interconnecting Chromel and Alumel leads.
The feedback of the potentiometer and output
Power for the indicator-amplifier is from the
of the compensator go to the chopper, where it
essential 115-volt ac bus.
compares the inputs. The chopper provides a
400-hertz error signal to the servo system. The
Both exhaust gas temperature indicators are
chopper output (signals relative to temperature
on the pilot's main instrument panel. They
change and potentiometer versus compensator
provide a visual indication of the engine exhaust
difference) goes to a servo amplifier. The servo
temperatures. Each instrument is a hermetically
amplifier modifies the signals to drive the
sealed unit with a single receptacle for a mating
servomotor. The shaft of the motor couples to
plug electrical connection. The instrument scale
the rebalance potentiometer and indicator tape
ranges from 0C to 1,200C. There is a vernier
through the gear train.
dial in the upper right corner of the instrument
As the amplifier error drives the motor, the
face. A power-off warning flag is in the lower
rebalance potentiometer goes in a direction that
portion of the dial.
reduces the error signal, nullifying the condition.
Internally, the indicator contains a simulated
The tape shows temperature, on the front scale
thermocouple cold junction with compensating
of the indicator, relative to thermocouple output.
resistors, a reference voltage source, and a
When supplied with 28 volts dc, a test circuit in
dc-to-ac modulator. It also contains a transistor
the indicator energizes a relay, disconnecting the
power output stage, miniature ac servomotor, and
thermocouple input. Then, it substitutes a test
the power-off warning flag. The temperature
signal of specific value to be processed and to
indicator contains range markings on the
drive the indicator tape.
instrument face.
EXHAUST GAS TEMPERATURE INDI-
The thermocouples convert engine exhaust gas
CATING SYSTEM. --The exhaust gas tempera-
temperature into millivolts. The voltage from the
thermocouples go directly to the indicator-
ture (EGT) indicating systems provide a visual
temperature indication in the cockpit of the engine
amplifier through the Chromel and Alumel leads.
exhaust gases. The following is a discussion of a
The voltage is amplified and drives a small
typical exhaust gas temperature indicating system.
servomotor. The motor, in turn, drives the
The aircraft contains two separate but
indicator pointer. The thermocouple harness
identical exhaust gas temperature indicating
consists of two halves, each containing six
systems (fig. 6-52), one for each engine. Each
dual-loop thermocouples. The assembled halves
system has 12 dual thermocouples, a combination
make up two independent thermocouple systems,
indicator and transistorized amplifier, and the
each consisting of 12 thermocouples connected in
Figure 6-52.-Exhaust gas temperature indicating system.
6-48
