Improvements in the measurement of the immersion profile of water triple points using bushes

Abstract

The triple point of water is the most important fixed point, both used as the defining fixed point for the definition of the Kelvin and also the pivotal fixed point for the realization of the International Temperature Scale of 1990 (ITS-90). As such, obtaining the best performance from the water triple point is vital. This comparative study focuses on the benefits of using long bushes (that cover most of the length of the thermowell) for improving the repeatability of the immersion profile for water triple point cells. It is shown that the use of long bushes helps by improving a) the thermal contact between the thermometer and the ice sheath and b) the repeatability of locating the thermometer inside the thermowell. To investigate the reproducability of the immersion profile for triple point of water cells, two different types of Standard Platinum Resistance Thermometers (SPRTs) have been used. The SPRTs have different designs, different length of sensing element and different self heating effects. These experiments were carried out using the same water triple point cell to identify thermometer specific effects. To measure the immersion profile, the following factors have to be taken into account: a) the effect of hydrostatic pressure, since the measurements are made at different depths below the water surface; b) the effect of heat flux along the thermometer stem; c) the need of long thermal stabilization at every immersion depth. The experimental results reported here show that repeatability in the determination of immersion profiles was significantly improved (by a factor of at least two) through the use of long bushes and when measured with a SPRT with a low self-heating effect.