Complete Calibration Of Pyrometers

: The Working Of Steel

For the complete calibration

of a thermo-couple of unknown electromotive force, the new couple

may be checked against a standard instrument, placing the two bare

couples side by side in a suitable tube and taking frequent readings

over the range of temperatures desired.

If only one instrument, such as a millivoltmeter, is available,

and there is no standard couple at hand, the new couple may be

> calibrated over a wide range of temperatures by the use of the following


Water, boiling point 212 deg.F.

Tin, under charcoal, freezing point 450 deg.F.

Lead, under charcoal, freezing point 621 deg.F.

Zinc, under charcoal, freezing point 786 deg.F.

Sulphur, boiling point 832 deg.F.

Aluminum, under charcoal, freezing point 1,216 deg.F.

Sodium chloride (salt), freezing point 1,474 deg.F.

Potassium sulphate, freezing point 1,958 deg.F.

A good practice is to make one pyrometer a standard; calibrate it

frequently by the melting-point-of-salt method, and each morning

check up every pyrometer in the works with the standard, making the

necessary corrections to be used for the day's work. By pursuing

this course systematically, the improved quality of the product

will much more than compensate for the extra work.

The purity of the substance affects its freezing or melting point.

The melting point of common salt is given in one widely used handbook

at 1,421 deg.F., although chemically pure sodium chloride melts at

1,474 deg.F. as shown above. A sufficient quantity for an extended

period should be secured. Test the melting point with a pyrometer

of known accuracy. Knowing this temperature it will be easy to

calibrate other pyrometers.