(2)
Using low voltage ohmmeter, check insulation for short circuits, excessive leakage, and internal insulation chafing discrepancies. Check proper operation of instrument by touching test prods together and noting needle deflection to read zero ohms.
NOTE: Low voltage ohmmeter utilizing less than 40 volts (dc) and maintained at an accuracy of five percent shall be used for determining insulation resistance. Megger or other high voltage test shall not be used under any circumstances.
(3)
Place one lead of ohmmeter in good contact with lug A. Place other lead in contact with wire braid.
(4)
Read ohmmeter. Resistance reading should not be less than 50,000 ohms. If value below limit is recorded, presence of carbon or excessive moisture may be indicated. Carbon yields fairly steady reading. Moisture tents to produce readings which waiver or drift after five to 30 seconds. Presence of moisture may be corrected by baking affected part of EGT system at 200°-250°F (93°-121°C) for one hour and checking again.
NOTE: False drift may be due to variations in applied voltage. To check for false drift, place prods in contact with each other for ten seconds. Readings must not wander from full-scale deflection.+
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77-21-0 Page 604 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Sep 20/82
EXHAUST GAS TEMPERATURE INDICATING SYSTEM (SERVOED) -DESCRIPTION AND OPERATION
1. General
A. The exhaust gas temperature (EGT) indicating system measures the engine exhaust gas temperature and displays the temperature value on indicators in the control cabin. The system, for each engine, consists of eight temperature sensing probes, a harness and lead and a temperature indicator (Fig. 1). Chromel and alumel wires are used from the temperature sensing probes to the temperature indicator.
B. Engine exhaust gas temperatures are sensed by the eight thermocouple probes. The heat of the exhaust gases causes the thermocouples to generate DC electrical signals which are averaged by a thermocouple cable assembly. The averaged exhaust gas temperature signal is transmitted to the EGT indicator.
2. Exhaust Gas Temperature Thermocouple Probe
A. The thermocouple probe is a temperature sensing device that senses the temperature of the engine exhaust gases. It consists of two stud terminals, a head, and a thermocouple measuring junction and leads enclosed in a cylindrical shield (Fig. 1). The thermocouple junction leads and terminals are made of chromel and alumel material. The alumel terminal (-) has a larger diameter stud than the chromel terminal (+). Each probe is provided with five gas inlet ports and two gas exhaust ports. The correct orientation of the probe with respect to an engine is obtained with an index slot in the probe.
B. Engines incorporating an EGT boost circuit (JT8D-17A) utilize a thermocouple lead featuring circuitry that artificially raises the EGT readout by 50°F (28°C) to an indicated EGT of 1274°F (690°C). 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.
C. Eight probes are mounted on the turbine exhaust section fan discharge inner duct of each engine and project into the engine exhaust gas path. The probes are connected in parallel to obtain the average value of the exhaust gas temperatures sensed by the eight probes.
3. Exhaust Gas Temperature Indicator
A. The exhaust gas temperature indicator is an independently powered servoed indicator 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 indicator is powered by 115 volts AC from the AC standby bus. The instrument dial indicates a temperature range from 0 to 850°C, with normal and dangerous operating temperatures marked in a color code. The temperature range 500-700° is expanded to facilitate readings within that range.
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May 20/82 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 77-21-01 Page 1
4. 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.
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