Types of Thermocouples

A wide variety of thermocouple probes is available, suitable for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they have the correct type of connector. The two common types of connector are ‘standard’ with round pins and ‘miniature’ with flat pins. This causes some confusion as ‘miniature’ connectors are more popular than ‘standard’ types.

When choosing a Thermocouple consideration should be given to the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings.

Type K (chromel / alumel)
Type K is the ‘general purpose’ thermocouple. It is low-cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the –200 °C to +1200 °C range. Sensitivity is about 41 µV/°C.

Type E (chromel / constantan)
Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.

Type J (iron / constantan)
Limited range (–40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept ‘modern’ thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (nicrosil / nisil)
High stability and resistance to high-temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an ‘improved’ type K, it is becoming more popular.

Thermocouple types B, R and S are all ‘noble’ metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10 µV/°C) they are usually only used for measuring high temperatures (> 300 °C).

Type B (platinum / rhodium)
Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.

Type R (platinum / rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.

Type S (platinum / rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

Thermocouple type Overall range °C

• B 20 to 1820
• E –270 to 910
• J –210 to 1200
• K –270 to 1370
• N –270 to 1300
• R –50 to 1760
• S –50 to 1760
• T –270 to 400

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