(5)  
The collimator fits around the radioactive source, and directs a conical shaped beam of low level radiation through the fuel to the two detectors in the densitometer electronics unit. The distance through fuel from the radioactive source to each detector is different.

Densitometer Emitter
Figure 6

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757
MAINTENANCE MANUAL

 F. Center Tank Densitometer Fuel Retention System (Fig. 6A)
______________________________________________
 (1)  
A fuel retention system is installed on the center tank densitometer and consists of a retention canister and a float valve.

 (2)  
The system allows the center tank densitometer to be covered with fuel before the center tank fuel level is high enough to cover the densitometer. This allows the center tank densitometer to determine fuel density, while refueling, at an earlier time than would be possible without the system.

 (3)  
At low center tank fuel volume, the float valve is open allowing fuel from the fueling manifold to flow into the retention canister. The canister, when filled with fuel, keeps the densitometer emitter covered with fuel.

 (4)  
As the level of fuel in the center tank increases, the float valve shuts off flow of fuel from the manifold to the retention canister. This allows the fuel in the cannister to drain, then refill with center tank fuel as the center tank fuel level rises.

 G.
Fuel Quantity Indicator (Fig. 7)

_______________________
 (1)  
A fuel quantity indicator is installed on the overhead panel, P5. It provides digital liquid crystal displays of individual fuel tank, and total fuel tank quantities; and fuel temperature.

 (2)  
The fuel quantity indicator is contained in an aluminum housing with a front panel which has five liquid crystal display windows. The back panel contains three electrical connectors. Inside the fuel quantity indicator are four printed circuit boards, and twelve lamps for internal lighting.

 (3)  
28V dc is supplied to the fuel quantity indicator from the FQIS processor. 5V ac for internal lighting is supplied from aircraft lighting circuits.

 (4)  
The fuel quantity indicator can be tested by putting the FUEL QTY test switch on the P61 panel to FUEL QTY. The total fuel display should show 188.8. The individual fuel tank displays should show

 88.8. The fuel temperature indicator should show -185|2. Returning the test switch to the neutral position causes the displays to show the quantity of usable fuel.
　
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