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. The current from the constant source may be increased when the resistor is connected in line such that the current through the diodes is substantially the same as the mean current during a.c. operation. The measuring unit may provide an indication of temperature derived from the voltage drop across the diodes.
