Over the years, many types of thermocouples have been developed in order to solve measurement problems.
Noble metal thermocouples, types B, R and S, based on platinum or platinum/platinum rhodium thermocouples share similar properties.
The diffusion of metal vapour at high temperatures can change the calibration of the platinum wires; therefore they should only be used inside a non-metallic sheath e.g made from high purity ceramic. An exception to this rule is a platinum sheath, but that is an expensive option.
Thermocouples based on platinum are by far the most stable of all. The type S is so stable that it is used as a standard for calibrating the temperature between the solidification point of antimony (630.74°C) and that of gold (1064.43°C).
The type B thermocouple is the only common thermocouple which shows a double evaluation ambiguity.
Non-precious metal thermocouples
In contrast to noble metal thermocouples, non-precious metal thermocouples do not have a precise chemical composition. It is possible to use any combination of metals which give temperature curve results within standard error limits. This results in a number of rather interesting combinations. For example, as we saw above, constantan does not have defined proportions, but is a generic name for copper-nickel alloys.
The type E thermocouple is suitable for low temperature measurements due to its high Seebeck coefficient (58 µV/°C), low conductivity and corrosion resistance. The Seebeck coefficient for the type E is the highest of all standard thermocouples, which makes it useful for detecting small changes in temperature.
Iron, the positive element in a type J thermocouple is an inexpensive metal, but one which is rarely very pure. The properties of type J thermocouples may vary due to impurities in the iron. Nevertheless, type J thermocouples are popular due to their high Seebeck coefficient and low cost. A type J thermocouple should never be used above 760°C because of sudden magnetic transformation which can cause decalibration even on return to lower temperatures.
The type T has a copper wire, a significant advantage in the monitoring of temperature differences in that enables two type T thermocouples to be installed in series thus eliminating the need for cold junction compensation.
Types K and N
Type K thermocouples have always been widely used. They are suitable for measuring high temperatures due to their resistance to oxidation. The type N is becoming increasingly popular as a replacement for the type K. The result is slightly below that of the type K (lower Seebeck coefficient), but its resistance to oxidation is even greater. The yield curve of the type N thermocouple depends on the wire gauge and there are two distinct characteristic Nicrosil-Nisil curves depending on the wire gauge.
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