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Thermocouples

Thermocouples are essentially distinguished by their design and the conductor material used.

2 Principles

We have been working with every possible combination of thermocouple for over 25 years, and to provide you with a better understanding of this subject, we'd like to explain the functional principle behind them. A thermocouple is made up of two metallic conductors. The conductors are made up of different materials that are connected at both ends (Figure 1). At one end is a measuring point and at the other the reference junction.

If there is a difference in temperature between the measuring point and the reference junction, the material pairing causes a temperature-dependent voltage between conductor A and conductor B known as thermoelectric voltage. This thermoelectric effect was discovered in 1821 by Thomas Johann Seebeck and now bears his name (Seebeck effect). The thermocouple's measuring point must accept the temperature of the medium being measured in order to measure its temperature. If it does, the voltage measured can be used to determine the temperature. For correct measurement, the temperature of the reference junction must remain constant in order to measure any change in thermoelectric voltage following a change in temperature at the measuring point. To ensure correct measurement, modern-day measuring devices use three different methods.

  • The first method uses a reference junction thermostat which maintains the reference junction at a constant temperature of 50 °C, for example. A second thermocouple with the same metal pairing is installed as the reference junction.
  • The compensation method is based on temperature-dependent bridging, which permanently establishes a reference junction voltage equal to that of a thermocouple at 20 °C.
  • There is also the option of digital correction. In this case, the temperature of the reference junction is measured with a temperature-dependent resistor and the device numerically corrects the temperature measured at the thermocouple.

The choice of thermocouple, adapted to the measuring conditions, is crucial for successful and accurate temperature measurement. Thermocouples are available with various material pairings, and for simplification each material pairing has been assigned a typification letter. To find the thermocouple appropriate for the application, a type is first determined using the temperature range to be measured.

2.1 Thermocouple temperature ranges according to DIN EN 60 584

Standard

Type

Material

Maximum Temperature Class 1

Defined up to

DIN EN 60 584

J

Fe-CuNi

+750 °C

+1,200 °C

 

T

Cu-CuNi

+350 °C

+400 °C

 

K

NiCr-Ni

+1,000 °C

+1,372 °C

 

E

NiCr-CuNi

+900 °C

+1,000 °C

 

N

NiCrSi-NiSi

+1,000 °C

+1,300 °C

 

S

Pt10Rh-Pt

+1,600 °C

+1,768 °C

 

R

Pt13Rh-Pt

+1,600 °C

+1,768 °C

 

B

Pt13Rh-Pt6

+1,700 °C (Class 2)

+1,820 °C

The most commonly used type of thermocouple is type K since it covers a wide range of temperatures and the limit deviation is low. The procurement costs of the type K element are comparatively low. Thermocouple types R and S, due to the more expensive precious metals and lower thermoelectric voltage used in them, are only used for temperatures over +1,000 °C. Type B elements are used only from +1,300 °C, and in a protective tube they can even be used at temperatures up to +1,800 °C.

Visit our online shop, where we offer a wide selection of thermocouples. If you should have any further questions, please contact us. Our many years of experience means that we are experts at designing thermocouples. We are therefore more than happy to help you when it comes to choosing the right thermocouple for your application. If you have any special requests, we can even develop customised products that are adapted to your measuring conditions for you.

Typical area of application

  • No details given

Pros

  • High temperatures
  • Rapid response times

Cons

  • Reference junction
  • Ageing drift at high temperatures

Images of thermocouples