Abstract: A fuel-gauging system has a capacitive probe comprising an outer tubular electrode and a coaxial inner electrode. The outer electrode is supplied with an alternating signal from an oscillator, the inner electrode being connected to a detector unit via a resistor. The resistor is mounted between the electrodes about midway along the length of the probe, the resistor having a value that is selected to give a substantially linear output of the probe and is at least 0.1 of the reactance of the probe, preferably being about 0.3595 of the reactance. The resistor may be connected to a phase-sensitive circuit that is controlled by a phase-shift circuit which receives the input signals to the probe and shifts their phase by 90.degree., so as to produce an output that is in phase with the probe output. In this case the value of the resistor is preferably 0.2178 of the probe reactance.

Abstract: Apparatus for measuring mass flow or density of fluid has a turbine mounted resiliently by torque rods within the fluid-flow line. The turbine has a number of vanes that extend radially to define passages parallel to the flow of fluid in the fluid-flow line. A torque driver causes oscillatory rotation of the turbine so as to give fluid in the passages a component of displacement at right angles to the passages. The turbine is driven simultaneously at two frequencies, displacement being sensed by a pick-off. The first frequency f.sub.90 is adjusted so that a 90.degree. phase difference is produced between the drive signal and the turbine displacement--the density of fluid being derived from this frequency. The second frequency f.sub.D is a fixed fraction F of the first frequency, the phase difference between the second frequency and turbine displacement being used to derive an indication of mass flow.