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allow the exhaust gas to flow across the junction.
The materials from which the thermocouples wires
are made are usually nickel-chromium and nickelaluminium
alloys.
30. The probes are positioned in the gas stream so
as to obtain a good average temperature reading
and are normally connected to form a parallel circuit.
An indicator, which is basically a millivoltmeter
calibrated to read in degrees centigrade, is
connected into the circuit (fig. 12-8).
31. The junction of the two wires at the thermocouple
probe is known as the ’hot’ or ’measuring’ junction
and that at the indicator as the ’cold’ or ’reference’
junction. If the cold junction is at a constant
temperature and the hot junction is sensing the
exhaust gas temperature, an electromotive force
(E.M.F.), proportional to the temperature difference
Controls and instrumentation
139
Fig. 12-6 Variable-reluctance speed probe
and phonic wheel.
of the two junctions is created in the circuit and this
causes the indicator pointer to move. To prevent
variations of cold junction temperature affecting the
indicated temperature, an automatic temperature
compensating device is incorporated in the indicator
or in the circuit.
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 temperaturesensitive
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 dialtype
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
amount of change is proportional to the pressure
applied at the transmitter.
39. The transmitter may be of either the direct or the
differential pressure type. The latter senses the
pressure difference between engine feed and return
Controls and instrumentation
140
Fig. 12-7 Turbine thermocouple installation.
oil pressures, the return oil being pressurized by
cooling and sealing air (Part 9) from the bearings.
40. In addition to a pressure gauge operated by a
transmitter, an oil low pressure warning switch may
be provided to indicate that a minimum pressure is
available for continued safe running of the engine.
The switch is connected to a warning lamp in the
flight compartment and the lamp illuminates if the
pressure falls below an acceptable minimum.
Fuel temperature and pressure
41. The temperature and pressure of the low
pressure fuel supply are electrically transmitted to
their respective indicators and these show if the low
pressure system is providing an adequate supply of
fuel without cavitation and at a temperature to suit
the operating conditions. The fuel temperature and
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