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Wednesday, 13 November 2013

Construction and Working of Pyrometer

Construction and Working of Pyrometer
Points : Construction and Working of Pyrometer, Temperature Measurement Temperature Measurement The heat treater must, of course, know the temperature of a furnace which he is operating. The most ancient pyrometer is the human eye, and heat treaters still rely on their observation of “color” of the metal or furnace, when no better means of temperature indication is available.
Modern parametric equipment, of course, is quite elaborate and complex and is often coupled with control mechanisms for maintenance of temperature in the furnace, and also with recording devices so that the record of the treatment may be readily reviewed. It is beyond the scope of this book to deal at length with this subject. With the exception of instruments of the observation type, called radiation and optical pyrometers. the most widely used type of pyrometer consists of a thermocouple and its indicating or measuring instrument. The thermocouple consists of two dissimilar wires welded together at one end; this particular weld is known as the hot junction. This end is inserted in the furnace at the place where a temperature measurement is to be made. Obviously, many thermocouples may be spotted about the furnace if needed. The open end of the thermocouple is connected to suitable conducting wires. The point of union of each of the wires of the thermocouple to its lead wire is the cold junction. The temperature at the cold junction is usually maintained constant. The thermocouple is, in essence, a heat engine which delivers an electric current that in amount is a function of the difference in temperatures between the hot and cold junctions. Since the device produces a small electric current (direct current), it can be used to actuate control mechanisms and recording devices.

The three most common kinds of thermocouples are those in which the component wires are: (1) Alumel and Chromel, (2) iron and Constantan, and (3) platinum and an alloy of platinum. The first is especially for oxidizing conditions, the second for reducing conditions, and the third for higher temperature ranges than the other two since platinum has a higher melting point. To prolong the life of the thermo couple elements, protection tubes of refractoriness or. of heat resistant metals commonly surround the wires. Naturally the wires are insulated from each other in assembling the thermocouple. gives the relationship between the emf development and the temperature for several common thermocouples.

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