Authors: J. van Geel, R. Bosma, J. van Wensveen & A. Peruzzi
Reference: Int. J. Thermophys. (2015) 36:569-576.
In 2011, VSL initiated the development of a facility for a relative humidity between -40 °C and +180 °C for calibrating high-temperature relative humidity sensors at pressures other than atmospheric. The setup for calculating the relative humidity uses the dew-point temperature, measured by a chilled mirror hygrometer, and the temperature distribution in the chamber, measured by a series of thermistors. This paper describes the results of thermal tests performed on the thermistors to ensure that they meet the requirements of the humidity calibration facility. Different types of thermistors were evaluated up to 105 °C , and the selected type showed a short-term drift of less than 2 mK. Exposure of these thermistors to temperatures up to 180 °C gave an initial hysteresis of 40 mK, but after this initial hysteresis, the hysteresis, over the range from -40 °C up to 180 °C , was less than 10 mK. Use of a digital multimeter, with a low-power option, limited the self-heating of the thermistors, over the range from -40 °C up to 160 °C , to less than 5 mK. During use in the new setup, the thermistors were exposed to changing humidities between 1 %Rh and 90 %Rh and temperatures up to 180 °C , showing drifts of less than 10 mK.
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