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Extension thermoelectrode and error of the thermocouple
Extension thermoelectrodes. The loose ends of the thermocouple must be at a constant temperature (Fig.1). However, it is not always possible to make thermocouple thermoelectrodes so long and flexible that its free ends are stirred at a sufficient distance from the working junction.
In addition, when using precious metals to make long thermoelectric economically unprofitable, so you have to use wires from another material. Connecting wires A1 and B1, (Fig.1), coming from the clamps in the thermocouple head to the vessel volume U, the temperature in which it is desirable to maintain a constant, called extension thermoelectric. Further, for connection to the measuring device, you can use conventional wires.
At include extension thermoelectrodes from materials different from materials of the main thermoelectrode, no change of the thermal EMF of the thermocouple, it is necessary to fulfill two conditions. First — the point of attachment of the extension thermoelectrodes the main thermoelectrode in the head of the thermocouple must be at the same temperature. And the second-extension thermoelectrics must be thermoelectrically identical to the main thermocouple, ie, have the same thermo-EMF in the range of possible temperatures of the junction of thermoelectrodes in the thermocouple head (approximately in the range from 0 to 200 °C).
For thermocouple platinoidy — platinum apply extension thermoelectrode of copper and alloy thermocouples that form the thermocouple, the thermocouple termoelastyczne platinoidy — platinum in the range of 150 °C. For thermocouples chromel — alumel extension thermoelectrode made of copper and Constantan. For thermocouple chromel-Copel extension are the main thermoelectric, but made in the form of flexible wires.
Improper connection of the extension thermoelectrodes there is a very significant error.
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Fig.2. Scheme of chromel-alumel thermocouple with extension wires made of copper and Constantan
Fig.1. The free ends of the thermocouple are at a constant temperature
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