The capacitance-type fuel gauging system measures the volume of fuel by measuring the capacitance of the tank which depends on the proportion of dielectrics (fuel and air) within the tanks. A compensating unit is located at the bottom of the tank in the unusable fuel area. It is always immersed in fuel so it will measure the actual capacitance of the fuel. This will allow the compensating (reference) unit to convert the measured volume of fuel into a mass and display it on the cockpit instrument.