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Steel For Chisels And Punches
The highest grades of carbon or tempering steels are to be re...

Piston Pin
The piston pin on an aviation engine must possess maximum res...

Phosphorus is one of the impurities in steel, and it has been...

The Care Of Carburizing Compounds
Of all the opportunities for practicing economy in the heat-t...

Separating The Work From The Compound
During the pulling of the heat, the pots are dumped upon a ca...

Silicon prevents, to a large extent, defects such as gas bubb...

Heat-treating Department
The heat-treating department occupies an L-shaped building. ...

ANNEALING can be done by heating to temperatures ranging from...

The Thermo-couple
With the application of the thermo-couple, the measurement of...

Heat Treatment Of Punches And Dies Shears Taps Etc
HEATING.--The degree to which tools of the above classes shou...

The Packing Department
In Fig. 56 is shown the packing pots where the work is packe...

Carbon Tool Steel
Heat to a bright red, about 1,500 to 1,550 deg.F. Do not ham...

Steel Before The 1850's
In spite of a rapid increase in the use of machines and the ...

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

Air-hardening Steels
These steels are recommended for boring, turning and planing...

Instructions For Working High-speed Steel
Owing to the wide variations in the composition of high-speed...

Liberty Motor Connecting Rods
The requirements for materials for the Liberty motor connecti...

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

PHOSPHORUS is an element (symbol P) which enters the metal fr...

Ebbw Vale And The Bessemer Process
After his British Association address in August 1856, Besseme...

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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