A fuel gauge (or gas gauge) is an instrument used to indicate the level of fuel contained in a tank.[1] Commonly used in cars, these may also be used for any tank including underground storage tanks.
As used in cars, the gauge consists of two parts:
The sensing unit usually uses a float connected to a potentiometer. As the tank empties, the float drops and slides a moving contact along the resistor, increasing its resistance.[2] In addition, when the resistance is at a certain point, it will also turn on a "low fuel" light on some vehicles.
Meanwhile, the indicator unit (usually mounted on the dashboard) is measuring and displaying the amount of electrical current flowing through the sending unit. When the tank level is high and maximum current is flowing, the needle points to "F" indicating a full tank. When the tank is empty and the least current is flowing, the needle points to "E" indicating an empty tank.
The system is fail-safe; a fault that opens the electrical circuit causes the indicator to show the tank as being empty (which will provoke the driver to refill the tank (in theory)) rather than full (which would allow the driver to run out of fuel with no prior notification). However this system has a potential risk associated with it. An electric current is sent through the variable resistor to which a float is connected, so that the value of resistance depends on the fuel level. In most of automotive fuel gauges such resistors are on the inward side of gauge i.e. inside fuel tank. Sending current through such a resistor has fire hazard (and an explosion risk) associated with it. Therefore there is demand for another safer (and cheaper) method to be invented.
Systems that measure large fuel tanks (including underground storage tanks) may use the same electro-mechanical principle or may make use of a pressure sensor, sometimes connected to a mercury manometer.
Many aircraft use a different fuel gauge design principle. An aircraft may use a number (around 30 on an A320) of low voltage tubular capacitor probes where the fuel becomes the dielectric. At different fuel levels, different values of capacitance are measured and therefore the level of fuel can be determined. In early designs, the profiles and values of individual probes were chosen to compensate for fuel tank shape and aircraft pitch and roll attitudes. In more modern aircraft, the probes tend to be linear (capacitance proportional to fuel height) and the fuel computer works out how much fuel there is (slightly different on different manufacturers). This has the advantage that a faulty probe may be identified and eliminated from the fuel calculations. In total this system can be more than 99% accurate. Since most commercial aircraft only take on board fuel necessary for the intended flight (with appropriate safety margins), the system allows the fuel load to be preselected, causing the fuel delivery to be shut off when the intended load has been taken on board.
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