Clean baseline pressure
Also called: clean pressure, clean starting pressure, filter baseline, starting PSI.
Clean baseline pressure is the reading on a pool filter's pressure gauge right after the filter is freshly serviced, with the air bled out and the pump at its normal running speed. Every cleaning rule is relative to it: clean the filter when pressure runs about 8 to 10 PSI above that baseline.
Why it matters
A gauge reading on its own tells you nothing, because two healthy pools can sit at 10 and 18 PSI when clean. Service the filter, bleed the air, let it settle, then write the number on the tank in marker with the date. Air in the tank inflates the reading, so a baseline taken before bleeding is too high and you will under-clean all season. Most clean residential sand filters sit around 10–15 PSI. Re-set the baseline every time you deep-clean or change media. A baseline that creeps up year over year is a symptom of calcified sand. Gauges fail too, usually by sticking or reading low.
In numbers
On a variable-speed pump, pressure changes with RPM: a baseline read at 3,000 RPM is meaningless when you run at 1,800, so note the speed beside the number.