How do I know which COAX® cartridge to use?

The easiest way is to use the selection guide in the pump section. MICRO, MINI and MIDI describe size and capacity where MICRO is smallest and MIDI the largest. Each are available in a few different characteristics, one for extra high flow (Si), one for extra high vacuum level (Xi), one for handling low of fluctuating compressed air feed-pressure (Bi/Pi) and finally an extra dirt tolerant design (Ti).