The Humidity Calibration Facility of the National Metrology Institute of South Africa (NMISA)

Abstract

The NMISA Humidity Laboratory maintains the South African national measurement standards for humidity and provides traceability for commercial calibration laboratories. The laboratory performs both relative humidity and dew-point calibrations. The laboratory has been accredited since January 2004 for both parameters by the South African National Accreditation System (SANAS). Industries such as the pharmaceutical industry, power stations, and the food industry make use of the calibration services provided by the laboratory. The facilities and the measurement capabilities of the laboratory are presented in this paper. Two calibration methods are used for the calibration of relative humidity measuring instruments (RH meters). Depending on the type of instrument, they are calibrated against aqueous salt solutions or by comparison with reference RH meters. Measurements are performed over the humidity range 5–95 %rh (Best Measurement Capabilities: 0.4–1.4 %rh). Two chilled-mirror hygrometers are used as reference standards for dew-point calibrations over the range from −75°Cfp to +17°Cdp (Best Measurement Capabilities: 1.5°Cdp to 0.4°Cdp). The laboratory has participated in two international interlaboratory comparisons, one for each accredited parameter. The NMISA Humidity Laboratory is working to reduce its calibration uncertainties for relative humidity instruments: since first accredited in 2004, the best measurement capabilities have been improved from 2.1 %rh to 0.4 %rh for some humidity ranges.