Page 150 - The Jet Engine

Controls and instrumentation
Fig. 12-7 Turbine thermocouple installation.
of the two junctions is created in the circuit and this amount of change is proportional to the pressure
causes the indicator pointer to move. To prevent applied at the transmitter.
variations of cold junction temperature affecting the
indicated temperature, an automatic temperature
compensating device is incorporated in the indicator differential pressure type. The latter senses the
or in the circuit.
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32. The thermocouple probes may be of single,
double or triple element construction. Where multiple
probes are used they are of differing lengths in order
to obtain a temperature reading from different points
in the gas stream to provide a better average reading
than can be obtained from a single probe (fig. 12-7).
33. The output to the temperature control system
can also be used to provide a signal, in the form of
short pulses, which, when coupled to an indicator,
will digitally record the life of the engine. During
engine operation in the higher temperature ranges,
the pulse frequency increases progressively causing
the cyclic-type indicator to record at a higher rate,
thus relating engine or unit life directly to operating
temperatures.
34. Thermocouples may also be positioned to
transmit a signal of air intake temperature into the
exhaust gas temperature indicating and control
systems, thus giving a reading of gas temperature
that is compensated for variations of intake
temperature. A typical double-element thermocouple
system with air intake probes is shown in fig. 12-8.
Oil temperature and pressure
35. It is essential for correct and safe operation of
the engine that accurate indication is obtained of
both the temperature and pressure of the oil.
Temperature and pressure transmitters and
indicators are illustrated in fig 12-9.
36. Oil temperature is sensed by a temperature-
sensitive element fitted in the oil system. A change in
temperature causes a change in the resistance value
and, consequently, a corresponding change in the
current flow at the indicator. The indicator pointer is
deflected by an amount equivalent to the
temperature change and this is recorded on the
gauge in degrees centigrade.
37. Oil pressure is electrically transmitted to an
indicator on the instrument panel. Some installations
use a flag-type indicator, which indicates if the
pressure is high, normal or low; others use a dial-
type gauge calibrated in pounds per square inch
(p.s.i.).
38. Electrical operation of each type is similar; oil
pressure, acting on the transmitter, causes a change
in the electric current supplied to the indicator. The
39. The transmitter may be of either the direct or the
pressure difference between engine feed and return