2548H INSTRUCTION MANUAL
56-101-00420 Rev. C
Ch. 3
Nov
/
07
/
14
Page 11 of 33
CHAPTER 3: OPERATION
1. GENERAL INFORMATION
Fuel Quantity Indicating Systems provide the crew with an accurate display of the remaining
fuel. The quantity of fuel shows on the fuel quantity indicator(s) in the flight compartment.
The Fuel Quantity Indicating System includes the amplifier or signal conditioners (also
known as the
amplifiers
) and the probe(s)/transmitter(s) that are installed in each tank (also
knows as the
Tank Unit
). A Tank Unit is a capacitor. It consists of two (2) conductive tubes
separated by an air gap, or a combination of air and fuel. The Unit is connected to terminals
that are, in turn, connected to the amplifier. This unit measures the fuel mass or weight.
The volume or weight is determined by the dielectric constant of the insulating air or fuel, or
air/fuel mixture in the tank; then, fed to other units for refinement or amplification before
being displayed on the indicator.
Note: For more complete and detailed information on specific Fuel Quantity Systems, refer
to the appropriate Aircraft/System Maintenance Manuals.
A. Stray Capacitance Compensation
A certain amount of stray capacitances exists within the Test Set (T/S). This could be
caused by exposure of unshielded lead wire at connecting points or capacitance
between switch contacts and between any HI-Z to LO-Z conductors (which cannot be
effectively shielded from one another.
These stray capacitances change from one function to another due to the addition or
subtraction of circuitry and, due to the differences in the mechanical position of movable
contact switches. A mounted circuit on the meter PC board compensates these stray
capacitances. To ensure accurate results, adapter harness stray capacitances should
be subtracted from the measured values and calibration settings.
