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June, 1925
so that as the latter is adjusted to keep the galvanometer
at zero, the pen follows its motion. The pen
corresponds to the index on the indicating potentiometer.
The deflection of the galvanometer pointer
automatically controls the movement of the slide wire
and pen by an ingenious arrangement of cams and
levers. The galvanometer is not required to do any
work. The record strip is moved forward at a uniform
rate by means of a small electric motor whose
speed is accurately governed. This motor also furnishes
the power for operating the cams and levers
which move the slide wire and pen. The battery circuit
is adjusted by hand. The cold junction temperature
is compensated automatically by the electrical
method described for the indicating instrument. A recorder
of this type is illustrated in Fig. 101.
Where a record of more than one thermocouple is
called for, the pen is replaced by a print wheel, which
prints a series of dots corresponding to the temperature
of any couple. A number is printed alongside
the dot for each couple, so that the record of each couple
may be readily identified. This instrument automatically
adjusts the battery circuit, as well as compensating
for the cold junction temperature. Its chart
is particularly wide and clear, which facilitates accurate
readings. A commutating switch, operated by
the motor and geared with the print wheel, connects
one couple after another into the circuit.
Controllers.
It is but a step in mechanical or electrical design
from an instrument which records the temperature of
a furnace to one which automatically controls that
temperature within desired limits.
For furnaces heated electrically, this simply necessitates
opening and closing a switch or circuit breaker
by means of a suitable relay. The relay is operated
by a contact controlled by the pointer or index of the
pyrometer. When the latter goes above a certain predetermined
temperature, it makes an electrical contact
which operates a relay that shuts off the power. The
furnace then begins to cool, but as soon as the temperature
has fallen a few degrees, to a predetermined
low limit, another contact is made which causes the
current to be turned on again. The same principle is
applied to gas and oil fired furnaces. Instead of opening
and closing an electric switch, the relays operate
the valves in the fuel and air lines. Controllers of
various makes are capable of holding the temperature
of a furnace constant within a few degiees Centigrade.
Automatic controllers find their chief application for
furnaces which must operate for long periods at uniform
temperature.
PART 7. — CALIBRATION
Thermocouples, guaranteed to be within a certain
accuracy, may be purchased from manufacturers of
pyrometric equipment. A chart or conversion table
is furnished, giving the temperature-e.m.f. values over
the working range for a certain type of couple, for
example, iron-constantan, or chrome-alumel, and the
couples are guaranteed to correspond with the chart
within a specified tolerance. If the couples are intended
for use with an instrument which reads directly
in degrees Centigrade or Fahrenheit, the chart is
not necessary, the couples simply being guaranteed
Forging- Stamping - Heat Treating
105
to give correct readings with the instrument in question.
A pyrometer whose scale is graduated in degrees
must be used only with the particular type of couple
with which it was calibrated.
Many users prefer to make up their own thermocouples,
from wires furnished for the purpose. Certified
thermocouple wire, guaranteed to produce couples
of a specified accuracy, is available. This wire requires
great care in manufacture, to insure correct
composition and exclude impurities. Otherwise it will
deviate from the standard temperature e.m.f. relation.
Every coil of certified wire must be checked by the
manufacturer, and this increases its cost. It generally
pays to buy certified wire, to insure interchangeability
of couples.
FIG. 99—Circular chart recording pyrometer,
millivoltmeter type.
It is often desirable to check the manufacturer's
calibration or to make an independent calibration of
thermocouples or thermocouple wires. Methods of
standardization may be divided into two classes, (1)
direct standardization, in which the couple is calibrated
directly in terms of certain fixed temperatures,
such as melting or freezing points of a series of chemically
pure metals, and (2) comparison standardization,
in which the couple under test is compared writh another
couple which has been calibrated as above. Direct
standardization requires special apparatus and a
considerable amount of skill. The U. S. Bureau of
Standards is prepared to do such work for a nominal
fee, and the cost is usually less and the results more
satisfactory than when attempted by an industrial
plant. The best plan for an industrial plant which
requires a large number of working couples, is to keep
either two or three noble metal couples for "primary
standards" and to have these calibrated by the Bureau
of Standards. One of these couples is then used to
check thermocouple wire, or to calibrate a few base
metal couples as "secondary standards" which are, in
turn, used to check the working couples in the plant.