Bilateral comparison of relative humidity standards between NMISA and MIKES

This article describes a bilateral comparison of the relative humidity measurement standards of the National Metrology Institute of South Africa (NMISA) and the Centre for Metrology and Accreditation (MIKES-Finland). The purpose of this comparison was to test the relative humidity calibration capabilities of NMISA, particularly at temperatures other than ambient (19 °C to 25 °C). A lack of participation in top level relative humidity intercomparisons at NMISA (identified at a previous assessment), also necessitates this comparison. This article will present the results of the comparison, as well as further measurements performed to determine the causes of differences between laboratories, revealed by the comparison. Five thermohygrometers with capacitive sensors were used as comparison artifacts. The comparison range was 10 %rh–95 %rh at temperatures from 5 °C to 55 °C. NMISA calibrated the artifacts against reference unsaturated salt solutions, while MIKES calibrated the artifacts in a climatic chamber using two chilled mirror hygrometers as reference standards. Results obtained by the laboratories agreed fairly well (differences of approximately 1.5 %rh or less) for one thermohygrometer, which had a wire mesh filter, but measurement points performed with the other four, which had less porous filters, showed significant differences. Measurements at NMISA were performed at constant humidity and rising temperature, in static conditions (no air flow), without removing the filters from the sensors, while MIKES performed measurements at constant temperature and rising humidity, in flowing air, without filters removed. Further measurements are being performed at NMISA to evaluate the comparison differences. With these measurements, the effects of filters, air flow, and differences in measurement order are investigated.