Practical study of psychrometer calibrations

Psychrometers remain the most widely used instruments for controlling the humidity in climatic test chambers, yet the calibration of these instruments is particularly challenging. Psychrometer calibrations require careful consideration of influence variables such as the fitting and cleanliness of the wick, the effect of the calibration chamber on the air flow past the sensors, on radiation incident on the sensors, and on the dissipation heat from the built-in fan (if included). In addition, uncertainty requirements for calibration of such psychrometers are typically around 1 %rh to 2 %rh, i.e., close to the best calibration and measurement uncertainties (CMCs) claimed by national metrology institutes (NMIs). As well as their role in supporting CMCs, inter-comparisons provide a good test-ground to ensure all influence variables are controlled or otherwise accounted for in the uncertainty budget. This paper presents the results of a comparison of psychrometer calibrations performed by the NMIs in Denmark, Slovenia, and Finland. The comparison was carried out under EURAMET Project No. 1033 with the aim to investigate the equivalence of psychrometer calibrations performed at the highest level and to gather practical experience to be used in similar comparisons in the future. An aspirated electro-psychrometer was used for the comparison, and calibrations were carried out in the range from 15 %rh to 93 %rh in a temperature range from 15 °C to 70 °C. While the results show good agreement at high relative humidity, significant differences at low relative humidity are reported. It is suggested that the differences are caused by a combination of psychrometer wick contamination and a difference in the wick-wetting methods used by the participant laboratories.