Contact - electrical thermometers
2. Thermocouples
Thermocouples are by far the most common temperature sensors in industrial use.
A thermocouple is a temperature sensor based on the Seebeck effect - the generation of a voltage in electrical conductors along which there is a temperature difference (gradient).
A simple thermocouple consists of two wires joined at the end where the temperature is to be measured, the other ends being connected to a voltmeter. The two wires must be of different materials, because the measured voltage is the difference between the voltages produced in each wire separately.
Voltage is built up along the lengths of the wires where the temperature gradient is, not at the junctions, which are only needed to make the electrical connections, and should be kept free of any temperature gradients. As a result the measured voltage depends on the temperatures at both ends of the wires.
In common practice the thermocouple is simply connected to the measuring instrument, which applies compensation for its ‘cold-junction’ temperature.
For more accurate use the reference junctions are controlled or fixed, typically using melting ice at 0 °C, and copper wires then connect to the instrument.
In the illustration most of the voltage is generated where the wires pass through the temperature drop across the furnace wall, and ideally there are no temperature gradients near the hot junction.
- Simple
- Rugged in protective metal cables
- Small
- Inexpensive
- Wide temperature range
The reliability of the sensor is a key factor in the performance of a contact thermometer, but as with any material device operated at elevated temperatures, they are liable to drift.
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