Pyrometry: A Practical Treatise on the Measurement of High Temperatures

by Charles R. Darling

Fig. 65.—Specific Heat of Nickel over Ranges from 0° C.

Regnault, who first suggested the calorimetric method for high temperature measurement, attempted to measure the specific heat of iron over different ranges, with a view to using this metal in the process. Owing to the absence of reliable means of determining the experimental temperatures, however, Regnault’s values were considerably in error. For the range 0 to 1000° C. he gave the average specific heat of iron as 0·126, a figure much below the truth. Thus, if a piece of iron be heated to 970° C., as measured by the thermo-electric method, and dropped into water, the temperature calculated from an assumed specific heat of 0·126 will be found to be 1210°, or 240° too high. The values now employed are obtained by experiments with a thermo-electric pyrometer, so that temperatures deduced by the calorimetric method agree, within the limits of manipulative error, with those of the standard scale. The accompanying curve, [fig. 65], shows the average specific heat of nickel over all ranges between 0° and 1000° C., and from this curve the correct figure to use in the calculation for any range may be determined. Thus for a furnace between 800° and 900° C. the specific heat would be taken as 0·136; and although the choice of the value to be taken involves a knowledge of the temperature within 100°, no difficulty arises in practice, as it is easy to judge this limit by experience at temperatures below 1000° C In the most approved forms of calorimetric pyrometers for industrial purposes the temperature of the hot metal may be read directly from a scale, prepared in accordance with the value applying to the specific heat at various ranges.

Copper and iron are still used to a limited extent in these pyrometers, but lose continuously in weight by oxidation, the scales of oxide falling off when quenched, necessitating weighing before each test to ensure accuracy. Nickel oxidises very little below 1000 C., and as the thin film of oxide which forms does not readily peel off, the weight may increase slightly. Quartz would probably be more suitable than metals, not being altered by heating and quenching, but does not appear to have been tried for this purpose. Another possible material is nichrom, which resists oxidation below 1000° C. The weight of the solid should be at least 1/20 of that of the water, in order to ensure a tangible rise in temperature, and the thermometer should be capable of detecting 1/20 of a degree C. The rise in temperature should not be so great as to cause the water to exceed the atmosphere in temperature by more than 4° or 5° C., as otherwise radiation losses would have a marked effect. The limits of accuracy of the method will be shown by reference to examples.

Example I.—A piece of nickel, weighing 100 grams, is placed in a furnace, and after heating dropped into 2000 grams of water at 10° C., contained in a vessel of water equivalent 50 grams. The temperature rises to 16·25° C. The specific heat of nickel for the range is 0·137. To find the temperature of the furnace and the limits of accuracy, the thermometer being readable to 1/20° C. Equating heat lost by the nickel to that gained by the water and vessel:—

100 × 0·137 × (x - 16·25) = 2050 × (16·25 - 10·0)

from which x = 952° C.

If the error in each thermometer reading amounted to ¹⁄₄₀° the maximum difference in the above calculation is obtained by introducing the altered values as under:—

100 × 0·137 × (x - 16·225) = 2050 × (16·225 - 10·025)

when x = 944° C.