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

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

Annealing Of High-speed Steel
For annealing high-speed steel, some makers recommend using g...

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

It is considered good practice to quench alloy steels from th...

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

Alloying Elements
Commercial steels of even the simplest types are therefore p...

Introduction Of Carbon
The matter to which these notes are primarily directed is the...

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

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

William Kelly's Air-boiling Process
An account of Bessemer's address to the British Association w...

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

Open Hearth Process
The open hearth furnace consists of a big brick room with a l...

Annealing In Bone
Steel and cast iron may both be annealed in granulated bone. ...

Cyanide Bath For Tool Steels
All high-carbon tool steels are heated in a cyanide bath. Wi...

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

Detrimental Elements
Sulphur and phosphorus are two elements known to be detrimen...

Tungsten, as an alloy in steel, has been known and used for a...

Complete Calibration Of Pyrometers
For the complete calibration of a thermo-couple of unknown e...

Fatigue Tests
It has been known for fifty years that a beam or rod would fa...

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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