Thermocouples are the temperature sensors which are extensively used for the measurement of the temperature variations. They sense the temperature and the temperature is further measured by other instruments after sensing it.
As they convert a non-electrical quantity (temperature) into voltage (electrical quantity) so they are transducers also. Since the do not require any external power source to operate, so they are active transducers.
A thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction.
The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore the thermocouple measures unknown temperature of the body with reference to the known temperature of the other body.
Working Principle
The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:
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, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.
3) Thomson effect: As per the 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 the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.
How it Works
The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises of 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 not 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 temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit.
The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.
The device for measuring the current or emf is connected within the circuit of the thermocouple. 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 figure 3 the two junctions of the thermocouple and the device 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.
Devices Used for Measuring EMF
The amount of emf developed within the thermocouple circuit is very small, usually in millivolts, therefore highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit.
Two devices used commonly are the ordinary galvanometer and voltage balancing potentiometer. Of those two, a manually or automatically balancing potentiometer is used most often.
Figure 3 shows the 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 4). This device can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.
The e.m.f. generated in a thermocouple is given by:
E = a(∆θ) + b(∆θ)2
∆θ = difference in temperature between two junctions (in Degree Celsius)
a, b = constants
generally, ‘a’ is very large as compare to ‘b’ so ‘b’ can be neglected. Therefore, the above expression can be approximated as under:
E = a(∆θ)
∆θ = E/a
Types of Thermocouple
T – Type Thermocouple
Positive wire — Cu
Negative wire — Constantan
It can be used up to 350 C. It is very stable and inexpensive. Generally, it is used for very low-temperature applications.
E – Type Thermocouple
Positive wire — Chromel
Negative wire — Constantan
It can be used up to 850 C. It is most sensitive thermocouple. It generates a high output voltage.
J – Type Thermocouple
Positive wire — Iron
Negative wire — Constantan
It can be used up to 1000 C. It is a very common type of thermocouple. Its stability is high.
K — Type Thermocouple
Positive wire — Chromel
Negative wire — Alumel
It can be used up to 1200 C. It is a widely used type of thermocouple. It is a cheaper type as compared to other types.
S – Type Thermocouple
Positive wire — Platinum 10% Rohodium
Negative wire — Platinum
It can be used up to 1400 C. It has very high precision thus used for very high accuracy requirements.
The current will flow from + ve marked lead to the – ve marked lead. In thermocouples, negative lead is generally a red colored wire. The color of positive lead will be according to its type.
