Pyrometry And Pyrometers

: The Working Of Steel

A knowledge of the fundamental principles of pyrometry, or the

measurement of temperatures, is quite necessary for one engaged

in the heat treatment of steel. It is only by careful measurement

and control of the heating of steel that the full benefit of a

heat-treating operation is secured.

Before the advent of the thermo-couple, methods of temperature

measurement were very crude. The blacksmith depended
n his eyes

to tell him when the proper temperature was reached, and of course

the color appeared different on light or dark days. Cherry

to one man was orange to another, and it was therefore almost

impossible to formulate any treatment which could be applied by

several men to secure the same results.

One of the early methods of measuring temperatures was the iron

ball method. In this method, an iron ball, to which a wire was

attached, was placed in the furnace and when it had reached the

temperature of the furnace, it was quickly removed by means of

the wire, and suspended in a can containing a known quantity of

water; the volume of water being such that the heat would not cause

it to boil. The rise in temperature of the water was measured by a

thermometer, and, knowing the heat capacity of the iron ball and

that of the water, the temperature of the ball, and therefore the

furnace, could be calculated. Usually a set of tables was prepared

to simplify the calculations. The iron ball, however, scaled, and

changed in weight with repeated use, making the determinations

less and less accurate. A copper ball was often used to decrease

this change, but even that was subject to error. This method is

still sometimes used, but for uniform results, a platinum ball,

which will not scale or change in weight, is necessary, and the

cost of this ball, together with the slowness of the method, have

rendered the practice obsolete, especially in view of modern

developments in accurate pyrometry.