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C.  Engines incorporating an EGT boost circuit (JT8D-15A) utilize a thermocuple lead featuring circuitry that artificially raises the EGT readout by 500F (280C) to an indicated EGT of 12740F (6900C). A proportionately lesser quantity of boost will occur at a lower indicated temperature. This is accomplished by isolating the output of two selected thermocouple probes, reducing the averaged output of these two probes to the desired value of the boost signal, and adding this boost signal to the averaged EGT output of the remaining six thermocouple probes. The result is a boosted EGT reading for cockpit use. The thermocouple lead associated with this boost circuit contains an integral fixed resistor to reduce the boost signal to its desired value.
3.  Exhaust Gas Temperature Thermocouple Balancing Resistor
A.  The exhaust gas temperature thermocouple balancing resistor permits adjustment of circuit resistance. The resistor consists of a spool of No. 24 constantan wire, coarse adjustment terminals and vernier adjustment terminals. Since primary adjustment of the resistance is made by cutting off lengths of the wire, a spare resistance spool is included for each engine system. The thermocouple resistors are located in a box in the P6 panel area and are connected into the constantan lead of the circuit.
5C2 
Sep 20/82  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  77-21-0 Page 1 

Exhaust Gas Temperature Indicating System  500 
77-21-0  Figure 1  Feb 20/86 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

4.  Exhaust Gas Temperature Indicator
A.  The exhaust gas temperature indicator is a specially built, sensitive millivoltmeter with a dial, graduated in degrees centigrade, housed in a hermetically sealed case. Two indicators, one for each engine, are mounted on the center instrument panel. Two terminals, marked plus (+) and minus (-), on the back of the indicator connect the meter unit to the thermocouple circuit. The instrument dial indicates a temperature range from 00 to 8500C, with normal and dangerous operating temperatures marked in a color code.
5.  Exhaust Gas Temperature Thermocouple Harness and Lead
A.  The thermocouple harness is a flexible electrical conduit mounted on the circumference of the engine exhaust fan discharge inner duct. A two-wire thermocouple lead from the harness is routed forward to an electrical terminal strip on the fan discharge diffuser outer duct. The harness is connected to the lead with a nine pin electrical plug or junction box, located on the engine exhaust section fan discharge outer duct at approximately, the 7 o'clock position.
6.  Operation
A.  Each engine exhaust gas indicating system consists of eight thermocouple probes connected in parallel to an indicator and operate on self-generated power. The airplane power is used only for integral lighting of the indicators (Fig. 2).
B.  The difference in temperatures between the hot junction at the thermocouple probes and the cold junction at the indicator causes electrical signals to be generated in the thermocouple circuit. These electrical signals are proportional to the temperature difference between the two junctions and are measured by the indicator. Since the generated signals correspond only to the temperature difference, correction due to control cabin ambient temperature variation must be made to this measured temperature difference to obtain the actual temperature of the exhaust gases; the indicator does this correction automatically. The control cabin temperature where the indicator is located, is reasonably constant; however, small temperature changes in the indicator are compensated with a bimetal (thermostatic) spring, connected to the indicator control mechanism. The indicated exhaust gas temperature is therefore, an accurate indication irrespective of prevailing ambient temperatures.
C.  The thermocouple probes are connected in parallel to obtain the average temperature of the exhaust gases. Another advantage of the parallel connection is that if a probe burns out or is damaged, thermocouple circuit is not disrupted.
500 
Nov 15/69  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  77-21-0 Page 3 

Exhaust Gas Temperature Indicating System Circuit  507 
77-21-0  Figure 2  Oct 20/86 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

EXHAUST GAS TEMPERATURE INDICATING SYSTEM - TROUBLE SHOOTING EXHAUST GAS TEMPERATURE INDICATING SYSTEM - ADJUSTMENT/TEST

500 
May 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  77-21-0 Page 101 
　
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