PQ Index

The Swansea Tribology Center used the knowledge that iron particles in lubricant can change its magnetic properties to create the particle quantifier (PQ). In order to establish the amount of magnetisable components in a sample, the sample is left upside down for a long period of time, in which the heavy components such as iron particles collect in the lid of the sample. The sample is then passed over a calibrated sensor.

An elevated number of iron particles is shown by a higher reading than the PQ index. All magnetisable components in the sample are recorded.

The PQ index is used to detect all magnetisable wear particles, irrespective of their size, in contrast with AES (atom emission spectroscopy), which cannot detect iron particles > 5 µm.

The difference between the AES iron content and the PQ index result is diagnosed. If the iron value in mg/kg is high, but the PQ index is low, the iron particles have been caused by corrosion. Rust is barely magnetisable so produces a low PQ index. A high PQ index value occurring in conjunction with low AES iron values always indicates acute wear e.g. scuffing.