A capacitive sensor is shielded against external electric fields to maximize measurement accuracy. The sensor may include two coaxial, tubular conductors with a chamber between them into which a material sample to be measured is introduced. A measurement circuit connected to the sensor may include a reference oscillator that oscillates at a
constant frequency and a test oscillator that oscillates at a frequency responsive to sensor
capacitance. The circuit may display a value responsive to the difference between the test and reference frequencies. The circuit may also reverse the polarity of the
signal applied to the sensor to minimize charge buildup, a phase-locked loop frequency measuring circuit, a temperature compensating circuit, and a non-
linearity compensating circuit. The indicating circuit that displays a representation of the measured
capacitance may display a continuous-scale or proportional representation or may display a binary or "go-nogo" representation. An alternative sensor of the
system includes coaxial, finned inner and outer conductors that intermesh to maximize capacitive surface area. Another alternative sensor includes two parallel, plate-like conductors. The
sensor system may be used to measure volume
charge density of a fluid or parameters responsive to changes in volume
charge density, such as flow velocity.