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Thermocouples. Constructions, types, properties

Thermocouples are widely used for temperature measurement due to their properties. These means provide high measurement accuracy, allow making them in a wide temperature range, have a rather simple construction and are rather reliable.

Среди большого количества типов термопар стоит выделить термопары Chromel-alumel, chromel-copel, WR5/WR20 are the most highly-demanded thermocouples due to their properties.

Thermocouple work principle and construction

The simplest thermocouple is a set of two heterogenous conductors that form a complete electrical circuit. For forming such a circuit the tails are coupled by soldering, welding or spiraling.

If one end (junction) is put into a medium with temperature T1 and another one - with temperature T2, the thermo-emf will initiate a current. This phenomenon is called Seebeck effect. The thermo-emf depends on the junction temperature difference and conductor material only. So the change of thermo-emf value can be used for determining the temperature change. Usually, conductors are called thermocouples and the conductor connections are called junctions.

Diagram of a basic thermocouple

Diagram of a basic thermocouple. t1 > t2. А - positive thermocouple wire, В - negative thermocouple wire. Junction with t1 temperature - hot junction (tip), with t2 - temperature - cold junction (loose end). The arrows show the current direction.

Practically measurements are performed with a thermocouple thermometer where the thermocouple serves as the sensing element. Such system also contains other components used for measuring thermo-emf and converting the values into degrees.

The most important factors that determined the thermocouple construction are its operation conditions. The main ones are the range of measured temperatures and properties of the environment where the measurements are made. These factors affect the thermocouple connection in the hot junction, thermocouple insulation and device protection.

Thermocouple connection can be made with welding, soldering or spiraling. Depending on the operating conditions the thermocouples can be insulated from each other with air or special ceramic tubes. Depending on the properties of the measurement medium thermocouples can have a protective casing made of steel, special heat-resistant alloys, porcelain or refractory alloys.

Thermocouple configuration

Thermocouple configuration. 1 - protection sleeve, 2 - stationary fitting (some designs with movable fitting), 3 - head, 4 - insulation socket with clamps for connecting thermocouples and extending wires, 5 - nozzle with a gland seal, 6 - connection pipe, 7 - thermocouple wires.

Thermocouple types and properties

The most common classification of thermocouple is by type of thermocouple wire material (e.g., precisious, refractory metals, etc.). Below the thermocouples classified by such principle are given.

Thermocouples of base metals

Thermocouples made of base metals are the most common ones. Chromel-alumel, chromel-copel, iron-constantan are the most widely used ones. The maximum short-term use temperature for the sensors is given below in brackets.

Chromel-alumel (CAT, K type)
  • Used for measuring temperatures from -200 °С to +1100 (+1300) °С.
  • A differential gap occurs within the range from 200 to 500 °С when the readings are different for heating and cooling. Sometimes the differential reaches 5 °С.
  • Works in neutral or oxygen-rich atmosphere.
  • After thermal ageing the values decrease.
  • Thermo-emf can change in rare atmosphere as chromium can be extracted from Ni-Cr output (so-called migration). In such case the thermocouple shows an underestimated temperature.
  • Sulphur atmosphere is harmful for the device as affects both electrodes.
Chromel-copel (CCT, L type) and chromel-constantan (CCnT, E type)
  • Used for measuring temperatures from -200 °С to +800 (+1100) °С.
  • It is the most sensitive industrial thermocouple.
Iron-constantan (ICT, J type)
  • Measurement range: -203 °С - +750 (+1100) °С.
  • Works in reduction and oxidation media.
  • Efficiently works in rare atmosphere.
  • If the temperature is higher than 500 °С and the measurement media contains sulphur, the thermocouple should have a gasproof protection.
  • Highly-sensitive.
  • Is rather cheap due to iron content.
  • The iron electrode may get corroded due to moisture condensation.
  • The values increase after thermal ageing.
Copper-constantan (CCnT, Т type) and copper-copel (CCT, M type)
  • Measurement range: -250 °С - +400 (+600) °С.
  • Works in oxidation and reduction atmosphere so as in vacuum.
  • The most precise thermocouple for measuring temperatures from 0 °С to 250 °С.
  • Not recommended to use under temperature over 400 °С.
  • Not sensitive to increased humidity.
  • Both thermocouples can be annealed for removing materials that cause thermocouple heterogeneity.
Nichrosil-nisil (NNT, N type)
  • Working temperatures - up to +1200 (+1250) °С.
  • It is a relatively new type based on K thermocouple. It can be easily “contaminated” with admixtures under high temperatures. For preliminary “contamination” of the thermocouple both electrodes are soldered with silicon thus reducing the risk of further “contamination” during work.
  • High stability within the range from 200 to 500 °С (a much lower differential gap than for K type thermocouple).
  • It is considered to be the most precise device among those made of base metals.

Thermocouples of refractory metals

This class includes thermocouples for measuring high temperatures.

WR5-WR20 (WRT, A type)
  • Measured range: 1300-2500 (3000) °С.
  • It can work in inert atmosphere or vacuum.
  • It has good mechanical properties under high temperatures.
Wolfram-molybdenum (WMT)
  • Used for measuring high temperatures from +1400 °С to +1800 (+2400) °С.
  • Works in inert or hydrogen medium or vacuum.
  • Is rather cheap if compared with other thermocouples of this class.
  • Has a low sensitivity.

Thermocouple made of precious metals

Sensors made of these materials are the most precise ones and are often applied as reference ones.

Platinum platinum-rhodium (PPT, S, R types)
  • Measuring range: 300 °С - +1400 (+1600) °С.
  • Works in oxidation and inert atmosphere. It can be used in reduction media if equipped with protection.
  • It is not recommended to use it under temperature below 300 °С as the thermo-emf in this range is very low and non-linear.
  • Precise measurements.
  • Has a reproducible and stable thermo-emf.
  • Used as a reference thermocouple.
  • Has a high cost.
  • Sensitive to chemical contaminations with metal and non-metal admixtures.
Platinum-rhodium-platinum-rhodium (PRT, B type)
  • Working range - 600 °С - +1600 (+1800) °С.
  • Works in oxidation and neutral medium, can be applied in vacuum. Can be used in reduction media with protection.
  • It is not recommended to use it under temperature below 600 °С, where thermo-emf is very low and non-linear.
  • Very precise measurements.
  • Has a reproducible and stable thermo-emf.
  • Used as a reference thermocouple.
  • Rather expensive.
  • Sensitive to chemical contamination with metal and non-metal admixtures.


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