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Thermocouple Types

What is a Thermocouple?
A thermocouple consists of two dissimilar metals, joined together at one end, which produce a small voltage when heated (or cooled). This voltage is measured and used to determine the temperature of the heated metals. The voltage for any one temperature is unique to the combination of metals used.
 
Are There Standards Governing Types of Thermocouple?
British Standards Specification, BS 1041, Temperature Measurement provides guidance for the selection and use of devices for measuring temperature.
ASTM Standard E230 provides specifications for the common industrial grades, including letter designations.
 
Why are there different types?
Thermocouples are available in different combinations of metals, usually refered to by a letter, e.g. J, K etc. Each combination has a different temperature range and is therefore more suited to certain applications than others. Although it is worth noting that the maximum temperature varies with the diameter of the wire used in the thermocouple.
 
Summary of Thermocouple Types
 
Type Conductor
Combination
Temperature Range
°F °C
B Platinum 30% Rhodium /
Platinum 6% Rhodium
2500 to 3100 1370 to 1700
E Nickel-chromium / Constantan 32 to 1600 0 to 870
J Iron / Constantan 32 to 1400 0 to 760
K Nickel-chromium / Nickel-aluminum 32 to 2300 0 to 1260
N Nicrosil / Nisil 32 to 2300 0 to 1260
R Platinum 13% Rhodium /
Platinum
1600 to 2640 870 to 1450
S Platinum 10% Rhodium /
Platinum
1800 to 2640 980 to 1450
T Copper / Constantan -75 to +700 -59 to +370
 
Type B
Type B thermocouples can be used up to 1600°C with short term excursions up to 1800°C. They have a low electrical output, therefore are rarely used below 600°C. In fact the output is virtually negligible up to 50°C, therefore cold junction compensation is not usually required with this type.
 
Type E
Type E thermocouples are often referred to as Chromel-Constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required.
Note - Constantan is Copper-Nickel.
 
Type J
Type J thermocouples degrade rapidly in oxidising atmospheres above 550°C. Their maximum continuous operating temperature is around 750°C though they can with stand short duration excursions to 1000°C. They are generally not used below ambient temperature due to condensation forming on the wires leading to rusting of the iron.
Note - Constantan is Copper-Nickel.
 
Type K
Type K are the most widely used thermocouples in the Oil & Gas, and refining industries due to their wide range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration.
 
Type N
Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They offers many advantages over Type R & S at a tenth of the cost, therefore prove to be popular alternatives.
 
Type R
Type R thermocouples cover similar applications as Type S but offers improved stability and a marginal increase in range. Consequently, Type R tend to be used in preference to Type S.
 
Type S
Type S thermocouples can be continually at temperatures up to 1450°C. They can with stand short duration excursions up to 1650°C. They need protection from high temperature atmospheres to prevent metallic vapour ingress to the tip resulting in reduction of emf generated. Protection commonly offered is high purity recrystallised alumina sheath. For most industrial applications, thermocouples are housed in a thermowell.
 
Type T
Type T thermocouples are rarely used in industrial applications, and lend themselves more to use in laboratory situations.
 

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Further Reading


17th Edition IEE Wiring Regulations: Explained and Illustrated
 

Electrical Installations
 

Plumbing NVQ
Type J thermocouple Type K thermocouple
 

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Wires and Cables
 

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