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

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

Steel Worked In Austenitic State
As a general rule steel should be worked when it is in the a...

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

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

Protective Screens For Furnaces
Workmen needlessly exposed to the flames, heat and glare from...

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

Reheating for machine ability was done at 100 deg. less than ...

Chromium when alloyed with steel, has the characteristic func...

Crucible Steel
Crucible steel is still made by melting material in a clay or...

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

High-chromium Or Rust-proof Steel
High-chromium, or what is called stainless steel containing f...

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

Hardness Testing
The word hardness is used to express various properties of me...

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

Furnace Data
In order to give definite information concerning furnaces, fu...

Take Time For Hardening
Uneven heating and poor quenching has caused loss of many ve...

Properties Of Steel
Steels are known by certain tests. Early tests were more or l...

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

Care In Annealing
Not only will benefits in machining be found by careful anne...

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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