Susceptance measuring system for indicating condition of a material

Claim 1

A condition measuring system for detecting changes in the condition of a material by observing changes in the susceptance exhibited by a capacitive probe immersed in said material or in proximity thereto, said system being particularly adapted for measuring the condition of a material of relatively high conductivity such that a capacitive probe immersed therein exhibits not only a susceptive component related to the condition to be measured, but also a substantial conductive component which may be unrelated to the condition to be measured, said system comprising in combination a series circuit including: a. a capacitive probe for immersion in said material whose condition is to be measured, b. an impedance which is very low compared to the impedance of said probe, and c. a source of an alternating voltage for energizing said series circuit, said source being adapted to provide a voltage of substantially constant amplitude and frequency to said series circuit even though the conductive component exhibited by said probe may exceed substantially the susceptive component exhibited thereby, said source comprising a highly stable oscillator and an amplifier connecting the output of said oscillator to said series circuit, said amplifier having a high open loop gain and high feedback such as to provide an output impedance for said amplifier which is low compared to the impedance presented by said series circuit, said oscillator comprising a second amplifier having an output connected to a tuned circuit, means for comparing the signal developed in said tuned circuit with a reference signal to develop a difference signal, and means for supplying said difference signal to the input of said second amplifier, said second amplifier having a gain low compared to the gain of said first amplifier, and means for deriving from said serially connected impedance a signal representative of the susceptive component of the current In said probe to the substantial exclusion of the conductive component of current therein.