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747-400
MAINTENANCE MANUAL

TANK UNIT

COMPENSATOR
DENSITOMETER

Fuel Quantity Indicating System Component Location
Figure 1 (Sheet 6)

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747-400
MAINTENANCE MANUAL

DENSITOMETER
H

TANK UNIT (TYPICAL)
I

COMPENSATOR J Fuel Quantity Indicating System Component Location Figure 1 (Sheet 7)
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MAINTENANCE MANUAL

 3.  Fuel Quantity Compensator (Fig. 1)
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 A.  The fuel quantity compensator serves as another sensing element for each subsystem. The unit consists of four metallic electrodes, a terminal block and two mounting clamps. The parts are mounted on a metallic tube. The four electrodes form a capacitor. The compensator mounts at the bottom of the fuel tank so that it is completely covered by fuel until the tank is almost empty. As a result, the compensator capacitance is affected only by changes in the dielectric constant of the fuel.
 B.  One compensator is mounted in each fuel tank and electrical connections are made at the terminals using two shielded leadwires. The shielded high impedance leadwire is common to both the compensator and the tank units in the tank.
 4.  Fuel Quantity Densitometer (Fig. 1)
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 A.  The fuel densitometer measures the density of the fuel in the tank. One densitometer is installed in each main tank and in the center wing tank. The reserve tanks use fuel density data from the outboard main tanks.
 B.  The densitometer is composed of a nickel alloy wiring-wrapped spool, an excitation coil, a sensing coil, and eight calibration resistors, all contained within a vented housing. The housing vents allow fuel to enter the housing and allow air to escape. The densitometer has two terminal blocks, each with four terminal mounting studs. Different sized studs on each terminal block ensure that the wiring harness is connected properly.
 C.  Fuel density is determined by causing the spool to vibrate, then measuring the frequency of vibration. The fuel quantity processor unit sends a signal at a given frequency through the excitation coil of the densitometer. The field produced by the coil causes the spool to vibrate at its resonant frequency. A signal is induced in the sensing coil and returned to the processor. The processor uses the frequency of the return signal to compute fuel density. Fuel density is proportional to the resonant frequency of the spool. If the fuel density changes, the spool will vibrate at a different resonant frequency, inducing a different return signal to the processor.
 5.  Fueling Quantity Indicator (Fig. 1)
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 A.  The fueling quantity indicator converts a digital signal from the fuel quantity processor into an indication of fuel quantity. If signal from the processor becomes invalid or not available, the indicator will display fuel quantity for approximately 4 seconds, then blank.
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MAINTENANCE MANUAL

 B.  Each indicator contains two four-digit LCDs installed in a hermetically sealed case. For the total and center wing indicator, the upper LCD displays actual fuel quantity and the lower LCD displays preselected fuel quantity. The other indicators display fuel quantities for two tanks each. Fuel quantity is displayed on the indicators in LB X 1000 or KG X 1000. A fuel quantity below 1000 lbs (1000 kgs) is displayed on an indicator with a leading zero - 0.5 represents 500.
　
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