Water Flowing to Atmosphere – If the water coming from a cartridge filter goes directly to atmosphere, a downstream pressure gauge would rarely go much above zero and so it can be eliminated. The pressure drop across a cartridge in this type of service can be determined by simply taking a dynamic reading from the inlet pressure gauge.
As particles accumulate on a filter cartridge, flow through the filter becomes restricted. This restriction reduces the flow rate through the cartridge and increases backpressure. (It also increases the effectiveness of the filter, so cartridges shouldn’t be changed unnecessarily.) The difference in pressure before and after a cartridge filter is the pressure drop, or △ (Delta P).
Different filter configurations and micron ratings have different △P replacement specifications. For example, pleated membrane filters normally require replacement at a △P of 15 psig, while string filters are still going strong at a △P of 25 psig. Consult the manufacturers △P recommendations for the specific filter cartridge being used.