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ASHRAE Standard 41.6-2025 -- Standard Methods for Humidity Measurement (ANSI/ASHRAE Approved), 2025
- ASHRAE Online Bookstore
- Addenda
- Errata
- Return to Previous Page
- ANSI/ASHRAE Standard 41.6-2025 [Go to Page]
- Contents
- Foreword
- 1. Purpose
- 2. Scope [Go to Page]
- 2.1 This standard applies to the measurement of humidity of moist air from sea level to 3048 m (10,000 ft), within the dry-bulb temperature range of –50°C to 160°C (–58°F to 320°F), and within the dew-point temperature range of –50°C to 99...
- 2.2 This standard applies to methods for the measurement of wet-bulb temperature, dew-point temperature, and relative humidity.
- 3. Definitions
- 4. Classifications [Go to Page]
- 4.1 Humidity Measurement Methods. Humidity measurement methods that are within the scope of this standard are the methods that are listed below and are described in Section 7.
- 4.2 Humidity Measurement Applications. Humidity measurement applications that are within the scope of this standard shall be classified as one of the types in Sections 4.2.1 and 4.2.2.
- 5. Requirements [Go to Page]
- 5.1 Test Plan. The test plan shall be one of the following options:
- 5.2 Values to be Measured and Reported if Specified in the Test Plan in Section 5.1
- 5.3 Steady-State Test Criteria. Humidity test data shall be recorded at steady-state conditions if specified in the test plan in Section 5.1.
- 5.4 Unsteady Humidity Measurements. If required by the test plan in Section 5.1, humidity test data shall be recorded under the following conditions:
- 6. Instruments and Calibration [Go to Page]
- 6.1 Instrumentation Requirements for All Measurements
- 6.2 Time Measurements. Time measurement system accuracy shall be within ±0.5% of the elapsed time measured, including any uncertainty associated with starting and stopping the time measurement, unless (a) otherwise specified in the test plan in Sect...
- 7. Humidity Measurement Methods [Go to Page]
- 7.1 Aspirated Wet-Bulb Psychrometer. ASHRAE Research Report RP-1460 4 provides design specifications for a wet-bulb aspirator apparatus that measures wet-bulb temperatures in a device that is operating at a constant velocity of 3.5 ± 0.25 m/s (690 ...
- 7.2 Chilled-Mirror Dew-Point Hygrometer. Chilled-mirror dew-point hygrometers directly measure the dew point and frost point for a moist air sample. The methods for chilling the mirror include (a) evaporation of solvent, (b) adiabatic expansion of ...
- 7.3 Polymer Film Hygrometer. The sensing element in a polymer film hygrometer consists of a hygroscopic organic polymer deposited with thin or thick film-processing technology onto a water-permeable substrate. There are two types of polymer film hygr...
- 7.4 Aluminum Oxide Hygrometer. The sensor in an aluminum oxide hygrometer consists of an aluminum strip that is anodized to provide a porous oxide layer. A very thin coating of gold is evaporated over this structure. The aluminum base and the gold la...
- 7.5 Porous Ceramic Hygrometer. Porous ceramic hygrometers use either ionic or electrical measurement techniques to relate adsorbed water to relative humidity. Ionic conduction is produced by dissociation of water molecules forming surface hydroxides....
- 7.6 Silicon Sensor Hygrometer. The sensing element is a semiconductor that acts as a moisture detector. Moisture in the air sample is detected as an impedance change that is a function of the vapor pressure of water in the sample air.
- 7.7 Dunmore Hygrometer. Dunmore, at NIST, developed the first lithium chloride resistance hygrometer in 1938. The instrument operates on the principle that a lithium chloride solution immersed in a porous binder changes its ionic conductivity dependi...
- 7.8 Electrolytic Hygrometer. The electrolytic humidity sensor utilizes a bifilar winding coated with a thin film of phosphorus that is wrapped around a nonconductive tube. A constant incoming flow rate of moist air is absorbed by the phosphorous desi...
- 7.9 Ion Exchange Resin Electric Hygrometer. A conventional ion exchange resin hygrometer consists of a high-molecular-weight polymer that contains polar groups of positive or negative charge configured in a cross-linked structure. The polar groups co...
- 8. Uncertainty Analysis [Go to Page]
- 8.1 Post-Test Uncertainty Analysis. A post-test analysis of the measurement system uncertainty, performed in accordance with ANSI/ASME PTC 19.1,1 shall accompany each refrigerant mass flow rate measurement if specified in the test plan in Section 5.1...
- 8.2 Method to Express Uncertainty. All assumptions, parameters, and calculations used in estimating uncertainty shall be clearly documented prior to expressing any uncertainty values. Uncertainty shall be expressed as shown in Equation 8-1:
- 9. Test Report [Go to Page]
- 9.1 Test Identification
- 9.2 Unit Under Test (UUT) Description
- 9.3 Instrument Description
- 9.4 Measurement System Description
- 9.5 Test Conditions
- 9.6 Test Results if Specified in the Test Plan in Section 5.1
- 10. References
- Informative Appendix A: Bibliography
- Informative Appendix B: Primary Humidity Calibrators [Go to Page]
- B1. NIST Gravimetric Hygrometer
- B2. Primary Standard Humidity Generators
- B3. Two-Pressure Humidity Generator
- B4. Two-Temperature Humidity Generator
- B5. Divided-Flow System
- B6. Saturated Salt Solutions
- Informative Appendix C: User Guidance Regarding Humidity Measurments [Go to Page]
- C1. Guidance for Below-Freezing Measurements with Wet-Bulb Sensor [Go to Page]
- C1.1 When it is necessary to measure wet-bulb temperature at or below freezing in a moving airstream, the problem of ice forming on the wet-bulb sensor is eliminated by continuously withdrawing a sample of air and adding heat. The dry-bulb temperatur...
- C1.2 Where it is necessary to directly measure wet-bulb at temperatures below freezing, a wet-bulb temperature sensor with an ice-coated bulb (0.5 mm [0.02 in.] thickness), without a wet-bulb sock, is used. The wet-bulb sock, at below freezing temper...
- C1.3 As a result of the reduced vapor pressure at low temperatures, a longer time is necessary to reach equilibrium than at higher temperatures. This condition is offset, however, by the ice remaining on the bulb for a much longer time.
- C1.4 The ice film is best formed by dipping the chilled temperature sensor into distilled water at 0°C (32°F). The temperature sensor is then removed from the water, and the film is allowed to freeze. The process is repeated to build up a film thic...
- C1.5 Air velocity, radiation shields, and other geometry described in Informative Appendix E must be maintained.
- Informative Appendix D: Humidity Measurement Methods Accuracies and Operating Ranges
- Informative Appendix E: Tips to Specifying the Accuracy Requirement in Section 5.1
- Informative Appendix F: Example of an Uncertainty Estimate for a Wet-Bulb Measurement [Go to Page]
- F1. Problem Statement and Assumptions
- F2. Method
- F3. Uncertainty Procedures in SI and I-P Units [Go to Page]
- F3.1 Uncertainty Procedures 1 in SI units
- F3.2 Uncertainty Procedures4 in I-P Units [Go to Page]
