Pressure tank drawdown — only about a quarter is usable water
Only the water between cut-in and cut-out is usable — about 28% of the tank on a 40/60 switch. Boyle’s Law: the air cushion does the pushing.
What this diagram shows
Cutaway of a bladder well-pressure tank showing why only part of the tank is usable water. A compressed air cushion fills the top; water fills the bladder below. Between the pump cut-in pressure (40 psi) and cut-out pressure (60 psi), the air expands to push out only the "drawdown" band — roughly 28 percent of the tank on a 40/60 switch. The air pre-charge is set about 2 psi below cut-in.
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Well Pressure Tank Calculator
Free well pressure tank calculator — Boyle's Law drawdown from pump GPM and switch settings, acceptance factor, and the tank size that stops short-cycling.
