Temperature Measurement
The temperatures of oven and sample are usually measured by thermocouples, although radiation thermometers and resistance thermometers have been used in some instruments. Probably, if higher temperatures are to be employed, the Platinum/30% Rhodium thermocouples would be the most common and would measure temperatures up to 1700oC.
There are usually two thermocouples placed in the oven, one close to the sample (to measure sample temperature which is the most difficult to obtain accurately) and the other is placed in the oven proper to control oven temperature. Unfortunately, there is always a difference between the oven temperature and the sample temperature and it is important to design the overall system so that this difference is made as small as possible. In practice there is also a difference between the actual sample temperature and the measured sample temperature, the difference being largely controlled by the position of the measuring thermocouple and to a lesser extent the program rate.
The difference between sample and oven temperature and the accurate measurement of sample temperature increases with the program rate of the analysis. In practice either the sample temperature [sample controlled thermal analysis (SCTA)] or the oven temperature can be used to control the temperature program, the choice depending on the nature of the sample and program rate. SCTA is considered by some to provide more accurate and consistent results.