Fuel Measuring Systems- Tank Unit
The tank unit is the component which gives the system its name “Resistive Fuel Quantity Measuring System”. The tank unit is comprised of a float which is mechanically linked to a variable resistive element. The float rides on top of the fuel and will move up or down depending on the level of fuel within the tank. The floats movement is coupled via a linkage arm and gear assembly to a variable resistive element which then mimics the movement of the float.
The variable resistive element is comprised of two parts; a resistive strip, typically manufactured with Nichrome wire, and a wiper assembly. Nichrome wire has a predictable and stable resistance to electricity per inch of wire. Therefore it is possible to cut and form a piece of nichrome wire to a very exact electrical resistance value simply by adjusting the length of wire.
The wiper assembly is a moveable electrical contact which slides across the length of the nichrome wire while making direct physical contact with the wire. In our fuel measuring system one end of the nichrome wire will be connected to the aircraft power source and the other end will be connected to ground. The wiper will tap a varying amount of electricity off of the nichrome due to its physical contact. The amount of electricity that is tapped is determined by the overall resistive value of the wire and precisely where on the wire that the wiper is making physical contact. If the wiper has moved 24% down the length of the wire then 75% of the electricity is tapped off; at 50% movement then 50% is tapped off; at 75% movement then 25% is tapped off. This relationship is very predictable and very consistent and operates precisely under the principles of Ohm’s Law (E=I/R).
The electricity that is tapped off by the wiper is then transmitted by wire directly to the Fuel Quantity Indicator and is used to drive the pointer indicating fuel qty.
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