscopic material such as glass. A metal or
9.2 The container should be small to minimize the influence
plastic container is acceptable if the solution is retained in a
of any temperature variations acting upon the container and
dish or tray made of appropriate material. Refer also to 9.2 for
3 2
contents. A maximum proportion of 25 cm volume/cm of
size restrictions.
solution surface area is suggested, and overall container
headspace volume should be no larger than necessary to
8. Reagents and Materials
confine a stored item.
8.1 Purity of Reagents—Reagent grade chemicals shall be
9.3 Measurement accuracy is strongly dependent on the
used for preparation of all standard solutions. Unless otherwise
ability to achieve and maintain temperature stability during
indicated, it is intended that all reagents conform to the
actual use of any solution system. Temperature instability of
specifications of the Committee on Analytical Reagents of the
60.1 °C can cause corresponding instabilities in generated
American Chemical Society where such specifications are
values of relative humidity of 60.5 %.
available. Other grades may be used, provided it is first
ascertained that the reagent is of sufficiently high purity to
9.4 The compatibility of any constant relative humidity
permit its use without lessening the accuracy of the determi-
system used for instrument calibration testing should be
nation.
confirmed by reference to the instrument manufacturer’s in-
8.1.1 Saturated salt solutions may be prepared using either
structions.
amorphous or hydrated reagents (that is, reagents containing
9.5 Important considerations leading to stability should
water of crystallization). Hydrated reagents are often preferred
include (but are not necessarily limited to) the following:
to amorphous forms for their solvating characteristics.
9.5.1 Elimination of leakage paths.
8.2 Purity of Water—Reagent water produced by
9.5.2 Elimination of heat sources or heat sinks, or both, for
distillation,orbyionexchange,orreverseosmosisfollowedby
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM 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.
Designation: E104 − 20 E104 − 20a
Standard Practice for
Maintaining Constant Relative Humidity by Means of
Aqueous Solutions
This standard is issued under the fixed designation E104; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Scope*
1.1 This practice describes one method for generating constant relative humidity (rh) environments in relatively small containers.
1.2 This practice is applicable for obtaining constant relative humidities ranging from dryness to near saturation at temperatures
spanning from 0 °C to 50°C.50 °C.
1.3 This practice is applicable for closed systems such as environmental conditioning containers and for the calibration of
hygrometers.
1.4 This practice is not recommended for the generation of continuous (flowing) streams of constant humidity unless precautionary
criteria are followed to ensure source stability. (See Section 9.)
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 Warning—Saturated salt solutions are extremely corrosive, and care should be taken in their preparation and handling. There
is also the possibility of corrosive vapors in the atmospheres over the saturated salt solutions.
1.7 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 information see 1.6 and 10.1.
1.8 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.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
This practice 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 March 1, 2020Nov. 1, 2020. Published April 2020December 2020. Originally approved in 1951. Last previous edition approved in 20122020
as E104 – 02 (2012).E104 – 20. DOI: 10.1520/E0104-20.10.1520/E0104-20A.
Opila, R., Jr., Weschler, C. J., and Schubert, R., “Acidic Vapors Above Saturated Salt Solutions Commonly Used for Control of Humidity,” IEEE Trans. Components,
Hybrids and Manufacturing Technology, Vol 12, No. 1, March 1989, pp. 114–120.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’sstandard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E104 − 20a
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
3. Terminology
3.1 Definitions—For definitions of other terms used in this practice, refer to Terminology D1356.
3.1.1 non-hygroscopic material, n—material which neither absorbs nor retains water vapor.
4. Summary of Practice
4.1 Standard value relative humidity environments are generated using selected aqueous saturated salt solutions.
5. Significance and Use
5.1 Standard value relative humidity environments are important for conditioning materials in shelf-life studies or in the testing
of mechanical properties such as dimensional stability and strength. Relative humidity is also an important operating variable for
the calibration of many species of measuring instruments.
6. Interferences
6.1 Temperature regulation of any solution-head space environment to 60.1°C 60.1 °C is essential for realizing generated relative
humidity values within 60.5 % (expected).
6.2 Some aqueous saturated salt solutions change composition following preparation by hydrolysis or by reaction with
environmental components (for example, carbon dioxide absorption by alkaline materials). These solutions should be freshly
prepared on each occasion of use.
7. Apparatus
7.1 Container—The container, including a cover or lid which can be secured airtight, should be made of corrosion resistant,
non-hygroscopic material such as glass. A metal or plastic container is acceptable if the solution is retained in a dish or tray made
of appropriate material. Refer also to 9.2 for size restrictions.
8. Reagents and Materials
8.1 Purity of Reagents—Reagent grade chemicals shall be used for preparation of all standard solutions. Unless otherwise
indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American
Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the
reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
8.1.1 Saturated salt solutions may be prepared using either amorphous or hydrated reagents (that is, reagents containing water of
crystallization). Hydrated reagents are often preferred to amorphous forms for their solvating characteristics.
8.2 Purity of Water—Reagent water produced by distillation, or by ion exchange, or reverse osmosis followed by distillation shall
be used. See Specification D1193.
8.3 Nomenclature for Salts—The proper nomenclature for the Humidity Fixed Point (HFP) salt chemicals and the corresponding
acceptable temperature ranges are provided in Table 1. The scale encompasses relative humidity ranges from 2 % to 98.5 % and
a temperature range from 5°C to 80°C. 5 °C to 80 °C. When the salt solutions are used at temperatures above 40°C, 40 °C, the
risk of salt crystals settling on the surface rather than being immersed in the solution must be taken into account.
9. Technical Precautions
9.1 Although a container capable of airtight closure is described in Section 7, it may be desirable to have a vent under certain
ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference Materials, American Chemical Society, Washington, DC. For
suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and
the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
E104 − 20a
TABLE 1 Humidity Fixed Point (HFP) Salt Solutions
HFP Designation Salt Name Chemical Symbol Temperature Range (°C)
HFP 4 Caesium fluoride CsF 15 to 80
HFP 7 Lithium bromide LiBr 5 to 80
HFP 12 Lithium chloride LiCl 5 to 80
HFP 23 Potassium acetate CH COOK 10 to 30
HFP 33 Magnesium chloride MgCl 5 to 80
HFP 43 Potassium carbonate K CO 5 to 30
2 3
HFP 59 Sodium bromide NaBr 5 to 80
HFP 70 Potassium iodide KI 5 to 80
HFP 75 Sodium chloride NaCl 5 to 80
HFP 85 Potassium chloride KCl 5 to 80
HFP 98 Potassium sulfate K SO 5 to 50
2 4
TABLE 1 Humidity Fixed Point (HFP) Salt Solutions
HFP Designation Salt Name Chemical Symbol Temperature Range (°C)
HFP4 Cesium fluoride CsF 15 to 80
HFP7 Lithium bromide LiBr 5 to 80
HFP12 Lithium chloride LiCl 5 to 80
HFP23 Potassium acetate CH COOK 10 to 30
HFP33 Magnesium chloride MgCl 5 to 80
HFP43 Potassium carbonate K CO 5 to 30
2 3
HFP53 Magnesium nitrate Mg(NO ) 5 to 50
3 2
HFP59 Sodium bromide NaBr 5 to 80
HFP70 Potassium iodide KI 5 to 80
HFP75 Sodium chloride NaCl 5 to 80
HFP85 Potassium chloride KCl 5 to 80
HFP94 Potassium nitrate KNO 5 to 50
HFP98 Potassium sulfate K SO 5 to 50
2 4
conditions of test or with some kinds of containers (changes in pressure may produce undesirable cracks in some types of
containers). The vent should be as small as practical to minimize loss of desired equilibrium conditions when in use.
9.2 The container should be small to minimize the influence of any temperature variations acting upon the container and contents.
3 2
A maximum proportion of 25 cm volume/cm of solution surface area is suggested, and overall container headspace volume
should be no larger than necessary to confine a stored item.
9.3 Measurement accuracy is strongly dependent on the ability to achieve and maintain temperature stability during actual use of
any solution system. Temperature instability of 60.1°C 60.1 °C can cause corresponding instabilities in generated values of
relative humidity of 60.5 %.
9.4 The compatibility of any constant relative humidity system used for instrument calibration testing should be confirmed by
reference to the instrument manufacturer’smanufacturer’s instructions.
...