Influence of Lead Temperature on the Accuracy of Various Stainless-Steel Sheathed, Mineral-Inulated Nickel-Chromium/Nickel Aluminium Thermocouples

摘要

Samples of three types of stainless steel sheathed MI thermocouples, such as are currently used in fire and furnace tests of transport flasks, have been subjected to high lead temperatures while the thermojunctions were kept at a constant low temperature. Both the lead temperature and the length of lead at temperature have been varied. As the lead temperature rises from ambient to a selected value, the emf output from the thermocouple initially decreases and then increases, taking up a final value dependent on the particular conditions. Below a threshold lead temperature, no significant steady state error occurs and the negative transient is generally negligible. Each thermocouple has its own threshold temperature, the lowest found being about 600 exp 0 C, although the average lies at about 750 exp 0 C. Above the threshold lead temperature, the thermal emf can be in error by the equivalent of more than 100 exp 0 C, the highest error found being nearly 230 exp 0 C at a temperature 250 exp 0 C above threshold. The same thermocouple showed a negative transient of 13 exp 0 C 3 minutes after start of heating to 890 exp 0 C. It is probable that the steady state error arises because of the degradation of the thermocouple mineral insulation at elevated temperatures and recommendations are made on the use of such thermocouples in fire and furnace tests. The cause of the initial negative transient error has not been identified, but ways of minimising any resultant errors are suggested. (Atomindex citation 09:365731)