NCSL International has published a Recommended Practice for Intrinsic/Derived Standards (RISP-5) on the two-pressure, two-temperature humidity generator. The origin of the commercial device in use today was a device developed in 1948 by E.R. Weaver and R. Riley at the National Bureau of Standards (now the National Institute of Standards and Technology - NIST) that utilized pressure rather than water vapor for the generation and control of humidity. The Riley-Weaver two-pressure device utilized air or some other gas, saturated with water vapor at a high pressure and then expanded to a lower pressure while kept at a constant temperature. The resulting relative humidity of the gas was the ratio of the lower pressure to the higher pressure. That method was improved upon by A. Wexler and R.D. Daniels, also at NBS, in 1951 with the addition of temperature control. Using temperature control enabled Wexler and Daniels to saturate a gas with water vapor at a given temperature and then raise the temperature to a higher value, allowing the measurement of temperature and pressure to be used to determine the relative humidity. The combined two-pressure, two-temperature humidity generators in commercial production today allow independent control of temperature and pressure. This device has been identified by NCSLI as an intrinsic/derived standard since the value of relative humidity is a mathematical relationship based on pressure and temperature.
RISP-5 gives a detailed description of the principles of operation and traceability of the two-pressure, two-temperature humidity generator, as well as a practical guide to its use, maintenance, personnel safety, data acquisition, and measurement uncertainty. The RISP is available from NCSL International, www.ncsli.org., tel 303-440-3339, fax 303-440-3384. A full list of NCSLI Recommended Practices and other metrology training information is available at
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