A simple humidity generator for relative humidity calibrations

作者：Liedberg H G^*; Mnguni M R; Jonker D

来源：International Journal of Thermophysics, 2008, 29(5): 1660-1667.

DOI：10.1007/s10765-008-0423-z

• calibration
• dew point
• humidity generator
• relative humidity

At present, the South African national humidity measurement standards, maintained by NMISA, consist of two chilled-mirror hygrometers operating from (-75 to 20)degrees 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)degrees C, equivalent to a dew-point range of (-30 to 59) degrees 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 70l 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:
comparison of the output when the air stream was oversaturated before entry to the saturator to that when it was dry,
comparison with a chilled-mirror hygrometer from (-25 to 20)degrees Cdp, and
comparison with relative humidity hygrometers that had previously been calibrated against salt solutions.

• 出版日期2008-10