Measuring Flow Rate of Wells
What is your well’s flow rate capability?
The flow rate capability of your well should be measured accurately because many backwashing water filters require a flow rate that is adequate to keep the media clean. Timing how long it takes to fill up a measured bucket is an inaccurate method of attaining flow rates unless you have a “constant pressure” well that delivers water at a more or less fixed rate. For conventional pressure tank wells, the single-shot bucket method is not accurate.
The proper well water flow rate is determined by counting the gallons drawn down and the time between cut in and cut off cycle of the well pump. To do this, you’ll need some kind of timing device, like a stop watch, plus a container of known size to catch water in.
- Allow the well pump to build to full pressure, the shut off the main water valve to the building to assure that no water is being used.
- Then, open a spigot below the pressure tank, capture the water, and measure the number of gallons drawn down from the pressure tank until the well pump turns on. You can measure in a small bucket, because it’s OK to turn the water off while the bucket is being empties.
- When the pump turns on, immediately close the spigot and time the period it takes for the well pump to recover, that is, see how much time lapses between when the pump turns on and when it turns off.
When you have this information, the formula for determining the flow rate is gallons drawn down that were measured above, divided by the seconds required for recovery, then multiplied by 60. (Gallons / Seconds) x 60 = Gallons per Minute (gpm) flow. For example, if 16 gallons are drawn down and it takes 90 seconds to build pressure back up, then: 16 divided by 90 = .177. Consequently, .177 x 60 = 10.6 gallons per minute flow rate.
What you are calculating is the sustained flow rate of the well–the gpm rate that the well can put out over the time necessary to backwash a filter. This can differ considerably from the “first bucket out” rate taken when the pressure tank is full.
Backwashing filters need sustained flow for several minutes to complete their cycle, and a filter should not be installed on a well that will not supply enough gpm flow to backwash it.