5. Exhaust Gas Temperature Indicator
A. The EGT indicator is a specially built, sensitive millivoltmeter with a dial, graduated in degrees centigrade, in a hermetically sealed case. An electrical plug on the back of the indicator connects the meter unit to the thermocouple circuit. The instrument dial indicates a temperature range from 0 to 850°C, with normal and dangerous operating temperatures marked in a color code.
6. Operation (Fig. 2)
A. The APU engine EGT indicating system consists of a thermocouple probe connected to an indicator and operates on self-generated power. The airplane power is used only for lighting of the indicator.
B. The difference in the temperatures at the probe (the hot junction of the thermocouple) and the indicator (the cold junction of the thermocouple) 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, the control cabin ambient temperature must be added to this measured temperature difference to obtain the actual temperature of the exhaust gases; the indicator does this addition 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 EGT is therefore an accurate indication irrespective of prevailing ambient temperatures.
APU EGT Indicating System Circuit 500
49-71-0 Figure 2 Feb 20/91
Page 4
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
APU EXHAUST GAS TEMPERATURE INDICATING SYSTEM - ADJUSTMENT/TEST
1. General
A. The APU exhaust gas temperature (EGT) indicating system test consists of two separate tests:
(1) an insulation resistance test of the complete EGT circuit, including the EGT thermocouple probe; (2) a test of the installed EGT harness including balancing resistor. Tests may be performed independently or as a complete system test.
B. When performing the insulation resistance test, the following information must be kept in mind: On the APU engine, the leads are Alumel and Chromel. The Alumel lead is magnetic and negative and is color coded green; the chromel lead is nonmagnetic and color coded white. In the fuselage, the leads are copper and constantan. The copper lead is positive and color coded red; the constantan lead is negative and color coded yellow.
C. The exhaust gas temperature (EGT) balancing resistor is located on the control cabin structure directly under the right No. 3 window. The circuit resistance is adjusted by moving the constantan leads from terminal to terminal on the coarse or vernier adjustments. The resistance increment between two coarse adjustment terminals is 0.15 ohm and between two vernier adjustment terminals is 0.05 ohm. External circuit resistance excluding the indicator and engine harness is 13.50 ohms nominal. To decrease the circuit resistance beyond coarse and vernier adjustment range, the wire on resistance spool is shortened. To increase the resistance beyond coarse and vernier adjustment range, the spare or new resistance spool is used and adjusted.
2. Insulation Resistance Test
A. Equipment and Materials
(1) Ohmmeter - capable of measuring resistance to 100,000 ohms
B. Prepare for Insulation Resistance Test
(1)
Position BATTERY SWITCH and APU START SWITCH to OFF and placard that APU maintenance is "in work."
(2)
Open the circuit breaker on M280 module-APU accessory unit on E3-3 electronic shelf.
(3)
Disconnect plug from APU EGT indicator on forward overhead panel.
(4)
Open APU compartment access door latches and open access door. Install door support rods.
(5)
Support lower shroud, open shroud latches, and remove shroud.
CAUTION: EXERCISE EXTREME CARE NOT TO DAMAGE SHROUD DRAIN LINES WHEN HANDLING SHROUD.
(6)
Disconnect engine EGT lead at receptacle in upper shroud.
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Nov 15/76 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 49-71-0 Page 501
C. Test Insulation Resistance
(1)
Test Insulation Resistance between EGT Indicator Plug and receptacle in upper shroud.
(a)
Connect one side of ohmmeter to receptacle pin A in upper shroud and other side of ohmmeter to ground.
(b)
Test lead for short to ground. Resistance must be more than 100,000 ohms.
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