There is a trade-off between the oxidation effect and element contamination in metal-sheathed standard platinum resistance thermometers (MSPRTs). Excessively high O 2 partial pressure causes the platinum sensor to oxidize, and excessively low O 2 leads to sensor contamination. The oxygen content in a thermometer may become unknown after a period of operation due to slow oxidation of the metal sheath and consequent loss of oxygen in the MSPRT. This can significantly affect the thermometer's performance. Our recent research has shown that a thermometer may eventually become contaminated due to a deficiency of oxygen surrounding its element. In order to research this phenomenon and improve the stability of MSPRTs, ten MSPRTs were specially manufactured for testing. In this paper, the construction of these MSPRTs is described. A series of experiments and their results are presented. Based on the experimental results, a feasible solution (having the element sealed separately from its sheath) is put forward. This solution can resolve the conflict between the oxidation effect and element contamination and improve the long-term stability of MSPRTs. The R tp and W(Al) stabilities of the MSPRTs with this new design can be as good as 1 mK and 2 mK respectively after operation at high temperature over 1000 h.

To prevent short circuits, to improve stability, and to raise the upper temperature limit to the freezing point of copper (1084.62 • C), the high-temperature standard platinum resistance thermometer (HTSPRT) was redesigned. The most important change was an improvement in the structure of the sensor support. The strip support was replaced by a new specially designed cross support. The structure and design of the new HTSPRT are briefly described in this article. The test results of a group of thermometers are presented. The test included long-term drifts of the thermometers at the triple point of water and freezing point of silver during a period of a few hundred hours operation at 1085 • C, the short-term stability of R (tpw) and W (Ag) in a period of 5 days, and thermal cycles between 22 • C and 1085 • C. The test results show that the thermometer performance is improved, and the new HTSPRT can operate up to the freezing point of copper.