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Calibration Of Pyrometer With Common SaltCategory: PYROMETRY AND PYROMETERS An easy and convenient method for standardization and one which does not necessitate the use of an expensive laboratory equipment is that based upon determining the melting point of common table salt (sodium chloride). While theoretically salt that is chemically pure should be used (and this is neither expensive nor difficult to procure), commercial accuracy may be obtained by using common table salt such as is sold by every grocer. The salt is melted in a clean crucible of fireclay, iron or nickel, either in a furnace or over a forge-fire, and then further heated until a temperature of about 1,600 to 1,650 deg.F. is attained. It is essential that this crucible be clean because a slight admixture of a foreign substance might noticeably change the melting point. The thermo-couple to be calibrated is then removed from its protecting tube and its hot end is immersed in the salt bath. When this end has reached the temperature of the bath, the crucible is removed from the source of heat and allowed to cool, and cooling readings are then taken every 10 sec. on the milli-voltmeter or pyrometer. A curve is then plotted by using time and temperature as cooerdinates, and the temperature of the freezing point of salt, as indicated by this particular thermocouple, is noted, i.e., at the point where the temperature of the bath remains temporarily constant while the salt is freezing. The length of time during which the temperature is stationary depends on the size of the bath and the rate of cooling, and is not a factor in the calibration. The melting point of salt is 1,472 deg.F., and the needed correction for the instrument under observation can be readily applied. It should not be understood from the above, however, that the salt-bath calibration cannot be made without plotting a curve; in actual practice at least a hundred tests are made without plotting any curve to one in which it is done. The observer, if awake, may reasonably be expected to have sufficient appreciation of the lapse of time definitely to observe the temperature at which the falling pointer of the instrument halts. The gradual dropping of the pointer before freezing, unless there is a large mass of salt, takes place rapidly enough for one to be sure that the temperature is constantly falling, and the long period of rest during freezing is quite definite. The procedure of detecting the solidification point of the salt by the hesitation of the pointer without plotting any curve is suggested because of its simplicity. Next: Complete Calibration Of Pyrometers Previous: The Pyrometer And Its Use
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