Each filter cartridge specification and pricing page has one or more flow rate graphs at the bottom of the webpage. These charts are also on the second page of the corresponding datasheet. Below is a copy of the chart from the LOFTREX-C.
Seven retention sizes are listed and you can see how the differential pressure increases as the flow rate increases.
Example. The 1 micron cartridge will have a 2 PSI differential at a 10 GPM flow rate; at 15 GPM the differential pressure increases to 3 PSI.
Notice that an increase in flow rate of 50% (from 10 to 15 GPM) corresponds to an increase in differential pressure of 50%.
This differential pressure (ΔPSI) is based upon the initial "clean state"; as particles become trapped within the filter cartridge, the open area available decreases and therefore the corresponding differential increases exponentially.
Testing is performed in a laboratory with ambient water and with a 10" cartridge; so if you have a 20" cartridge, the differential pressure values would be half of what is shown in the graph; OR to state it another way, a 20" cartridge has the capacity double the 10" size for a given differential pressure.
Example. A 20" 1 micron LOFTREX-C cartridge would have a clean differential pressure of 2 PSI for 20 GPM OR only 1 PSI for 10 GPM.
The "clogging curve" illustrates the exponential increase in differential pressure as the media becomes clogged. This is a reflection of the "increase in efficiency" related to the clogging of open area and ability of retaining finer particles.
In practice this means that, assuming a consistent particle size and volume, the amount of time required to become 75% clogged is only 1/2 the time required for the initial 50% clogging. Stated another way, once you reach 50% clogged, the clogging rate doubles.
Besides the physical cost of replacement filter cartridges, you should consider the frequency of replacement with regards to labor costs, down time and annual disposal costs. Then consider using a more expensive cartridge which will provide you with higher particle loading capacity, thus reducing the frequency of change outs and your disposal costs. Usually the more expensive "engineered" cartridges end-up being a more cost effective product than the less expensive versions that are replaced more frequently; it's just a little harder to quantify.