As we already mentioned, thermocouples are the most often used temperature sensors.
Thermocouple is made of at least two metals that are joined together to form two junctions. One is connected to a body whose temperature will 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 the unknown temperature of the body with reference to the known temperature of the other body, which is in line with the Zeroth law of thermodynamics which states that :“When two bodies are separately in thermal balance with the third body, then the two are also in thermal balance with each other". Because of this, we need to know the temperature at the cold junction if we wish to have an absolute temperature reading. This is done by a technique known as cold junction compensation (CJC).
Typically CJC temperature is sensed by a precision RTD sensor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures.
Understanding cold junction compensation is important, since any error in the measurement of the cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip. As well as dealing with the CJC, the measuring instrument must also compensate for the fact that the thermocouple output is non-linear. The relationship between temperature and output voltage is a complex polynomial equation (5th to 9th order depending on thermocouple type). High accuracy instruments such as Dewesoft instruments store thermocouple tables in devices and compensate the results to eliminate this source of error.
Working principle of Thermocouples
Now let's take a look at working principle of every Thermocouple. The working principle is based on the Seebeck, Peltier or Thomson effect.
1. Seebeck effect prescribes that a circuit made from two dissimilar metal, with junctions at different temperature, induces a voltage difference between the junctions.
2. Peltier effect is the opposite of the Seebeck effect. Instead of using heat to induce a voltage difference, it uses a voltage difference to induce heat.
3. Thomson effect states that if an electrical current flows along a single conductor while a temperature difference exist in the conductor, thermal energy is either absorbed or rejected by the conductor, depending on he flow of the current. More specifically heat is liberated if an electric current flows in the same direction as the heat flows; otherwise it is absorbed.
The circuit of every Thermocouple must be composed of two dissimilar metals, for example, A and B. These two metals are joined together to form two junctions, p, and q, which are maintained at the temperatures T1 and T2 respectively. Let us not forget, that thermocouple cannot be formed if there is just one junction. If the temperature of both the junctions is the same, equal and opposite electromotive force 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 electromotive force will not become zero and there will be a net current flowing through the circuit. The total electromotive force flowing through this circuit depends on the metals used in the circuit as well as the temperature of the two junctions. An ammeter is connected in the circuit of the thermocouple. It measures the amount of electromotive force flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures.