A thermocouple is a pair of different metals, which produces a thermoelectric effect (e. m. f. ), which is used to sense and measure a difference in temperature. A thermocouple consists of two different metal wires joined together. When there is a temperature difference between the two junctions, a thermoelectric emf is produced. A thermocouple is a source of a temperature – dependent emf that can be used to monitor or measure temperatures. When two different metals are connected together, electrons leave one metal and transfer to the other metal, causing a potential difference across the two junctions.
This is known as the Seebeck effect. This potential difference occurs because electrons can leave one of the metals more easily than the other so the first metal looses the electrons to become positive and the second metal gains electrons to become negative. If both junctions are at the same temperature, it causes an equal but opposite potential difference at the other junction. If the junctions are at different temperatures, then the potential difference across them can differ, giving rise to a net emf around the circuit. This is what is called as the thermoelectric effect.
The metals must be different and for a given temperature, the emf depends on the choice of the two metals. The potential difference is small, of the order of a few millivolts or less. However, the resistance of a thermocouple is generally small. So despite the small potential difference, it will give a measurable current. A problem with working with thermocouples is that it needs a sensitive voltmeter, able to detect a signal of the order of a thousandth of a volt. Too sensitive a detector, and there appears to be no temperature difference. Another problem is the problem of internal resistance of a source.
If the thermocouple is connected to a low resistance detector, the potential difference across it may be tiny, because most of the potential difference available is used up in driving a small current through the large internal resistance. Only if the detector has itself a very high resistance is there an appreciable potential difference seen as the output of the sensor. A thermocouple thermometer can be made using two similar wires and a different wire connected together to form two junctions acting in opposition to each other. The free end of each similar wire is connected to a microvoltmeter or to a d.c.
Amplifier with a voltmeter connected at its output. One of the junctions is maintained at a constant temperature and the other junction is used as a temperature prob. The meter reading changes according to the difference between the temperature of the probe and the reference junction. The probe is simply a junction between two thin wires and this makes it respond rapidly to change of temperature and has therefore a faster response time in liquid – glass thermometer. In addition, because a thermocouple generates an emf directly, it is used widely in control systems.