A Simple Humidity Generator for Relative Humidity Calibrations

At present, the South African national humidity measurement standards, maintained by NMISA, consist of two chilled-mirror hygrometers operating from (−75 to 20) °Cdp and unsaturated salt solutions covering the range (5 to 95) %rh. To reduce measurement uncertainties and to obtain traceability to local measurement standards for temperature and pressure, it is desired to replace the salt solutions by a dew-point generator for relative humidity calibrations from (5 to 95) %rh (5 to 60) °C, equivalent to a dew-point range of (−30 to 59) °Cdp. Required uncertainties in relative humidity (at a coverage factor of k = 2) are (0.1 to 1.2) %rh. This article describes the design and evaluation of a two-pressure humidity generator intended to satisfy this requirement. The saturator consists of a coil of stainless steel tubing immersed in a 70 l stirred water bath. Pressure reduction is accomplished using a throttling valve adjusted by a stepper motor, and hygrometers being calibrated are sealed into a small test chamber contained in a larger temperature-controlled chamber. The results of the following performance tests are presented:

(i)	comparison of the output when the air stream was oversaturated before entry to the saturator to that when it was dry,
(ii)	comparison with a chilled-mirror hygrometer from (−25 to 20) °Cdp, and
(iii)	comparison with relative humidity hygrometers that had previously been calibrated against salt solutions.