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How
to Determine Flow Rate (Gallons per Minute)
If you need to compute flow rate in gallons per minute (or liters per
minute) of a steady flow in to a holding tank, or open discharge:
you can measure the flow into a bucket, (a five gallon bucket for instance)
and time the flow with a watch or clock. For instance if it takes 1
minute to fill a five gallon bucket: 5 x 1 = 5 gallons per minute. If
it were to take 45 seconds: 60 seconds divided by 45 times 5 gallons
= 6.6 gallons per minute.
Alternatively, you
could install a water meter on the line and record the flow rate as
read from the water meter.
If
you have a combination well pump and pressure tank, then you need to
use this method:
How to Determine Gallons Per Minute (Flow Rate Available, Pump
Capacity) on Private Systems with a Pressure Tank:
- Open any hose
bib or faucet until pump turns on.
- Close hose bib
or faucet and let pump fill up pressure tank until it turns off.
- Using a 1 or
5 gal. bucket, open faucet, collect and measure all water discharged
until pump turns on.
- When pump turns
on, immediately close faucet and start timing pump cycle.
- When pump turns
off, record pump cycle time to refill pressure tank in seconds.
- Divide the number
of gallons collected in Step 3 by the number of seconds in Step 5.
- Multiply the
answer from Step 6 by 60.
- The answer in
Step 7 is the average pumping capacity of the pump in gallons per
minute (GPM).
Example: Number
of Gallons collected during draw-down of pressure tank (Step 3) = 20
Number of Seconds
in pump cycle to refill pressure tank (Step 5) = 95
Gallons Per Minute
(GPM) = (Gallons collected / seconds in cycle) x 60
GPM = (20 / 95) x 60
so: GPM = .21 x 60 so: GPM = 12.6
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