A large tank label does not equal usable water. Measure the switch range and precharge safely, then compare observed cycling with the pump maker’s limits.
1. Record what one complete cycle does
Choose a repeatable steady draw, note the pressure where the pump starts and stops, and time both the pump-off draw period and pump-on refill period. Record the delivered water if you can measure it safely.
Rapid starts can come from lost air charge, a damaged bladder, a restricted connection, an incorrectly placed pressure switch, leakage, unstable demand or a pump-control fault. Timing alone does not identify which one.
2. Check precharge only with water pressure removed
Isolate electrical power, prevent automatic starting and drain the water side to zero pressure before checking the air valve. Do not infer precharge from a reading while the tank is still pressurized with water.
Follow the tank and switch manufacturer’s stated relationship. Amtrol’s current support guidance, for example, specifies a precharge 2 psi below pressure-switch cut-in for its well tanks; that product instruction is not a universal substitute for the installed equipment manual.
Amtrol · Well tank support and precharge guidance ↗3. Separate nominal volume from drawdown
Only part of a precharged tank’s nominal volume is delivered between cut-out and cut-in. The drawdown calculator applies a simple gas-law estimate so you can screen an observation or compare operating ranges.
Use the manufacturer’s drawdown table for final selection. A wider pressure range can increase drawdown but must remain compatible with the tank, pump, switch, fixtures, treatment equipment and relief protection.
Amtrol · Well-X-Trol literature and sizing documents ↗4. Screen the demanding operating point
For a fixed-speed pump and steady flow below pump capacity, one cycle contains a tank draw period and a refill period. Cycling is often most frequent when demand is near one-half of pump delivery, so test more than one realistic flow.
Compare calculated and observed starts per hour and minimum run time with the pump and motor documentation. If demand can exceed pump delivery, this cycle model no longer applies; storage and source capacity require a separate peak-demand check.
5. Correct the cause, then repeat the observation
Have unsafe pressure, electrical defects, a waterlogged or damaged tank, leaking check valves and control faults corrected by a qualified person. Never bypass a relief valve or increase settings beyond the lowest-rated component.
After service, repeat the same flow test and keep the pressures, elapsed times and date with the equipment record. A trend is more useful than a single unexplained number.
Useful evidence from one cycle test
| Observation | What it helps reveal | Do not assume |
|---|---|---|
| Cut-in and cut-out | Actual operating pressure range | Gauge calibration |
| Delivered drawdown | Usable tank water | Nominal tank volume |
| Pump-off time | Demand versus drawdown | No hidden leakage |
| Pump-on time | Net refill rate | Unrestricted pump rating |
| Starts per hour | Cycling severity | Universal motor limit |
Worked example
Screen a fixed-speed pump at half its delivery
A tank provides 30 L between switch settings. Pump delivery at operating head is 40 L/min and the test draw is held at 20 L/min.
Pump-off draw time = 30 / 20 = 1.5 min Pump-on refill time = 30 / (40 − 20) = 1.5 min Cycle time = 3.0 min Starts per hour = 60 / 3.0 = 20
Compare 20 starts per hour and a 1.5-minute pump-on interval with the installed pump and motor requirements. The calculator does not decide whether those values are acceptable.
What to have ready
- Tank model, nominal volume and manufacturer drawdown table
- Verified pressure gauge and actual cut-in/cut-out readings
- Precharge procedure and pump/motor cycling limits
- Safe way to hold and measure a steady water draw
Common mistakes
- Calling the tank nameplate volume usable drawdown.
- Checking precharge while water pressure remains in the tank.
- Using free-flow pump capacity instead of flow at operating head.
- Changing pressure settings without checking every component rating.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.