Siemens SIMATIC ET 200AL, System Manual

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Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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Thermocouples
5
5.1 Selecting thermocouples
Introduction
Thermocouples are electrical devices for precise temperature measurement. They consist of
two different metals connected at one point. Temperature affecting this point generates a
voltage difference from which the temperature can be calculated.
Thermocouples can measure a wide range of temperatures and come in extremely robust
models. They are therefore frequently used in industrial applications. Below are some of the
criteria to consider when selecting the right thermocouple:
● Temperature range
● Atmospheric conditions
● Price
Grounded thermocouples
In grounded thermocouples, a conductive thermocouple wire is attached to the inside of the
sensor housing. This results in good heat transfer from the sensor housing to the
thermocouple measuring tip.
Non-grounded thermocouples
In non-grounded thermocouples, the thermocouple is not connected to the sensor housing.
The response time following temperature changes is longer than with grounded
thermocouples. The measuring point is galvanically isolated.