Calibration Of Pyrometer With Common Salt

: The Working Of Steel

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.