Abstract: Wear particle monitoring is carried out on samples of a lubricating oil extracted from lubricated equipment together with the naturally appearing burden of ferromagnetic particles. The extracted sample is collected in a bottle to a depth sufficient to exceed the flux field of a sensing coil to which the bottle is subsequently applied whereby the numerical value (PQ value) assigned to the particle burden is substantially insensitive of the volume of liquid in the bottle. From time dependent measurements information as to the size of the particles in the burden can be obtained.