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B.  Tank units are mounted to main tank wing ribs and to center tank spanwise beams. Access to the tank units is obtained through fuel tank access openings. Electrical connections to tank units are made at the tank unit terminals (Fig. 1). Tank unit leads, within a given tank, form a harness which terminates at a receptacle (Fig. 2). The receptacles for tanks No.1 and 2 are located in the respective wing front spar, and for the center tank in the main landing gear wheel well forward bulkhead on the left side. Mating with each receptacle is a bussing plug assembly. The plug assembly connects the tank units in parallel.
3.  Compensator Units
A.  The compensator unit in each tank acts as a sensing element to maintain a linear relationship between fuel dielectric properties and fuel density. Each compensator unit consists of a low impedance plate and two high impedance plates, both protected by a mechanical shield. The compensators are mounted to the same wing structure as tank units and are located so that they are completely submerged in fuel at all times. Access to the compensator units is obtained through the fuel tank access openings. Leads from the compensator units are contained in the same harness used for the tank units.
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4.  Fuel Quantity Indicators
A.  Fuel quantity indicators on the P2 panel show the weight of fuel in the tanks. Each indicator consists of the reference side of a self-balancing capacitance bridge, a power transformer, an amplifier, a reversible two phase induction motor, gear trains, pointers, a main dial, and a vernier dial. All indicators are hermetically sealed. Electrical connection, and full and empty adjustment screws are at the rear of each indicator.
B.  The fuel quantity indicators use 115-volt ac power for the bridge circuit through the transformer and for one phase of the induction motor. (The other motor phase is energized by amplifier output.) Indicator pointers are driven by the same shaft as the potentiometer wiper of the self-balancing bridge circuit and rotate with the wiper to provide continuous indication of the fuel quantity.
5.  Fuel Summation Unit
A.  The fuel summation unit on the P23 panel takes individual fuel quantity indicator voltage signals and sums them for an output voltage signal representative of the total fuel remaining. This output signal is one of several inputs to a performance data computer system which has a control display unit on the pilots' forward electronic control panel P9. The performance data computer converts these inputs into optimum airspeed and engine performance limits for climb, cruise, and descent but will also display the instantaneous total fuel remaining when the mode selector knob is positioned on STBY and key No. 6 is depressed. For information on the PDCS and CDU, (Ref Chapter 34).
B.  The fuel summation unit has calibration potentiometers consisting of one EMPTY potentiometer and an individual full potentiometer for each tank. Adjustment screws are accessible after removal of a protective cover on the top of the unit. Calibration adjustment allows precise setting of the input to the PDC such that it will always agree with the fuel sum total of the fuel tanks. All inputs and outputs are treated as voltage ratios thereby eliminating errors caused by bus supply voltage variations. The electrical connector to the fuel summation unit provides grounding connection for any tank input not in use.
6.  Fueling Quantity Indicators
A.  Fueling quantity indicators, on the P15 panel, show the weight of fuel in the tanks (Fig. 1). The fueling quantity indicators function as repeaters of the fuel quantity indicators located in the control cabin and aid the ground crew in fueling or defueling operations. The fueling quantity indicators are interchangeable with the fuel quantity indicators provided they are recalibrated to their new location. The fueling quantity indicators are operative only when the fueling station door is open. When the door is closed, power to the fueling quantity indicators is interrupted by a proximity switch (Fig. 2).
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Fuel Quantity Indicating System Schematic  5A6 
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526  Fuel Quantity Indicating System Schematic 
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7.  Measuring Sticks
A.  The measuring sticks are used to measure the fuel quantity in tanks No. 1 and 2 by mechanical means. Each main tank has five measuring sticks, calibrated in inches, located in wing fuel tank access panels No. 1, 4, 6, 8, and 10. The measuring stick unit of calibration is marked on each stick approximately one inch from end of stick.
　
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