Measuring the fuel level in a car’s petrol tank is not an exact science in the true sense of the word. The fuel level sensor sends a signal to the gauge which only gives readings between “Full” and “Empty” - with a varying degree of accuracy and without telling you how many litres are left in the tank.
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The fuel level sensor, commonly known as the “sender unit,” is situated in the fuel tank. This consists of three components: a float, a float arm consisting of a thin metal rod attached to the float, and a variable resistor connected to the working end of the float arm.
A resistor is a device that restricts the flow of electricity through an electrical wire. The higher the resistance, the lower the amount of electricity or “amperage,” flowing through the circuit. The variable resistor in your fuel tank is made from a strip of resistive material with one end connected to earth and the other end connected to your fuel gauge via a “wiper.” A wiper controlled by the float arm slides across this strip, and depending on the position of the float arm, transmits the amount of current flowing to the fuel gauge. This makes the needle on your gauge respond by moving between the “Full” and “Empty” marks.
When your tank is full, the float is submerged. This means the needle on your gauge will remain on the “Full” mark until the level drops to the point where the float is actually resting on the surface. Conversely, when the level drops to the point where the bottom of the float rests on the bottom of the tank, the gauge will show “Empty” even if there are several litres left between the top of the float and the bottom of the tank. This is why there’s a “Fuel Reserve” light on your dashboard telling you when to add more fuel.
Fuel tanks vary in shape and size because they are moulded to fit into tight spaces between the chassis, various components and the car’s body. For example, a narrow top and wider base will affect the variation in accuracy between “Full” and “Empty” on the fuel gauge. In addition, when the fuel reaches the halfway mark, there might be significantly more or less than half of the tank’s capacity remaining.
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