I echo Andrew's info except that the PQ Index can be considered unitless. It is an index and thus has no units. It is very useful to trend and use to determine if further analysis is required i.e. an actual visual debris analysis. But keep in mind it only measures "magnetic" metals so one could have a low PQ but still a lot of non-magnetic metallic particles present i.e. aluminium, brass, bronze etc. The good thing is that this is a rare occurence as most systems have at least one iron-based wearing part.