This British Standard specifies methods to assess the level of
particulate contamination in hydraulic liquids using an optical
microscope. It includes assessment by two manual methods using
either transmitted- or incident-lighting systems. It also includes
procedures for evaluating and controlling the accuracy of the
technique that ensure reproducible results.
This method is applicable to nearly all hydraulic liquids at a wide range
of contamination levels, provided the samples can be filtered. However,
the procedure requires modification for samples that are either heavily
contaminated (e.g. > ISO -/20/17 according to BS ISO 4406) or lightly
contaminated (< ISO -/12/10 according to BS ISO 4406), by decreasing or
increasing the volume of sample filtered.
This procedure is not applicable to samples that have high concentrations
of small (< 3 μm) particles (so called “silt”), gels or where overlapping
particles are present on the membrane filter.
The method detailed is a qualitative assessment only, not a quantitative
evaluation, and it is applicable to all samples where the data has to be
expressed in terms of contaminant codes.