Patch Test Method for Solid Contaminants:

mixed in a solvent like Hexane which is passed through a 47 mm dia 0.8µ membrane with the help of a vacuum pump.

The whole oil passes leaving behind the insoluble particles up to 0.8µ on the membrane forming a grey patch on it which is compared with a reference chart for evaluating the NAS or ISO value

The patch is also examined through a 100x microscope which shows the particles of all sizes that can be roughly counted