Pyrometry: A Practical Treatise on the Measurement of High Temperatures

by Charles R. Darling

The maximum error due to a possible incorrect reading of ¹⁄₄₀° is therefore less than 1 per cent.

Example II.—The loss of heat by radiation in transferring 100 grams of nickel at 927° C., possessing a surface of 30 square centimetres, and with radiating power 0·7 of a black body, may be shown by the fourth-power law to be 50 calories per second ([see page 139]). If two seconds were occupied in the transfer, the error from this cause would be 1 in 130; and adding this to the thermometric error, the total is less than 2 per cent.

Practical Forms of Calorimetric Pyrometers.—When required to estimate the temperature of a muffle furnace or other laboratory appliance, a sheet-copper vessel of about 1500 c.c. capacity may be used. This should rest on wooden supports in a second similar vessel, about 2 inches wider, which acts as a shield against radiation. A cylinder of nickel about 1½ inches long, and 1¼ inches in diameter, with a hole of ½-inch diameter in the centre, is suitable for test purposes. This may conveniently be heated in a nickel crucible; and when transferring to the water the crucible may be grasped with a pair of tongs, and tilted so as to allow the cylinder to drop into the water. When used in a tube furnace, a length of thin nickel wire may be attached to the cylinder to enable withdrawal to be accomplished rapidly, allowance being made for the weight of the heated wire. The transfer should be accomplished as speedily as possible, to avoid radiation errors. The figure to be used to represent the specific heat of nickel may be obtained from the curve ([fig. 65]), when the range to be measured is approximately known. The water equivalent of the vessel and thermometer should be determined as follows:—Place in the vessel one-half the quantity of cold water used in the experiment—say 750 c.c.—and note the temperature (t₁) after stirring with the thermometer. Then add an equal quantity of water at a temperature (t₂) about 10° higher than t₁ Mix thoroughly with the thermometer, and note the temperature of the mixture (t₃). Check results may be obtained by varying the proportions of cold and warm water, the total quantity always being equal to that used for quenching the hot nickel. If W₁ = the weight of cold water, and W₂ that of the warm, the water equivalent (x) is obtained from the equation

		W2 (t2 - t3) - W1(t3 - t1)
x	=	———————————— .
		t3 - t1

This figure represents the weight of water equal in thermal capacity to the vessel, and in a pyrometric measurement is added to the weight of water taken.

In industrial practice, it is desirable to dispense, if possible, with the necessity for calculations, so that a reading may be taken by an unskilled observer. The earliest form of calorimetric pyrometer, patented by Byström in 1862, consisted of a lagged zinc vessel into which a piece of platinum was dropped, and a table was provided from which the temperature of the furnace could be read by noting the rise in temperature of the water. The modern industrial form, made by Messrs Siemens, will now be described.

Fig. 66.—Siemens’
Calorimetric or
“Water” Pyrometer.