Fuel gauge

A fuel gauge

The empty fuel indicator displaying on the fuel gauge for a 2003 Honda Civic.

A fuel gauge (or gas gauge) is an instrument used to indicate the level of fuel contained in a tank. 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
• The indicator

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. 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 a 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.

Large airplanes use a different fuel gauge design principle. An airplane has a few (around 30 on an A320) low voltage capacitors where the fuel can go between them. At different fuel levels, there are different capacitances (i.e. capacitance proportional to the height of fuel) and therefore the level of fuel can be determined. For the aircraft pitch and roll attitudes the fuel computer works out how much fuel there is (slightly different on different manufacturers). In total this is more than 99% accurate.

See also

• Float switch
• Sight glass

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