The particle seaparators I'm familliar with on Bk117 and the Bell 212/412 have no filter element at all. The Bk has tiny little vortex generators in a panel, and has an extractor fan to suck out any sand or tiny particles that could get through the panel. The Bell style is just a straight duct with a door on it and when the separator is working, the door is open and any particle just keeps going through the duct by momentum, and the air is allowed to suck in from the bottom of the duct. The compressor inlet on both machines also have a wire screen fitted to collect any debris that may make it through.