I realized it would be much faster if we could simply see how long it takes to pump 100 Gallons of water and simply do the math to figure out how fast it was pumping - this can get pretty complicated so I decided to make an app for it.
I made the app as generic as I dared so that you can shift the numbers around to whatever you are measuring (you could even time your heart rate off of this).
HOW TO USE THE APP:
By default it is set to measure Gallons per Minute given how long it takes to pump 100 Gallons. To measure this press the big green PLAY button to start the test. Then as soon as you have pumped 100 Gallons hit the button again (which should now look like a repeat button). This will give you an instant reading of your gallons per minute at the bottom of the screen.
The timer will continue to count so you can take an average of several readings for increased accuracy. At the next 100 gallon point go ahead and hit the repeat button again - the label at the bottom will give you an updated estimate and note how many readings you have had. When you are done press the big red STOP button and your history will be cleared (though your last reading will still be displayed).
App icon By Damianadrian (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
* All other images were provided courtesy of Icons8 (www.icons8.com) modified to remove transparency.