How Thermocouples Work

Thermocouples sense temperature in such a way that it converts thermal energy into electrical energy. It is made by connecting wires with different materials to form a junction. The temperature change in the junction would result in a production of voltage and this phenomena is based on the Seebeck effect. The amount of voltage will greatly depend on the characteristics of metals used and the range of the temperature change.

In a junction formed by two wires, one of these is called the hot or measuring junction. This  is connected to the body of the thermocouple and is responsible for measuring the temperature. The second junction, on the other hand, is the reference junction and is attached to the body of a known temperature.

These devices are derived from three principles and these are the: Seebeck effect, Peltier Effect, and the Thomson effect.

The Seebeck effect is usually applied to thermal-to-electrical energy conversion.

It is an event where a voltage difference is produced from the temperature difference between two electrical conductors or semiconductors with different materials.

The Peltier effect can be seen when heat is developed at one side of the junction and absorbed at the other junction resulting from an electromotive force passing through the circuit.

Thomson effect is described when the temperature of the heat is related with the flow of current to the temperature along the rod.

Circuitry:

The circuit of this device can be described by two dissimilar wires joined to form a junction. The two junctions need to have different temperatures to generate the Peltier EMF in the circuit, in which it is the function of the temperature of two junctions.

If a certain length of copper wire is heated at one end, the electrons will move along the wire to the cooler end and result in a temperature gradient along the wire. The explanation behind this is that electrons carry heat and electricity. At this point, The heat has been changed into electric energy. This same principle was discovered by Volta and Seebeck and greatly applies to the operation of this device.

The current flow will be zero if the temperature of the junctions is the same. The current will be flowing through the circuit if the junctions have different temperatures and the EMF will not be zero. The electromotive force through the circuit will depend on the metals used and the difference in temperature of the two junctions measured by a meter.

To read the EMF, it is required to use a highly sensitive instrument because the value in the circuit is very small which is in millivolts. For this reason, galvanometers and voltage balancing potentiometers are normally used.

A potentiometer measures potential difference by comparing an unknown voltage to a known or defined voltage. It consists of three terminal variable resistors that acts as an adjustable voltage divider. This device may provide high precision measurements.

A galvanometer is used to measure null deflection or even zero current and can also measure very small electric currents.

A thermocouple must be referenced to a known temperature on the other end of the sensor in order to make an absolute measurement. The hot junction is the measuring member while the cold junction is the reference where a cold junction compensation chip is placed. To maintain a constant temperature for the cold junction, this may be fixed by plunging it in water. Ambient air, on the other hand, can influence the reference temperature and it may always need to be calibrated and adjusted by another compensation device.

Key Conclusions:

A thermocouple converts thermal energy into electrical energy and is characterized by joining dissimilar metals or semiconductors to form a junction. This device measures an unknown temperature and would compare it to a reference temperature making it a low cost temperature sensor. The types of these sensors vary depending on the types of alloys used for the wires used.