What Is a Fuel Gauge?

A fuel gauge is a device inside of a car or other vehicle that measures the amount of fuel still in the vehicle. This type of system can be used to measure the amount of gasoline or some other type of liquid. It will typically consist of a sensing or sending unit that measures the amount of fuel actually left, and a gauge or indicator that relays this information outside the fuel container. A fuel gauge can be designed in a number of different ways, and many gauges have several flaws that can make the readings less than accurate.
Also called a gas gauge, a fuel gauge is typically used to relay information about fuel storage to a person operating a vehicle or using a storage container. For a car, this gauge typically consists of two parts that work together to measure the amount of fuel and relay this measurement to a driver. The two parts of a fuel gauge are the sensing or sending unit and the indicator or gauge.
A sensing unit is the part of a fuel gauge found within or connected to the actual fuel storage container on a vehicle. On a car, for example, the sensing unit will consist of a float inside the fuel tank, which is connected to a metal rod that runs to a small electrical circuit. The float raises or lowers depending on the amount of gasoline in the fuel tank.
As the float moves, the arm it is connected to moves a wiper that is connected to a resistor in that electrical circuit. When the tank is full and the float is at the top, the resistor is moved to one side of the circuit and a large amount of current is able to pass through. As the tank empties and the float lowers, the wiper moves in the circuit, the resistance increases, and current reduces.
On the other side of the fuel gauge, there is an indicator or gauge that displays information about fuel levels to the driver of a car. This can consist of either a bimetallic strip or a microprocessor connected to the circuit in the sensing unit. As resistance changes in the circuit, current increases or decreases, and the bimetallic strip will bend or straighten out depending on changes in current that increase or decrease the temperature of the strip. As the strip changes shape, it will physically move a needle on a fuel display, relaying that the fuel tank is empty, full, or some amount in between. A microprocessor is able to process the changes in current and then sends a signal to a mechanism that moves a needle in a similar way.
The design of a fuel gauge, however, creates some potential for errors in accurate fuel measurement. For example, the float in the fuel tank can reach the top of the tank and will not go down until fuel has gone below the bottom of the float. This is why many fuel gauges will read full after some fuel has been used since the float has not begun to descend. Once the float reaches the bottom, it will display that the tank is empty, even if there is still a significant amount of fuel left in the container. The shape of a fuel tank can also affect the accuracy of a fuel gauge, since uneven shapes may not have easily measured volumes.