ASTM E2551-20

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Designation: E2551 − 18 E2551 − 20
Standard Test Methods for
Humidity Calibration (or Conformation) of Humidity
Generators for Use with Thermogravimetric Analyzers
This standard is issued under the fixed designation E2551; 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 Scope*
1.1 These test methods describe the humidity calibration (or conformance) of humidity generators for use with thermogravi-
metric analyzers and other thermal analysis apparatus. The humidity range covered is 5 % relative humidity (% RH) to 95 % RH
and the temperature range is 0 °C to 80 °C.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 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
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E473 Terminology Relating to Thermal Analysis and Rheology
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1142 Terminology Relating to Thermophysical Properties
These test methods are under the jurisdiction of ASTM Committee E37 on Thermal Measurements and are the direct responsibility of Subcommittee E37.10 on
Fundamental, Statistical and Mechanical Properties.
Current edition approved Oct. 1, 2018March 15, 2020. Published October 2018April 2020. Originally approved in 2007. Last previous edition approved in 20132018 as
E2551 – 13.18. DOI: 10.1520/E2551-18.10.1520/E2551-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’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
E2551 − 20
E1582 Test Method for Temperature Calibration of Thermogravimetric Analyzers
3. Terminology
3.1 Specific technical terms used in this standard are defined in Terminologies E473 and E1142. These terms include
first-deviation-from-baseline, isohume, relative humidity, thermal curve, and thermogravimetric analysis.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 water activity, n—the ratio of actual partial pressure of water to the saturated water vapor pressure at the same temperature,
expressed as a decimal fraction.
3.2.1.1 Discussion—
Water activity is also known as relative pressure in some applications areas.
4. Summary of Test Method
4.1 Humidity generators are devices aimed at producing a specific level of humidity in the purge gas used by thermogravimetric
analyzers or other thermal analysis apparatus. The requested humidity levels may be held constant (isohume) or increased or
decreased in a continuous or stepped fashion.
4.2 The humidified purge gas is submitted to a thermogravimetric analyzer in which the weight of a hygroscopic material is
observed. The relative humidity (or activity) of the moisture in the purge gas is stepped or scanned through a humidity range. At
a fixed humidity of the purge gas, the test specimen deliquesces and gains weight. In Test Method A, the humidity of the onset
of this weight gain is taken as the humidity calibration point. In Test Methods B and C, the rate of weight change is zero at the
humidity calibration point.
5. Significance and Use
5.1 These test methods calibrate or demonstrate conformity of the humidity level in a purge gas generated by a humidity
generator at a fixed temperature. Such calibration or demonstration of conformity may be required by quality initiatives.
5.2 Conformance demonstrates that the humidified purge gas is within some established limits.
5.3 Calibration provides an offset and or slope value that may be used for establishing the relative humidity scale of the
apparatus.
6. Interferences
6.1 Temperature regulation of any solution-head space environment to within 60.1 °C is essential for realizing generated
relative humidity values stable to within 61 % RH (expected).
7. Apparatus
7.1 The humidity generator that is the focus of this standard may be an accessory providing a humidified purge gas to some other
thermal analysis apparatus (typically a thermogravimetric analyzer) or it may be part of a self-contained instrument that includes
both the humidity generator and the thermal analysis apparatus. In the former case, some of the components described below may
be redundant.
7.2 Humidity Generator—The essential instrumentation required to provide the minimum humidity generator capability for
these test methods includes:
7.2.1 Temperature Sensor, to provide an indication of the purge gas temperature readable to within 60.1 °C.
7.2.2 Temperature Controller, capable of executing a specific temperature program by operating heaters or coolers between
selected temperature limits at a rate of temperature change of 0.5 °C/min constant to 60.1 °C/min or at an isothermal temperature
constant to within 60.1 °C.
7.2.3 Humidity Sensor, capable of indicating the humidity of the purge gas over the range of 5 % RH to 95 % RH readable to
within 60.1 % RH.
7.2.4 Humidity Controller, capable of executing a specific humidity program by operating purge gas humidifiers between
selected humidity limits at a rate of humidity change of 0.5 % RH/min constant to within 60.1 % RH or at an isohume relative
humidity to within 60.1 % RH.
7.2.5 Purge Gas Flow Sensor, capable of measuring purge gas flow readable to within 60.1 mL/min.
7.2.6 Purge Gas Flow Controller, capable of controlling purge gas flow readable to within 60.1 mL/min.
7.2.7 Humidifier Element, capable of generating purge gases with relative humidity continuously over the range of 5 % RH to
95 % RH.
7.3 Thermogravimetric Analyzer (TGA)—The essential instrumentation required to provide the minimum thermogravimetric
analytical capability for this method includes:
E2551 − 20
7.3.1 A furnace to provide uniform controlled heating or cooling of a specimen to a constant temperature or at a constant rate
within the applicable temperature range of this method.
7.3.2 A temperature sensor to provide an indication of the specimen or furnace temperature to within 60.1 °C.
7.3.3 A continuously recording balance to measure the specimen weight with a minimum capacity of 100 mg and sensitivity
of 610 μg.
7.3.4 A means of maintaining the specimen/container under atmospheric control at a purge rate of 10 mL/min to 200 mL/min
6 10 mL/min or 1 %, whichever is greater.
7.3.5 A temperature controller capable of executing a specific temperature program by operating the furnace between selected
temperature limits at a rate of temperature change of 0.5 °C/min constant to within 60.1 °C/min or to an isothermal temperature
that is maintained constant to within 60.1 °C for a minimum of 100 h.
7.3.6 Containers (pans, crucibles, etc.) that are inert to the specimen and that will remain gravimetrically stable within the
temperature limits of this method.
7.3.7 Data storage capable of storage of the weight and relative humidity signals.
NOTE 1—A data acquisition rate of 1 point per second has been found adequate. At rates may be used.
7.3.8 A display capable of plotting a thermal curve with weight on the ordinate (Y-axis) and relative humidity (or activity) on
the abscissa (X-axis) with a sensitivity of 10 μg for weight and 0.1 % RH, respectively.
8. Reagents and Materials
8.1 One or more inorganic salts taken from Table 1 selected to provide the humidity range of interest.
8.2 Purity of Reagents—Reagent grade chemicals (or better) shall be used for preparation of all standard solutions.
8.3 Purity of Water—Reagent water produced by distillation or by ion exchange, or reverse osmosis followed by distillation
shall be used (see Specification D1193).
9. Hazards
9.1 Salt solutions are extremely corrosive to apparatus if spilled. Care shall be taken in their preparation and handling to prevent
contact with apparatus.
10. Preparation of Apparatus
10.1 Perform any setup or calibration procedures recommend by the apparatus manufacturer in the operations manual.
10.2 Positioning of the Temperature Sensor—If the system employs a temperature sensor that is movable, it shall be located as
close to the specimen as possible without touching it or the balance pan. In addition, it must be located in exactly the same position
during calibrations as used during analytical determinations.
11. Calibration and Standardization
11.1 Calibrate the temperature display of the apparatus according to Test Method E1582 using a heating rate of 0.5 °C/min 6
0.2 °C/min.
TABLE 1 Humidity Fixed Points
NOTE 1—Greenspan, L., “Humidity Fixed Points of Binary Saturated Aqueous Solutions,” Journal of Research of the National Bureau of Standards
— A. Physics and Chemistry, Vol 81A, No. 1, 1977, pp. 89–96.
Temperature Lithium Potassium Magnesium Potassium Magnesium Sodium Strontium Sodium Potassium
(°C) Chloride Acetate Chloride Carbonate Nitrate Bromide Chloride Chloride Chloride
10 11.3 23.7 33.5 43.1 57.4 62.2 75.66 75.7 86.8
15 11.3 23.4 33.3 43.2 55.9 60.7 74.13 75.6 85.9
20 11.3 23.1 33.1 43.2 54.4 59.1 72.52 75.5 85.1
25 11.3 22.5 32.8 43.2 52.9 57.6 70.85 75.3 84.3
30 11.3 21.6 32.4 43.2 51.4 56.0 69.12 75.1 83.6
35 11.3 . 32.1 . 49.9 54.6 . 74.9 83.0
40 11.2 . 31.6 . 48.4 53.2 . 74.7 82.3
45 11.2 . 31.1 . 46.9 52.0 . 74.5 81.7
50 11.1 . 30.5 . 45.4 50.9 . 74.4 81.2
55 11.0 . 29.9 . . 50.2 . 74.4 80.7
60 11.0 . 29.3 . . 49.7 . 74.5 80.3
65 10.9 . 28.5 . . 49.5 . 74.7 79.9
70 10.8 . 27.8 . . 49.7 . 75.1
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