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June, 1925
referring to the calibration chart of the couple, it is
found that a temperature of 25 cleg. C. corresponds to
a certain e.m.f., which we may call "e" millivolts. The
observed e.m.f. is E1 millivolts. The true temperature
of the hot junction is then that corresponding to (E1
plus e) millivolts. As previously explained, the result
obtained by adding the cold junction temperature to
the temperature which corresponds to the observed
e.m.f. will not accurately give the hot junction temperature,
because the temperature-e.m.f. relation is not
strictly proportional throughout the range. If plotted
on cross section paper, with temperature horizontal
and e.m.f. vertical, it would not be a perfectly straight
line.
Calibration by Melting Points.
In the absence of a noble metal standard, a fairly
accurate check or calibration of base metal couples
may be made by comparison with the melting point
of one or more pure metals or salts. As a rule, the
freezing point is actually used, being identical with
the melting temperature, while the manipulation is
somewhat simpler.
The material to be used is melted in a crucible,
usually of graphite, and the couple to be standardized
is inserted in the liquid bath to a depth sufficient to
insure that its hot end will assume the temperature
of the bath and not be cooled by conduction of heat
along its wires or protection tube.
The protection tube should not be thicker nor larger
than necessary. A light steel tube with an oxidized
surface, will do for a bath of tin, lead, aluminum, or
salt. A graphite tube is sometimes used. A reducing atmosphere
should be maintained by excluding air and
floating a little powdered charcoal or graphite on the
surface of the bath. An electric crucible-type furnace
of sufficient depth is excellent for this work.
The molten bath is allowed to slowly cool and temperature
readings are taken at regular intervals of
time. When the bath begins to solidify or freeze, the
temperature will remain constant, until all, or nearly
all, has become solid. Then the temperature will begin
to decrease again, and the test may be stopped.
The bath is remelted, the couple removed, and the
operation repeated, if desired, with a material of different
melting point. The cold junction temperature
is taken care of in the same way as when calibrating
by comparison with a standard couple.
Freezing point temperatures may be taken automatically
by connecting the couple with a recording
pyrometer. When the freezing point is reached there
is a distinct jog or hump in the curve.
Various factors, such as rate of cooling, size of
bath, size of couple, specific heat of each, latent heat
of fusion of bath, rate of heat conduction through bath,
etc., will influence the length of time for which the
temperature remains constant. The presence of impurities
in the bath will lower its melting point, and
is likely to make melting or freezing extend over a
range of temperature instead of being a fixed point—
in other words, the liquid may pass through a mushy
state before solidifying. Great care must be taken to
obtain pure materials for the bath and to avoid contamination
during use.
The following materials are useful for melting
point standards, and give well distributed points:
Forging- Stamping - Heat Treating
197
Melting Temperature
Material Centigrade
Deg.
Tin 231.9
Lead 327.3
Zinc 419.4
Aluminum 6587
Sodium Chloride 800.0
Copper 1.083.0
Because of the difficulties involved, and the likelihood
of error from various causes, the method is less
desirable for standardizing purposes in industrial
plants than the method of comparison with a noble
metal couple described above. It is often useful as
a check and may be made quite accurate with proper
precautions. More detailed instructions will be found
in reference 10.
FIG. 101—Recording pyrometer, potentiometer type
Checking of Working Couples.
It is very important to check frequently the couples
used in heat treating operations. These couples
deteriorate with use and develop inhomogeneities along
their length. It does not pay to keep them after their
readings become 5 deg. or 10 deg. C. in error.
Neither the couple, the leads nor the pyrometer
should be moved or disconnected when making a
check. They should all be allowed to remain in their
usual working position. The secondary standard, connected
through its own leads to a portable pyrometer
of known accuracy, preferably of the potentiometer
type, should be placed alongside the working couple.
with its hot junction as close to the hot junction of
the working couple as possible. If the working couple
is installed in a furnace, a hole should be provided
close to the hole for the working couple through
which the checking couple may be inserted. Where
this is impracticable, the secondary standard may he
inserted through some other opening, or under the
slightly raised door, the open space being stopped with
fire brick or asbestos. The secondary standard may be
bare or may be protected by a light sheath of thin
seamless steel tubing.
It is usually sufficient to check the couple at one
temperature, which should be about the customary
operating temperature of the furnace. The furnace is
held at constant temperature until both couples with
their respective pyrometers have shown constant
readings for at least 10 minutes, to allow for lag, before
the check reading is taken. This method checks
not only the working couple but also the leads and