Introduction
Accurate temperature measurement is a very vital and common requirement in every industrial instrumentation process. It is however very difficult to achieve this objective. Unless the techniques used to measure temperature are proper and effective, massive reading inaccuracies can occur leading to the accumulation of useless data. This is where the thermocouple comes into play. This device is a temperature sensor that is widely used in all corners of the globe. It has very many favorable characteristics and these include: high inherent accuracy; wide temperature range suitability; high reliability; great application versatility, fast thermal response and relatively low cost. The thermocouple was discovered by Thomas J Seebeck in 1822. His discovery was purely accidental. Seebeck found that there was a voltage difference when one end of a wire was heated. With disregard to temperature, no voltage difference was observed when both ends were heated and attained the same temperature. In addition, Seebeck noted that no current flowed when the circuit was actually made with a wire of similar material. Therefore, current only flowed when two dissimilar metals were used to make a closed circuit and when a single junction of the closed circuit was heated.
In a thermocouple, current continues flowing if the two junctions are maintained at different temperatures. The direction and magnitude of this current is actually a function of the difference in temperature of the metal’s thermal properties and the junctions of the circuit. This is a very famous phenomenon called Seebeck Affect, which is named after the discoverer of the thermocouple, Thomas Seebeck.
The conductors used can be made of any two metals are dissimilar. When heat is applied on any one junction, the resultant current that flows actually be observed or measured using a milliamp meter. If the cold and hot junctions positioning is reversed, the resultant current will flow in opposite direction. As seen earlier, the low measurable current that is generated by the thermocouple circuit is a direct factor of temperature difference between the cold and the hot junction. Therefore, any unit change in the temperature difference between the two junctions produces a net voltage change.
The response of time of thermocouples also varies and is somehow dependent on the junction. The thermocouple that is fastest in responding is the one whose junction is exposed. The diameter of the probe sheath also determines the response time of the device. The smaller the diameter, the faster the response.
There are many types of thermocouples in the world today. Each has a different set of characteristics and attributes. This is in terms of durability, temperature range, application compatibility and chemical resistance and vibration resistance. In spite of these differences, the working mechanism similar to that described above.
Although the thermocouple is a very popular device, it is however plagued by a lot of mystique. For example, there is a lot of controversy surrounding the proper definition of a standard cold junction. A plant engineer may tend to ignore thermocouple tables that are based on reference or cold junction when he or she perceives that the values given are not really the standard ones.
In summary, a thermocouple is definitely one of the greatest science devices invented. The device has been helpful in industrial instrumentation where it has enabled the measurement of accurate temperature values. This would otherwise have been unachievable if the device had not been invented. The device has a particular advantage in that its sensing elements are very small such that they can be inserted into tiny spaces and consequently respond to temperature changes. It is however very paramount those individuals who use this device learn everything about its working mechanism to ensure that it is indeed used effectively.
Works Cited
"How sensors work - thermocouples." N.p., 12 Nov. 2012. Web. 1 May 2013.
Michalski, L. Temperature Measurement. Chichester: J. Wiley, 2001. Internet resource.
Smith, Ralph J, and Richard C. Dorf. Circuits, Devices, and Systems: A First Course in Electrical Engineering. New York: Wiley, 1992.
