The Use of Thermowell Bushes at the Triple Point of Water for Improving Repeatability
Water triple point cells are essential for realization of the International Temperature Scale of 1990 (ITS-90). There is some evidence that achieving the ultimate performance of water triple point cells may be restricted by the variation in the position of the platinum resistance thermometer at the bottom of the re-entrant well, and that the variation in position is not completely compensated for by correction to zero measurement sensing current. This comparative study focused on the use of quartz bushes (tubular sleeves around the thermometer) of two different lengths, to improve the thermal contact and to help locate the thermometer. It shows that an improvement in repeatability of the resistance readings was achieved. The experiments were conducted over a five-week period using a standard platinum resistance thermometer, a one water cell, and two different lengths of quartz bushes. The resistance measurements were performed using an Automatic Systems Laboratories F900 resistance bridge. A description of the experiment and results is given. Significant improvement in the repeatability of the measurement of resistance was observed (factor >2) when quartz bushes were used.