Inter-laboratory Comparison of Impedance-Type Hygrometer in the Range from 10 % to 95 % at $$5\,^{\circ }\hbox {C}$$ 5 ∘ C to $$55\,^{\circ }\hbox {C}$$ 55 ∘ C

Abstract

Traceability in the field of relative humidity (RH) measurements is typically assured indirectly through dew point and temperature scales. Conducting an inter-laboratory comparison at the national metrology institute (NMI) level, using a direct approach with a precision RH hygrometer as a transfer standard would, therefore, be of a particular interest, especially if the measurement setups were of a different type. This paper presents an RH comparison at the NMI level between the National Metrology Institute of South Africa (NMISA) and University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Metrology and Quality (MIRS/UL-FE/LMK). In scope of this inter-comparison, calibration of an impedance-type hygrometer in the range from 10 %rh to 95 %rh at air temperatures of \(5\,^\circ \hbox {C}\), \(25\,^\circ \hbox {C}\), and \(55\,^\circ \hbox {C}\), respectively, was performed. It was recommended that the participants use their standard procedure for the calibration of RH sensors and, at the same time, follow the specific criteria of the review protocol for uncertainty estimation accepted by Bureau International des Poids et Mesures (BIPM), marked as BIPM CCT-WG8/CMC-10. An interesting part of the comparison was the two different calibration methods which were used by the two partners and which also have different traceability routes. MIRS/UL-FE/LMK calibrated the sensor in the humidity generator by comparison against the reference chilled mirror hygrometer, which is traceable to the MIRS/UL-FE/LMK primary dew-point generator. NMISA calibrated the transfer standard against certified salt solutions, which were kept in a temperature-controlled chamber. Results showed acceptable agreement at all 15 calibration points.