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 sender unit
• The indicator

The sending unit usually uses a float connected to a variable resistor. When the tank is full, the resistor is set to its low resistance value. As the tank empties, the float drops and the slides a moving contact along the resistor, increasing its resistance, finally reaching its highest value when the tank is empty.

Meanwhile, the indicator unit (usually mounted on the instrument panel) 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) rather than full (which would allow the driver to run out of fuel with no prior notification).

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.

See also

• Float switch
• Sight glass

HVAC | Mechanical engineering | Switches | Auto parts