Is Type K The Right Choice?

The type K thermocouple is the most versatile of the several types in use. Type K has been around for a long time and is popular due to its accuracy and wide temperature tolerance. It supports a temperature range of -200 to 1250 ºC. It is also inexpensive. Its positive conductor is composed of chromel, a metal alloy consisting of 90% nickel to 10% chromium. Its negative conductor is alumel, which consists of about 95% nickel, 2% aluminum, 2% manganese and 1% silicon.

However, the type K thermocouple is not suitable for all applications. Its main constituent, nickel, is magnetic. This means that at a certain temperature, known as the Curie point, the thermocouple will change behavior by becoming paramagnetic. When it becomes paramagnetic the metal loses its magnetic properties in the absence of a magnetic field. If there is another magnetic field present, the metal will be weakly attracted to it. The Curie point occurs for type K thermocouples at around 350 ºC.

Type K also may not be sensitive enough for certain applications such as cryogenics. For example, the type J thermocouple has a much narrower temperature range but is more sensitive, making it more suitable for cryogenic applications. Further, type K has standard limits of error of the greater of 2.2 ºC or .75%. and specific standard limits of error of the greater of 1.1 ºC or .4%. Some highly specialized applications require greater accuracy. While this is hard to achieve with thermocouples, there are some newer formulations that provide a consistently smaller deviation.
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