What is a Thermocouple? How Thermocouple Works?

Thermocouple is the devise used extensively for measurement of the temperature of the body. Temperature is the fundamental property just like the mass and time and is frequently measured quantity. Thermocouple comprises of at least two metals joint together to form two junctions. One is connected to the body whose temperature is to be measured; this junction is called as hot or measuring junction. The other junction is connected to the body of known temperature; this is called as cold or reference junction. Thus the thermocouple enables measuring the unknown temperature of the body with reference to the known temperature of the other body.

The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. All these have been described in brief below.

1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.

2) Peltier effect: As per the Peltier effect when two dissimilar metals are joined together to form two junctions, the emf is generated within the circuit due to different temperatures of the two junctions of the circuit.

3) Thomson effect: As per Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.

In most of the cases the emf suggested by Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect play prominent role in the working principle of the thermocouple.

The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises to two dissimilar metals A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T₁ and T₂ respectively. Remember that the thermocouple cannot be formed if there are no two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperature, the emf’s will not become zero and there will be net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable devise.

For measurement of the temperature of the body, one junction of the thermocouple is connected to the body whose temperature is to be measured. This junction is called as hot junction or the measuring junction. The other junction is connected to the body whose temperature is known. This junction is called as cold or reference junction.

Within the circuit of the thermocouple the devise for measuring the current or emf flowing the circuit is connected. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In the figure 2 above the two junctions of the thermocouple and the devise used for measurement of emf (potentiometer) are shown.

Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

The amount of emf developed within the thermocouple circuit is very small, which is usually in millivolts, hence some highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit. The two devices used commonly for measuring emf within the thermocouple circuit are ordinary galvanometer and voltage balancing potentiometer. The manually or automatically balancing potentiometer is used more commonly.

The figure 2 above shows potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction q is the reference junction, whose temperature can be measured by the thermometer. In some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath (see figure 3). This devise can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.

Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck

1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck

2) http://www.tpub.com/content/doe/h1013v1/css/h1013v1_24.htm