Samenvatting

The thermally-sensitive resistors (thermistors) used in VSL’s water calorimeter to detect the radiation-induced temperature change in water are affected by self-heat. The measurement current used to determine their resistance causes their temperature to increase above that of their environment. The self-heat of the thermistors must be corrected for when performing water calorimetry to isolate the temperature change that is caused by the radiation.
In this study, the relationship between the self-heat temperature and the power dissipated by the thermistors is investigated. A measurement method and setup are designed to measure the thermistor’s temperature as a function of dissipated power in an automated fashion. The data is analyzed using two methods. The first is based on a linear fit of the data from the measurement setup, resulting in a self-heat temperature that is proportional to the dissipated power. The other method is based on a 2nd order polynomial fit of the same data, where non-linear influences which are assumed to be caused by convection are taken into account. Analysis of the data leads to the determination of a self-heat correction factor ksh, which is used to isolate the change in water temperature during water calorimetry.
The lead resistances of the measurement setup are incorporated in the calculations to increase the accuracy of the results. The new measurement setup and automated data acquisition program are successfully validated. With the new setup, the self-heat correction factor is determined for two thermistors. ksh is determined to be 1.00638 ± 0.00018 (k = 1) for thermistor VSL13T025 and 1.01016 ± 0.00018 (k = 1) for thermistor VSL13T033. With the automated method, the standard relative uncertainty of ksh is reduced to 0.018% while also requiring minimal human input, making it well-suited for on-site measurements of the self-heat correction factor.