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Steel Making

A combination of the characteristics of nickel and the charac...

Steel is hardened by quenching from above the upper critical....

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

The Quenching Tank
The quenching tank is an important feature of apparatus in c...

Carbon Steels For Different Tools
All users of tool steels should carefully study the different...

Heat Treatment Of Gear Blanks
This section is based on a paper read before the American Gea...

Pyrometry And Pyrometers
A knowledge of the fundamental principles of pyrometry, or th...

Carburizing Material
The simplest carburizing substance is charcoal. It is also th...

Steel Can Be Worked Cold
As noted above, steel can be worked cold, as in the case of ...

Application Of Liberty Engine Materials To The Automotive Industry
The success of the Liberty engine program was an engineer...

High-carbon Machinery Steel
The carbon content of this steel is above 30 points and is ha...

Composition And Properties Of Steel
It is a remarkable fact that one can look through a dozen tex...

For Milling Cutters And Formed Tools
FORGING.--Forge as before.--ANNEALING.--Place the steel in a ...

The crankshaft was the most highly stressed part of the entir...

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

Calibration Of Pyrometer With Common Salt
An easy and convenient method for standardization and one whi...

Cutting-off Steel From Bar
To cut a piece from an annealed bar, cut off with a hack saw,...

Vanadium has a very marked effect upon alloy steels rich in c...

Refining The Grain
This is remedied by reheating the piece to a temperature slig...

The Modern Hardening Room
A hardening room of today means a very different place from ...

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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