Well Pressure-Tank Drawdown and Sizing
Usable drawdown, runtime per cycle, and short-cycle flag for a well pressure tank.
Example
You enter
- Mode find-drawdown
- Tank total volume (gal) 44
- Cut-in pressure (psi) 40
- Cut-out pressure (psi) 60
- Pump capacity (gpm, optional) 10
You get
- Drawdown (gal) 11.35
- Runtime / cycle (min) 1.135
Details, formula, and sources
Usable drawdown, runtime per cycle, and short-cycle flag (or the required tank size) from cut-in/cut-out and precharge by Boyle's law.
Diaphragm-tank drawdown by Boyle's law on absolute pressures: drawdown = V_tank * (P_pre_abs/P_in_abs - P_pre_abs/P_out_abs), P_abs = P_gauge + 14.7, precharge ~ cut-in - 2 psi. Runtime = drawdown / pump_gpm; size mode inverts for V_tank.
Pressure-tank drawdown from Boyle's law on the diaphragm air charge and the anti-short-cycle minimum-runtime rule (about 1 min per cycle), per the published pump/tank engineering practice (Amtrol/WellMate and the WQA references), by name; first-principles gas law.
First-principles gas law. The pump manufacturer's minimum runtime and the installed precharge govern.
Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
Field names used by the API: mode, tank_volume_gal, cut_in_psi, cut_out_psi, pump_gpm, drawdown_gal, runtime_min
- Precharge cut-in minus 2 psi unless overridden; a precharge at or above cut-in will not draw downdiaphragm-tank practice