(2)  
Tank Unit Signals and Programming

 (a)  
Strategically located tank units measure the fuel quantity in each fuel tank. Each fuel tank also contains a compensator, which senses the dielectric of the fuel. Both the tank units and the compensator are used in computing fuel quantity.

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LOAD SELECT INDICATOR FQIS PROCESSORTEST SWITCH
Fuel Quantity Indicating System (FQIS) SchematicFigure 13
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 (b)  With power applied to the FQPU, an excitation signal of 5 volts peak, 18.75 khz is sent to the tank units to measure their capacitance value. This is accomplished by comparing the tank unit against a rebalance capacitor of known value, which increments or de-increments until a balance is reached. This operation is carried out for each tank unit, and a computation of tank volume made by the microcomputer as follows. 1) The computer program divides each tank unit into 12 equal
 sections, and assigns a value proportional to tank shape. Wetted sections of the tank units are summed, and the value of unusable fuel stored in the computer memory is subtracted from the total. Fuel quantity indicators show the quantity of usable fuel.
 2)  The values assigned tank unit sections vary with airplane attitude. The computer program contains four different characterizations for the ground conditions and four for the air condition.
 (c)  
The high frequency signal used for fuel measurement avoids interaction with other airplane systems. The signal also tolerates low insulation resistance, and is not sensitive to fuel contamination.

 (3)  
EICAS Messages

 (a)  
The FQPU sends signals of low fuel, fuel configuration, and system faults to the EICAS computer. 1) The EICAS computer will generate and display a LOW FUEL

 caution message and the FUEL CONFIG light will come on under the following condition: a) When the quantity of fuel in either fuel tank is below
 2200 lbs or 1000 kgs.
 2)  A fuel configuration signal is sent to EICAS and to the FUEL CONFIG light on the P5 panel if either of the subsequent conditions exist. The EICAS computer will generate and display a FUEL CONFIG advisory message. a) A fuel imbalance of more than 1800 lbs or 800 kgs
　
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