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Thermocouple and thermistor - Essay Example

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Awareness of heat and attempting to measure the intensity can be traced to ancient history in the development of humankind. The initial success in measuring heat or temperature was based on the phenomenon of thermal expansion…
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Thermocouple and thermistor
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Download file to see previous pages Awareness of heat and attempting to measure the intensity can be traced to ancient history in the development of humankind. The initial success in measuring heat or temperature was based on the phenomenon of thermal expansion. It has remained one of the simplest methods and the most popular means for temperature sensing and is the basis of the liquid-in-glass thermometers that find applications ranging from home to the science laboratories. (1). Advances in the field of science required more accurate thermal sensing then was possible with the best liquid-in-glass thermometers, which led to development of thermal sensing devices using different methods. The range of modern thermal sensing devices includes thermoelectric, resistive, semi-conductive, optical, acoustic, and piezo-electric detectors. (1). Thermoelectric and resistive devices are based on the transmission of a fraction of the quantity thermal energy of the object to a contact sensor that has the ability to convert the heat energy into an electric signal that can be used for measuring the temperature of the object. However, there arises an error issue in this method that requires the sensor to be placed on or in the object. The error issue pertains to the sensor, irrespective of its size, disturbing the point of contact or the measurement site, and thereby causing inaccuracy in the measurement of the temperature. These inaccuracies are applicable to any method of sensing, be it conductive, convective, or radiative. A key goal in engineering has been to minimize these inaccuracies in temperature measurements through appropriate design of the sensors and use of correct measurement techniques. (1). 2. Principle and Design of the Thermocouple Thermocouples are based on the discovery by Thomas Johann Seebeck of thermoelectric currents during his studies of electromagnetic effects using bismuth-copper and bismuth-antimony circuits. The observation of Seebeck in 1821 that small currents flow in circuits made up of two different conductors, when their junctions are maintained at dissimilar temperatures is called the Seebeck Effect and is the principle on which thermocouples are founded. (2). In case one junction of a circuit consisting of dissimilar metals is maintained at a hotter temperature than the other junction, an electro-motive force (emf) is generated, which is in proportion to the difference in temperature between the hot junction or the measuring junction and the cold junction or the reference junction. The emf produced in this condition is called the Seebeck emf, which is measured in millivolts, and the pair of conductors that make up the thermoelectric circuit is called the thermocouple. (2). Measuring of the Seebeck emf can be effected either through a closed circuit current or an open circuit current. The Seebeck voltage constitutes the net conversion of thermal energy to electric energy, which is seen in the electric current generated. There are two factors on which the direction and magnitude of the Seebeck voltage is dependent, and they are the temperatures of the two junctions and the materials that constituter the thermo couple. When the temperature of the references junction is available, it is possible to measure the temperature of the measuring junction through an accurate measurement of the Seebeck emf generated by the thermocouple. There are three possible emf-measuring instruments that can be used for this purpose, namely deflection meters, digital voltmeters, and potentiometers. Digital voltmeters and potentiometers are the preferred emf-measuring instruments, because of their higher accuracy. (2). There are four main types of thermocouple junctions. These are exposed or bare-wire junction, grounded junction, ungrounded or isolated junction, and reduced diameter junction. In the exposed or bare-wire junction the sheath and insulating material are removed causing exposure of the thermocouple wires and are joined to create a measuring junction. This type of ...Download file to see next pagesRead More
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