When it comes to thermocouple wire, there’s a veritable alphabet of calibration types available: K type thermocouple, J type thermocouple, T type and so on. We’re frequently asked: What do these designations mean and which type is right for a specific application?
First, you have to understand what is meant by the phrase ‘calibration type’. This refers to the designation name (or type) that defines the unique chemical composition of the dissimilar metals of the positive and negative thermo element bare conductors used in the thermocouple wire. Commercially available calibrations are grouped into two groups according to the conductor’s material characteristics.
There are currently five base-metal thermocouple types and three noble-metal thermocouple types defined and accepted by governing bodies such as ANSI (American National Standards Institute) and IEC (International Electrotechnical Commission). The base-metal types, J, K, N, E, T are nickel metal based. The noble-metal types R, S, B are platinum metal based.
- Type J thermocouple: Widely used because of its versatility and low metal cost. The positive thermoelement is 99Fe. The negative thermoelement is 44Ni/55Cu alloy. Type J is also known as Iron-Constantan. Type J bare conductors have an accepted upper temperature limit of approximately 1300°F (700°C).
- Type K thermocouple: Also known as Chromel-Alumel, Type K is widely used for its higher temperature range. The positive thermoelement is 90Ni/9Cr. The negative thermoelement is 94Ni with alloying constituents of Si,Mn,Al, Fe and Co. Type K bare conductors have an accepted upper temperature limit approximately 2350°F (1285°C).
- Type N thermocouple: Also known as Nicrosil - Nisil. Provides EMF stability superior to that of Type K at elevated temperatures. The positive thermoelement is 84Ni/14Cr/1.4Si. The negative thermoelement is 95Ni/4.4Si. Type N bare conductors have an accepted upper temperature limit of approximately 2350°F (1285°C).
- Type E thermocouple: Also known as Chromel – Constantan, Type E provides the largest EMF output of any base metal thermocouple. The positive thermoelement is 90Ni/9Cr. The negative thermoelement is 44Ni/55Cu. Type E bare conductors have an accepted upper temperature limit of approximately 1600°F (870°C).
- Type T thermocouple: is preferred for sub-zero cryogenic temperatures and up to 700°F (370°C). The positive thermoelement is oxygen free copper OFHC CU. The negative element is 44Ni/55Cu. Type T is also known as Copper – Constantan. Type T bare conductors are accepted temperature limits of cryogenic up to 700°F (370°C).
The following three types are also called Nobel metal thermocouples and consist of differing amounts of platinum and rhodium. All three are relatively expensive and susceptible to contamination at high temperatures.
- Type R and Type S thermocouples are used for high temperatures up to 2700°F (1480°C).
- Type B thermocouple can be used up to 3100°F (1700°C).
To determine which thermocouple wire is right for your application, select one that provides a temperature rating that meets (or exceeds) the temperatures to be measured in your application.
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