How to Measure the Output of a Residential Well

by Pure Water Annie


Gazette Technical Wizard Pure Water Annie Explains the How and the Why of Measuring Well Output



If you are thinking of installing a backwashing filter to treat well water issues like iron,  turbidity, or manganese,  the first thing you should consider is the output capacity of your well.  This is important because if you don’t have enough water–enough gallons per minute (gpm) flow–the filter will eventually fail.  If, for example, the filter that you install requires 8 gallons per minute (gpm) to backwash the media bed and your well is capable of only 6 gpm, the filter may work well for a few weeks or even months, but it will eventually lose its service flow and/or its effectiveness.

When a backwashing filter regenerates, it lifts and tosses the media bed in the empty upper part of the tank (freeboard space). If you don’t give the filter enough water to get sufficient lift in the bed, particles will remain after the backwash and eventually build up to clog the filter.  The filter has a flow control device installed by the manufacturer to prevent excess backwash flow, which would wash media out through the drain, but it is up to your well to supply a sufficient volume of water to keep the media clean. 

 How Much Water Do You Need?

Every filter medium has backwash requirements that are determined mainly by its density (weight).  Other factors that determine the backwash flow requirement are the diameter of the filter tank and the temperature of the water.  You can find a chart that lists the most common filter media and a complete explanation on Pure Water Products’ main website.

How to Measure the Output of Your Well

After you have determined the needed flow rate, here’s how you find out if your well has the needed output:

1. Start by closing off the water going to the building(s) served by the well so that no water can run to the building.  Then, run water through an outdoor spigot until your well pump comes on,  turn your spigot off, and let the tank fill completely.

2. With the tank full, run water into a measured bucket so that you can get an exact number of gallons that the well tank puts out before the pump turns back on.  If you have a small bucket and have to turn the water off to refill it several times, it doesn’t matter.  Just keep an accurate record of how many times you fill it.

3. When the pump comes on, immediately close your spigot and,  using a watch,  record the number of seconds it takes for the pump to turn off.

4. Now that you know the time between the pump’s cut-on and cut-off and the gallons it takes to fill the tank you can determine the flow rate of the well in gallons per minute.  The formula for determining the flow rate is the number of  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.

Another example:  The refill time is 110 seconds and the amount drawn from the tank was 22 gallons.  22 divided by 110 = 0.2, which multiplied by 60 gives a flow rate of 12 gallons per minute.

After you’ve done this simple calculation,  you will no longer have to embarrass yourself by admitting that you don’t know how many gallons per minute your well is capable of putting out.  It will improve your self esteem.


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