"Gary Slusser" wrote in message ...
"Adam E" wrote
So.... turn on your water at a faucet until the pump turns on, note the
pressure, then shut off the water and note the pressure the pump shuts
off at; I.E. 30/50. Then take a 5 gallon bucket and run water in it from
the drain on your pressure tank tee and time the pump run time and
measure how much water you get in 60 seconds. Maybe 2 or more 5 gal
buckets will be needed if you have a large pressure tank. You want the
pump to run at least 60 seconds (for proper cooling of the motor). The
gpm your 'system' delivers to the pressure tank in a minute is the gpm
of the system, regardless of what hp and gpm pump you have. Size the
tank, based on the drawdown at the pressure range you use, to provide
the manufacturers' minimum run time of one minute (60 seconds). This
drawdown is the amount of water between the pump shutting off and coming
on again. Another drawdown is how far the water level in the well falls
when you pump the well for say 15-30 minutes, and you don't need t
oknow that for this project.

Gary, great post - thanks!

Would it be possible to calculate the desired drawdown by using this
formula?

(new drawdown) = (existing tank drawdown) * 60 / (# seconds to
complete current cycle)

Essentially I'd like to 'cheat' and calculate the desired tank
drawdown capacity, by just assuming pump runtime would scale linearly
with tank drawdown capacity. Could it be this easy if we felt like
playing with numbers instead?

- Chris