INTRODUCTION :
In 1821 a German physicist named Seeback discovered the thermoelectric effect, which forms the basis of modern thermocouple technology. He observed that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures. The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions. If the same temperature exists at the two junctions, the voltage produced at each junction cancel each other out and no current flows in the circuit. With different temperatures at each junction, different voltages are produced and current flows in the circuit. A Thermocouple can therefore only measure temperature differences between the two junctions.
It is important to designate each of the junctions for practical purposes; the measuring junction (often referred to as the 'hot' junction) is that which is exposed to be measured temperature. The reference junction is the other junction that is kept at a known temperature; this is often referred to as the 'cold' junction. The term thermocouple refers to the complete system for producing thermal voltages and generally implies an actual assembly (i.e. a sheathed device with extension leads or terminal blocks).
The two conductors and associated measuring junction constitute a thermo element and the individual conductors are identified as the positive or negative leg.
Developments in theoretical aspects of thermoelectricity under the influence of solid-state physics have resulted in a rather different explanation of thermocouple activity. This is that the thermo electric voltage is generated in the thermocouple wires only in the temperature gradient existing between the 'hot' and 'cold' junctions and not in the junctions themselves. While this is a fundamental conceptual difference to the established theory, the way in which thermocouple are currently used is generally successful in practical terms. However, this explanation of thermocouple behavior must be born in mind when calibrating the sensor or indeed when using it for relatively high precision thermometry.
Thermo electric voltages are very small and at best attain a few tens of micro Volts per degree centigrade. In consequence, practical thermocouple are mainly used at elevated temperatures, above say 100C and at depressed temperatures, below -50C; however with appropriate measuring instrument they can be used at any value with in their operational range. In some application, the reference junction may be held at some temperature other than 0C, for example in liquid gas or a heated enclosure; in any event, the measured ''output'' will correspond to the difference temperature between two junctions.
Note :- Thermocouple is always formed when two metals are connected together. For example, when the Thermo element conductors are joined to copper cable or terminals, thermal voltages can be generated at the transition. In this case, the second junction can be taken as located at the connection point (assuming the two connections to be thermally common). The temperature of this connection point (terminal temperature) if known, allows computation of the temperature at the measuring junction. The thermal voltage resulting from the terminal temperature is added to the measured voltage and their sum corresponds to the thermal voltage against a 0C reference.
If the measuring junction is at 300Cand the terminal temperature is 25C, the measured thermal voltage for the type K thermo element (Nickel-Chromium v Nickel-Aluminium) is 11.18 mV. This corresponds to 275C difference temperature. Therefore a positive correction of 25C refers the temperature to 0C reference; 300C is thus indicated.
Important points to note at this stage are four-fold. Firstly, thermocouples only generate an output in the regions where the temperature gradient exist- not beyond. Secondly, accuracy and stability can only be assured if the thermoelectric characteristics of the thermocouple conductors are uniform throughout. Thirdly, only a circuit comprising dissimilar materials in a temperature gradient generates an output. And, fourthly, although the thermoelectric effects are seen at junctions, they are not due to any magic property of the junction itself.
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Fundamental of thermocouple
