The thermocouple is a thermoelectric transducer used for temperature measurements. It is an active transducer that converts the change in temperature (thermal energy) into electrical energy directly. It works on the phenomena of thermoelectric.
- In 1821 I.J. Seedback discovered that when two metals of different metals are joined together at there both ends such that there exist two junctions at both ends. Suppose if the two junctions are at different temperatures, an electromotive force exists across a junction and circulates a current in the closed-circuit formed by two metals.
- Later Prof. Peltier shows that there exists a temperature difference between two junctions formed by two dissimilar metals when an external electromotive force is connected between them. So that, one junction becomes hotter by absorbing heat and another junction becomes cold by liberating heat. This is called the Peltier effect.
- Again Prof. Thomson postulated that, the closed circuit formed by two dissimilar metals induces an emf and that due to temperature gradients along the conductors in the circuit. This is called the Thomson effect.
Both Peltier and Thomson's effects are a bit similar. Mostly Peltier emf is quite larger relative to the Thomson emf, but it also depends upon materials used. Hence, the above Seedback's, Peltier's and Thomson's effect form the basis for the thermoelectric transducer, the Thermocouple. Let us see the construction and working of a thermocouple.
Construction of Thermocouple :
Thermocouples are constructed by two different metals that exist in the form of wires. The ends of the two wires can be twisted, screwed, clamped, or melted together.
Generally, the two ends are joined by twisted weld and butt weld. In a twisted weld, the junction is formed by twisting the two wires and welded them together. In butt weld, the ends of the two wires are fused into a round bend. The below shows the thermocouple formed by two dissimilar metals.
A protective seating is provided around the junction and a portion of extension leads. If the temperature to be measured is high, the wire should be heavier. Also, the response time decreases with an increase in the size of the wire.
Hence, the selection of a transducer for a temperature measurement should be done keeping the above two conditions compromised. Generally, a diameter of wire ranging from 1.5 to 3mm is used for base metals and a diameter of 0.5mm wire is used for noble metals.
Working of Thermocouple :
In a thermocouple transducer, out of two junctions, one junction is referred to as hot junction or measuring junction, which is placed at the process media where the temperature is to be measured. Another junction is referred to as a cold junction or reference junction is maintained at a constant reference temperature.
When there exists a temperature difference between hot and cold junctions. An emf will be set up at the free ends due to temperature gradient and is measured by millivoltmeter. The amount of induced emf depends upon the difference in temperature between two junctions and the material used to build the thermocouple.
The temperature is determined by calibrating the millivoltmeter. Since the cold junction is at 0°C, the induced emf measured by the voltmeter is the function of the temperature of the hot junction. It is essential to keep the reference junction at 0°C to avoid errors due to change in room temperature.
Materials Used for Thermocouple :
The selection of a thermocouple for a particular application depends upon the various factors like temperatures to be measured, atmospheric conditions where the thermocouple to be used, output voltage required, and accuracy in the measurement. The different metals used to build thermocouples for different temperatures measurement is shown in below tabular form.
| Positive Wire | Negative Wire | Type of Thermocouple | Temperature Range |
|---|---|---|---|
| Copper | Constantan | T | -250°C to 400°C |
| Iron | Constantan | J | -200°C to 850°C |
| Chromel | Alumel | K | -200°C to 100°C |
| Chromel | Constantan | E | -200°C to 850°C |
| Platinum and Rhodium | Platinum | S | 0°C to 1400°C |
| Tungsten | Molybdenum | - | 0°C to 2700°C |
| Tungsten | Rhenium | - | 0°C to 2600°C |
From the above table shown, the output voltage of copper-constantan thermocouple is high with high sensitivity. The iron-constantan thermocouple is widely used for industrial applications, but iron can oxidize easily above 750°C temperature. The tungsten-molybdenum thermocouple can be used for measuring high temperatures. Hence, thermocouples can be used for wide range temperatures (-200°C to 2800°C).
Laws of Thermocouple :
1. Law of Homogeneous Circuits :
According to the law of homogeneous circuits, an electric current cannot flow in a circuit made of a single homogeneous metal when heat alone applied to it.
2. Law of Intermediate Metals :
The law of intermediate metals states that the emf developed in a circuit made of two dissimilar homogeneous metals with the junctions at two different temperatures will not get affected when a third homogeneous metal is introduced into the thermocouple circuit as long as the temperature of the two junctions formed by the third metal is same as the temperature of the thermocouple metals.
Practical Significance :- It allows the use of extension wires of metal, different from the metal used to form thermocouples.
- It allows the use of measuring instruments into the circuit without influencing or disturbing the emf generated by the thermocouple.
- It permits the use of joining materials or soldering materials to form the thermocouple junctions without affecting the performance of the junction.
3. Law of Intermediate Temperatures :
The law of intermediate temperature states that the emf developed by a thermocouple having junctions at temperatures T1 and T3 is equal to the sum of emf's developed by two thermocouples having junctions at temperatures T1 and T2, T2 and T3 respectively.
