<Li> A time - independent perturbation in thermal EMF in specific temperature ranges . This is due to composition - dependent magnetic transformations that perturb the thermal EMFs in type - K thermocouples in the range about 25--225 ° C, and in type J above 730 ° C . </Li> <P> The Nicrosil and Nisil thermocouple alloys show greatly enhanced thermoelectric stability relative to the other standard base - metal thermocouple alloys because their compositions substantially reduce the thermoelectric instabilities described above . This is achieved primarily by increasing component solute concentrations (chromium and silicon) in a base of nickel above those required to cause a transition from internal to external modes of oxidation, and by selecting solutes (silicon and magnesium) that preferentially oxidize to form a diffusion - barrier, and hence oxidation - inhibiting films . </P> <P> Type T (copper--constantan) thermocouples are suited for measurements in the − 200 to 350 ° C range . Often used as a differential measurement, since only copper wire touches the probes . Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics . Type - T thermocouples have a sensitivity of about 43 μV / ° C. Note that copper has a much higher thermal conductivity than the alloys generally used in thermocouple constructions, and so it is necessary to exercise extra care with thermally anchoring type - T thermocouples . </P> <P> Types B, R, and S thermocouples use platinum or a platinum / rhodium alloy for each conductor . These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 μV / ° C. Type B, R, and S thermocouples are usually used only for high - temperature measurements due to their high cost and low sensitivity . </P>

In a thermocouple thermo emf is produced when its