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

Annealing Alloy Steel
The term alloy steel, from the steel maker's point of view, r...

Critical Points
One of the most important means of investigating the properti...

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

Placing Of Pyrometers
When installing a pyrometer, care should be taken that it re...

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

Hardening High-speed Steel
In forging use coke for fuel in the forge. Heat steel slowly ...

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

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

Effect Of A Small Amount Of Copper In Medium-carbon Steel
This shows the result of tests by C. R. Hayward and A. B. Joh...

Quality And Structure
The quality of high-speed steel is dependent to a very great ...

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

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

The Effect Of Tempering On Water-quenched Gages
The following information has been supplied by Automatic and ...

Gas Consumption For Carburizing
Although the advantages offered by the gas-fired furnace for ...

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

Chrome-nickel Steel
Forging heat of chrome-nickel steel depends very largely on ...

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

Preparing Parts For Local Case-hardening
At the works of the Dayton Engineering Laboratories Company, ...

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

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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