Abstract: An apparatus and method for determining a parameter of a particle in a fluid is disclosed and, in particular, an apparatus and method for determining contamination or machine wear by the measurement of contamination or wear particles in a fluid used in the machine is disclosed. In a first embodiment, a measurement cell is provided with a fluid flow path. A first hall probe is arranged adjacent to the flow path for detecting dia-magnetic, para-magnetic, ferro-magnetic particles and a second optical sensor formed of a light emitting diode and phototransistor are provided for optically detecting the particles. A correlation between the hall probes and the optical detector enables an indication of the particles to be obtained so that a parameter of those particles can be determined to provide an indication of contamination or machine wear.