A thermocouple consists of
- the thermocouple (sensor)
- the mounting and terminal elements required.
The thermocouple consists of two wire elements made of different metals or metal alloys which are joined at one end, the measuring point, by soldering or welding:
Measuring principle of the thermocouple element
If the measuring junction is exposed to a temperature different from that at the free ends of the thermocouple, a voltage (the thermoelectric voltage, Seebeck effect) is produced at these free ends. The magnitude of the thermoelectric voltage depends on the difference in temperature between the measuring junction and the free ends, and on the combination of materials in the thermocouple. Thermocouples always record a temperature difference. It is therefore essential to hold the free ends of a reference junction at a constant and known temperature in order to be able to determine the temperature at the measuring junction.
Calibration data for thermoelectric voltages and permissible deviations
Calibration data for thermoelectric voltages and permissible deviations for commonly used thermocouples are defined in DIN EN 60 584, part 1 (see “Technical Data”, table “Calibration data for thermoelectric voltages and error limits”).
The thermocouples Cu-CuNi and Fe-CuNi to DIN 43 710 are used for replacement purposes. Thermocouples of class 2 are supplied as standard. For more accurate measurements, thermocouples of Class 1 are available with half the DIN tolerance or with a test certificate. The tolerances only apply to the condition upon delivery.
During operation at high temperatures, the tolerances of the thermocouples may change due to absorption of foreign matter, oxidation or evaporation of alloy components.
Mode of operation
The thermocouples are extended from their connecting point by means of compensating cables to a point with a temperature which remains constant as far as possible (reference junction).
The compensating cables have the same color code as the associated thermocouple elements; the positive pole is marked in red. Correct polarity must be ensured since otherwise large errors will occur. Up to 200 °C, the same calibration data and tolerances apply to the compensating cables as to the corresponding thermocouples.
The influence of temperature changes at the cold junction can be balanced by means of a compensating circuit, e.g. a compensating box. The reference temperature is 0 °C (32 °F) or 20 °C (68 °F).
It is also possible to keep the cold junctions at a constant temperature of 50, 60 or 70 °C (122, 140 or 158 °F) using a thermostat (for several measuring junctions).
The connections from the cold junction to the measuring or process instrument are made using copper leads. With energy-consuming instruments such as indicators or multipoint recorders, the complete measuring circuit (thermocouple, compensating cable and copper lead) must be balanced in the operating condition using a resistor. SITRANS T transmitters and compensating self-recording instrument KOMPENSOGRAPH for connection to thermocouple elements have a built-in compensating circuit for balancing the effect of the ambient temperature on the cold junction. Lead balancing is not necessary in this case because of the high input impedance.
Protection fitting/protective tubes
The thermocouple can be protected against mechanical stress and chemical attack by a ceramic or metal protective tube which may be mounted using flanges, screwed glands or by welding into the pipeline or tank. The thermocouple element terminates in the connection head.
Installation examples with specification of the recommended thermocouples and protective tube materials are listed on pages “Integration” and “Installation Examples”.
Owing to the different operating conditions, no guarantee can be given for protective fittings. The manufacturer is responsible for damages and measuring errors caused by wrong installation in compliance with the General Terms of Delivery if the instruments have been installed by the manufacturer and if the specifications for the operating conditions furnished by the customer were correct and sufficiently detailed.
Thermocouple elements are very compatible since it is almost always possible to adapt them in shape and size to the particular problem. The temperature-responsive part is almost point-shaped. Thermocouple elements are therefore particularly suitable for measuring rapidly changing temperatures.