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pyrometer. If. upon replacing an apparently defective
couple with a new one. the error still persists,
it probably exists in the line or instrument, which can
then be separately investigated.
PART 8. — GENERAL PRECAUTIONS
Although a pyrometer is not a complicated instrument,
there are many ways in which it can go wrong.
It is not, by any means, "fool proof", but if correctly
installed and given a reasonable amount of intelligent
care, it will render service many times repaying its
cost. The loss of a single charge through failure to
properly regulate the heat treating temperature, may
easily exceed the cost of installing a first class pyrometer.
It should be kept clearly in mind that a thermocouple,
at best, only indicates the temperature of its
rorging- Stamping - Heat Treating
June, 1925
position where its readings cease to increase. The hot
junction should come within an inch of the end of the
protection tube. It should not be located near any
part of the furnace that is likely to be at a different
temperature from the charge, nor placed where flames
will strike it.
The couple should be securely mounted so that its
position will not vary. The head should be far enough
from the furnace to be reasonably cool, and the hole
through which the couple is inserted should fit closely
or be covered by a flange, or caulked with asbestos,
to prevent hot gases from coming out of the furnace
or cold air from going in.
Care must be taken to see that the extension leads
are connected to their corresponding wire in the couple
(iron to iron, constantan to constantan, etc.).
Leads should be run in a workmanlike way, on porcelain
insulators or through metal conduits, the latter
Commutotinq Switch
(Enclosedfor Uniform Temperature}
Cold
/Vob/e lie to/ Standard Coup/e in Porce/ain Tube Junction
Base Meta/ Coup/e
Thick Meto7 Thimb/e
ELECTRIC FUI?Nr9CE
Potentiometer
FIG. 102—Calibration of couples.
Cold junction of noble n-.etal at 0 dig. C. Cold junct.on ol bas- metal couple at room temperature.
hot junction. If the hot junction is not properly
placed, or the furnace is not uniformly heated, the
charge may have a very different temperature. The
couple should be located with its hot junction close
to the work being treated. It is well to place the
couple so that its hot end may be observed through a
hole in the door or wall, and its brightness or color
compared with that of the charge. Small differences
in temperature between the work and the couple can
be detected in this way, provided there are no flames
or exceptionally bright surfaces to shed a false brightness
on either object.
To prevent its hot end from being cooled by conduction
of heat along the couple wires or sheath to the
outside of the furnace, the couple should extend into
the furnace a distance of at least 10 or 12 times the
diameter of its sheath. One way to insure sufficient
depth is to hold the furnace at constant temperature
by means of another couple, and gradually move
the furnace couple inward until it is well beyond the
being grounded to prevent leakage from electric light
or power circuits. A small leak will cause erratic readings,
and may damage the instrument. The leads
must be well protected against dampness. Joints in
leads should be avoided, as they may cause trouble.
Where connections are required, as at switches, they
should not be exposed to uneven temperature.
The indicating or recording instrument should be
securely mounted, where it will not be subjected to
excessive heat or cold, or severe jarring or vibration.
In most cases it is desirable to have the instrument
readily visible to the furnace operator, and where this
necessitates mounting it in a shop where there is much
dirt, smoke, or fumes, a protecting cabinet with a glass
door is a good investment.
Recording instruments are necessarily more complicated
than indicators, and their mechanism requires
occasional cleaning, oiling and adjustment. Millivoltmeters
should not be opened except by one experienced
in their care.