Photoacoustic express diagnostics of lubricating oil: Model investigations

The possibility of evaluating the size of metallic inclusions in lubricating oil by photoacoustic measurements in a gas-microphone cell is investigated. The photoacoustic spectra of suspensions of microscopic spherical steel particles with diameters from 5 to 70µm in an oil layer with a thickness of 20 µm are measured. A semiquantitative model of the photoacoustic effect in a suspension layer is proposed for inclusions of both small and large size in comparison with the layer thickness. It is demonstrated that the body of information obtained from the measurements considerably increases when the thickness of the oil layer is close to the size of inhomogeneous inclusions.