A method for estimating the composition and size of metal particles is described. An inductive coil sensor is used which, on the introduction of a metal particle, changes impedance. The nature of the impedance change, for a given coil, depends on the composition and size of the particle which can be detected by incorporating the coil into a marginal oscillator circuit and effecting amplitude and frequency demodulation. Experimental results are given for various materials and an application of the method to lubricant wear-debris condition monitoring is described.

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