IEC 60970 Insulating liquids - Methods for counting and sizing particles
10 Method B - Optical microscopy
10.1 Principle
A known volume of insulating liquid is filtered under vacuum conditions through a membrane filter to collect contaminants on the filter surface. The membrane is then mounted between glass slides and examined microscopically by transmitted or incident light to measure, count and size particles according to their largest dimension.
10.2 Procedure by transmitted light
Counting and sizing of particulate matter microscopically by transmitted light shall be made according to the procedure described in ISO 4407.
10.3 Procedure by incident light
Counting and sizing of particulate matter by incident light shall be made according to the procedure described in ISO 4407.
