(2)  
Each tank unit consists of two concentric aluminum tubes, a terminal block, two end caps, and two brackets.

 (a)  
The aluminum tubes are anodized and polyurethane coated to protect against corrosion. The tubes are factory calibrated. The distance between the tubes is relatively widely spaced to avoid electrical short circuiting, due to water in the fuel or fungus coating on the tubes.

 (b)  
The tank wiring harness attaches to the tank unit at the terminal block. The wiring terminals on the tank wiring harness and the terminal posts on the terminal block have different dimensions to prevent errors in installation.

 (c)  
The end caps are insulated to help prevent the tank unit from shorting to the ground. They also supply capacitance fringing control.

 (d)  
Two brackets made of nonferrous glass-filled nylon, attach each tank unit to the wing structure.

 (3)  
The tank unit measures the fuel level in the fuel tank. It is a linear, capacitance type sensor. Tank geometry is accounted for by computer software. The concentric aluminum tubes of the tank unit act as plates of a capacitor, and the dielectric is air and/or fuel depending on the quantity of fuel in the fuel tank. This air to fuel ratio gives a capacitance value which is used to calculate the volume of fuel.

 B.
Compensator (Fig. 3)

___________
 (1)  
One compensator is installed in each fuel tank. All compensators are the same. The compensators are almost the same in construction and installation provisions as the tank units.

 (2)  
Each compensator contains four metallic tubes, or electrodes, which attach to the support tube, a terminal block and two brackets. The four electrodes form a capacitor.

 (3)  
The compensator senses the dielectric constant of the fuel. It is a capacitance sensing device almost the same as the tank unit. The compensator is installed at the bottom of the fuel tank, completely covered by fuel until the fuel tank is almost empty. As a result, the compensator capacitance is affected only by changes in the dielectric constant of the fuel.

 C.
Densitometer (Fig. 4)

____________
 (1)  One densitometer is installed in each fuel tank. The densitometer measures the fuel density.
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 ú CONFIG ú 07 Page 3 ú Sep 20/93

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757
MAINTENANCE MANUAL

REAR SPAR TANK UNIT LOCATION - LEFT WING FQIS Tank Units Figure 2 (Sheet 1)
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