View Full Version : gpm???

07-10-2008, 11:03 PM
Ok, I was told to check the gpm by cutting into my main line (1 inch) just ahead of the water meter and filling a 5 gallon bucket out of the 1 inch pipe. I put a "T" in the line and ran a piece of 1 inch pipe down off that with a 1 inch stop valve to check the flow. I filled up a five gallon bucket in 8 seconds. By the math 5 gal/60 x 8= 37.5 gpm. This seems like alot. I was told that the max safe flow is around 18 gpm. What does this mean? Do I need to get my gpm down to 18gpm, and if so, how? I was going to run the system with 1 inch poly and 3/4 inch laterals. Any help would be appreciated. Also, my pressure is 58 psi.

07-11-2008, 01:28 AM
Welcome! Hey, did you mean AFTER the meter? Makes a difference, since meters eat pressure. Anyway, you sound as if you need some help, and also want to understand how it works. Jess Stryker has a free on-line tutorial site that will help out. Spend a little time there and you won't be sorry. http://www.irrigationtutorials.com/


07-11-2008, 01:35 AM
Before I get slammed, I do understand that static pressure is the same on either side of the meter. Jeff

07-11-2008, 01:45 AM
37.5GPM open discharge = 18GPM +/- usable - right on the money.

Where/how did you measure pressure?

07-11-2008, 07:55 AM
I am not sure I understand. How does 37.5 open discharge = 18 gpm usable. Please explain.

07-11-2008, 08:04 AM
If you cut in just downstream of the meter, and tee off and come up to a shutoff valve that has a pressure gauge on its upstream side (you can swipe the valve from a backflow preventer) you can repeat the bucket test at different pressures.

07-11-2008, 08:57 AM
Why would I want to redo the test? Are the numbers that I am coming up with ok or does something need to be done. Also, does anyone know what waterit means when he says: 37.5GPM open discharge = 18GPM +/- usable - right on the money.

Thank You

07-11-2008, 09:03 AM
Are you going to have a POC just downstream of the meter? If so, that's where you cut in to do the test. And with a gauge upstream of your added shutoff valve, you can flow enough water to get a certain pressure reading, say 50 psi, and see what the flow is. Now you have a precise number to work with, and no guesstimating required.

07-11-2008, 10:45 AM
Yes, but can I use the readings I have. The flow was 37.5 gpm at 58 psi.

07-11-2008, 10:48 AM
Yes, but can I use the readings I have. The flow was 37.5 gpm at 58 psi.Then what's the static (no-flow) pressure?

07-11-2008, 11:34 AM
The pressure with the line closed is 58 psi

07-11-2008, 11:40 AM
The pressure with the line closed is 58 psiThen you will not see 58 psi in the system when you have a flow of 37.5 gpm.

07-11-2008, 11:42 AM
So in simple terms, what do I do?

07-11-2008, 11:50 AM
If you do as already described, and make your actual tee-off for the system, you can take real operating measurements. The meter will take away pressure. The plumbing between meter and street will take away pressure. If your tee-off is piped to a ball/gate valve, and you have a pressure gauge just upstream of that valve, then you can open it enough to see the gauge pressure drop from 58 psi to 50 psi. What's the flow at 50psi? Open the valve some more, and get more flow+pressure readings. Now you know what you have to work with.

By the way, this is all assuming you have a streetside water meter.

07-11-2008, 12:06 PM
Wet boots,
So when I do what you suggest about opening the valve to see what the flow is at 50 psi, then say at 40 psi. Then what do I do with those numbers. What numbers do I use to configure the system. If the correct flow rate is say at 45 psi, how do I keep the system at 45 psi? I am very confused. When I did the flow test at a spigot outside the house that is close to the water meter, but is run with 1/2 inch copper, the flow test measures about 12 gpm.

07-11-2008, 12:14 PM
How do you get from 12 gpm from a faucet to 37.5 gpm? The same basic advice applies, whenever you have the ability to make your connection in advance of designing the system. Do the work, then measure flow+pressure. Otherwise, just take the 18 gpm guesstimate, and go from there, and be prepared to improvise if you have less than you expected.

07-11-2008, 12:59 PM
My water meter is 1 inch. I plan on running the system with a 1 inch main and 3/4 inch laterals. When I retest the pressure and gpm at the connection by the water meter, should I test with 1 inch or 3/4 copper because that is what the laterals will be.

07-11-2008, 01:06 PM
A one-inch meter in the basement would tee off to a one-inch line as a minimum. For long outdoor runs in a large lawn, you'd bump up the pipe size, because you don't have a lot of pressure to waste.

07-11-2008, 01:24 PM
So do you think I should use 3/4 laterals, and if so should I run the gpm test with 3/4. or 1 inch.

07-11-2008, 01:29 PM
You don't have even one foot of pipe buried in the ground before you test the flow+pressure from your connection. Use of 3/4-inch poly is always okay to feed one or two heads, but for pulling pipe, the extra work might not be worth it.

Mike Leary
07-11-2008, 05:10 PM
Everyone should have a Sentry1 Flow Meter.


07-11-2008, 06:02 PM
If you plan on using a 1" main, you should really stay within 13 - 14 GPM to stay within the 5 FPS rule. If you want to push 18GPM, upgrade your main to 1 1/4". Teh rule of thumb is that safe flow is 75% of the water meter capacity. A 1" meter can flow about 34GPM safely but a 1" Type K Copper main should stay in 17 - 18 GPM to stay close enough to 9 FPS to be safe. This is probably where the persont that told you 18GPM got it from but, like I said, if you're going to go there, you should run 1 1/4 main.