Abstract: An aircraft fuel gauging system has a capacitance level probe mounted in a fuel tank that receives an alternating signal from a drive unit. The output of the level probe is connected to two diodes one of which is connected in series to earth and the other of which is connected in the opposite sense to a measuring unit. The measuring unit includes a signal conditioning unit which is switchable between operating as a d.c. current to voltage amplifier and a d.c. voltage amplifier by switching a resistor out of or into series connection with the diodes. A constant current source backs off the current from the probe when the tank is empty. Periodically, the supply of signals from the drive unit is interrupted and the conditioning unit is switched to operating as a d.c. voltage amplifier, the measuring unit measuring the voltage drop across the two diodes caused by current from the constant source.

Abstract: A fuel-gauging system has a capacitive probe mounted in a fuel tank and supplied with an alternating voltage V.sub.s from a drive unit. The output of the probe is rectified by a diode mounted in the tank, prior to supply to a measuring unit. The voltage V.sub.D across the diode varies with temperature and is measured periodically by reducing the input voltage V.sub.s by a known factor X. The voltage V.sub.D is calculated by the measuring unit from the two output currents I.sub.1 and I.sub.2 produced respectively at the two different voltages V.sub.s and V.sub.s /X. The capacitance and hence the depth of fluid is calculated using the known voltage V.sub.D across the diode. The drive unit has a transformer that is used to reduce the voltage periodically by switching between different tappings of its secondary windings.