As used in cars, the gauge consists of two parts:
The sensing unit (sender)
The indicator unit (gauge)
The sensing unit (or sender) usually uses a float connected by a lever to a variable resistor 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 may 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. In recent years there have been electronic fual gauges incorporated into the instrument panel that display the fuel using a liquid crystal display, but behind the scenes they usually still use a resistive sender and measure the current flowing through it
This all sounds straightforward, but things are not as simple as they seem !
The sender may not be linear ! This is due to a combination of two things, The tank may not be a uniform cross section all the way. This has an effect on how the fuel gauge must work. As the fuel level rises the moving contact will not move uniformly along the resistor. Additionally, as the float drops, it moves along a curved path which also affects the way the contact moves along the resistor. Most manufacturers compensate for this non-linearity by winding the resistor around a specially shaped support, by varying the pitch of the resistor coils, or a combination of both.
The indicator may not be linear ! Older indicators were of the hot wire type. They have a thin wire connected to the pointer and as the current increases, the wire gets hot and stretches, thus moving the pointer. This in itself is fairly linear, but often additional components to slow the rate of movement, or compensate for a non-linear sender make the guage behave in a non-linear way.
The indicator may be connected to a voltage regulator in the dash.
There are several different standards for the amount of current needed to show full or empty.
A fuel gauge that responds immediately to a change in the sender float position is undesirable as it will show different amounts of fuel depending on whether the vehicle is accelerating, braking, climbing a hill, or rounding a corner.. Manufacturers usually build in something to reduce the speed the gauge reats to float level changes, this is called anti-slosh.
When an OEM gauge is used on an OEM tank with an OEM sender, usually, the gauge will show the correct amount of fuel in the tank. As soon as you change one of the components, it is possible that this is no longer the case.
Fitting a new fuel gauge in the dash. Aftermarket gauges usually work with senders of 0-100 Ohms or senders of 0-500 Ohms, yet many senders are 30-240 Ohms or some other value. Using these together will result in incorrect fuel level readings.
Fitting a race tank. Using an OEM sender in a race tank will result in incorrect fuel levels as the sender has been designed to work in a non-linear way to suit the original tank shape whereas the Race tanks are generally uniform in cross section and require a linear sender.
Fitting a linear sender in an OEM tank will result in incorrect fuel level readings as the cross section of the tank may not be linear, or the overall movement of the float may not be the same as the original equipment.
It is possible to correct the readings in all of these cases with our fuel guage compensator, which allows you to adjust the full and empty readings of the gauge, however, this does not compensate for non-linearity in the middle of the range, so the pointer may be slightly inaccurate in the middle of the range. Our LED gauges do not require compensation as they have individually settable switch points.
- Our 8 LED fuel gauge with anti-slosh is fully adjustable, each LED can be set to an actual fuel level in the tank, independently of the others. This is the most accurate fuel gauge possible as it even compensates for minor manufacturing and alignment differences in a particular tank and sender.
- Our 4 LED fuel gauge without anti-slosh is designed for competition or race use, it is designed for checking the fuel level in the paddock. It is the lightest and smallest of the units.
- Our Fuel Gauge Wizard fits between the sender and gauge. It can amplify or reverse or shift the signal from the sender so that gauges think they have the correct sender attached.
- We now have a fuel gauge specifically designed for Paramotors that combines a 4 LED gauge with our anti-slosh circuit in an extra slim case.