The NBS Two-Pressure Humidity Generator, Mark 2

A new humidity calibration facility which uses the two-pressure principle for generating gas of known humidity has been developed at NBS for calibrating and testing hygrometers. The relative humidity range of the two-pressure humidity generator is 3 to 98 percent for ambient temperatures −60 ° to 80 °C and test chamber pressures 5 to 200 kPa (absolute). This is equivalent to a nominal dew/frost point range of −80 ° to 80 °C. Intercomparison tests were made with the NBS standard gravimetric hygrometer over a portion of the generator’s operating range. The estimated maximum uncertainty (three standard deviations) is 0.2 percent RH for temperatures 0 ° to 80 °C which in units of dew point corresponds to an estimated maximum uncertainty of 0.04 °C for dew points −35 ° to 80 °C.