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Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

Temperatures To Use
As soon as the temperature of the steel reaches 100 deg.C. (...

Tool Or Crucible Steel
Crucible steel can be annealed either in muffled furnace or b...

Classifications Of Steel
Among makers and sellers, carbon tool-steels are classed by g...

Knowing What Takes Place
How are we to know if we have given a piece of steel the ver...

Carburizing Low-carbon Sleeves
Low-carbon sleeves are carburized and pushed on malleable-ir...

Annealing Method
Forgings which are too hard to machine are put in pots with ...

Tempering Round Dies
A number of circular dies of carbon tool steel for use in too...

Lathe And Planer Tools
FORGING.--Gently warm the steel to remove any chill, is parti...

Suggestions For Handling High-speed Steels
The following suggestions for handling high-speed steels are ...

Annealing Work
With the exception of several of the higher types of alloy s...

Bessemer Process
The bessemer process consists of charging molten pig iron int...

Hints For Tool Steel Users
Do not hesitate to ask for information from the maker as to t...

Annealing To Relieve Internal Stresses
Work quenched from a high temperature and not afterward tempe...

Effects Of Proper Annealing
Proper annealing of low-carbon steels causes a complete solu...

Correction By Zero Adjustment
Many pyrometers are supplied with a zero adjuster, by means ...

SILICON is a very widespread element (symbol Si), being an es...

Hardening High-speed Steels
We will now take up the matter of hardening high-speed steels...

Restoring Overheated Steel
The effect of heat treatment on overheated steel is shown gra...

Judging The Heat Of Steel
While the use of a pyrometer is of course the only way to hav...

Calibration Of Pyrometer With Common Salt


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.

Next: Complete Calibration Of Pyrometers

Previous: The Pyrometer And Its Use

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