1. General
A. The exhaust gas temperature (EGT) indicating system is tested using a Wheatstone Bridge. Tests include measurements of circuit resistance and insulation.
B. Two operational tests are provided which may be used optionally to establish integrity of exhaust gas temperature indicating system. Neither test provides sufficient accuracy for system calibration, and is to be used only to indicate whether system operation is or is not satisfactory. Operational test No. 1 is preferred if conditions allow.
2. Equipment and Materials
A. Wheatstone Bridge capable of measuring resistance to ± 0.01 ohm at 15 ohms
B. Low voltage ohmmeter capable of measuring resistance of 100,000 ohms. Meter should utilize less than 40 volts (dc) and should have accuracy of 5%
NOTE: Low-powered digital meters may give a misleading indication when attempting to measure extremely high or infinite resistances. Analog meters operate at higher voltages and are preferred to digital meters.
C. Low value load resistance variable to within ± 0.05 ohms
D. DC power source variable from 0 to 50 millivolts
E. DC millivolt meter capable of measuring to ± 0.01 millivolt
F. Portable container of ice or accurate thermometer
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Oct 20/90 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 77-21-0 Page 501
3. Test Exhaust Gas Temperature Indicating System Resistance
A. Disconnect plug from exhaust gas temperature indicator on engine instrument panel.
B. Connect Wheatstone Bridge to sockets in plug and measure circuit resistance. Resistance reading should be as shown:
SYSTEM RESISTANCE (OHMS)*[1] AMBIENT TEMPERATURE
15.00士
0.05 20。士 5。C 68。士 9。F
14.97士
0.05 10。士 5。C 50。士 9。F
14.94士
0.05 0。士 5。C 32。士 9。F
NOTE: If resistance is not within tolerance, check circuit for loose, corroded or shorted connections and defective thermocouples. Dissimilar materials at connections (nuts, screws, washers, and etc.) can cause incorrect circuit resistance. The following information should be kept in mind in checking this system:
(1)
On the engine the thermocouple leads are alumel and chromel in the high temperature region to the firewall disconnect. The alumel lead is magnetic and negative and color-coded green. The chromel lead is nonmagnetic and positive and color-coded white.
(2)
The leads from the firewall disconnect to the EGT indicator are copper and constantan. The copper is positive and color-coded red. The constantan is negative and color coded yellow.
*[1] System resistance may be calculated for other ambient temperatures by extrapolating from the above values by using 0.03 ohms/10。C change.
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77-21-0 Page 502 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Sep 20/81
C. Adjust resistor spool to bring circuit resistance within specified limits, if required (Ref 77-21-11).
D. Remove Wheatstone Bridge and reconnect plug to indicator.
4. Test Insulation Resistance
A. Check thermocouple leads, from indicator to engine terminal strip, for short to ground.
(1)
Disconnect plug from exhaust gas temperature indicator on engine instrument panel.
(2)
Open left engine cowl panel (Ref Chapter 71).
(3)
Disconnect indicator leads (Chromel - white and Alumel - green) from terminal strip (Fig. 501).
(4)
Connect one side of ohmmeter to one of the leads and the other side of ohmmeter to ground.
(5)
Check lead for short to ground. The resistance shall not be less than 100,000 ohms.
(6)
Repeat steps (4) and (5) for the other lead.
(7)
Remove ohmmeter and connect leads to terminal strip.
(8)
Connect plug to the indicator.
B. Check thermocouple harness and leads for shorts to ground.
(1)
Disconnect harness leads from the engine terminal strip (Fig. 501).
(2)
Connect one side of ohmmeter to one of the harness leads and the other side of ohmmeter to ground.
(3)
Check the thermocouple harness and lead for shorts to ground. Resistance shall not be less than 50,000 ohms.
(4)
Remove ohmmeter and reconnect harness leads to the engine terminal strip.
(5)
Close engine cowl panels.
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Jan 20/83 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 77-21-0 Page 503
Exhaust Gas Temperature Indicating System Terminal Strip 501
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