SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Hey Readdhead thanks for your input. Yeah, i remember what you said about those cheap gauges staying pegged to the initial pressure reading. I ran a test the first time I read your input. I turned the water on slowly and the gauge read about 52 psi. I then went in the house and turned on a few faucets and the the water in the bath tub and while they were running took another reading and the pressure was like 47. So I guess the gauge I got is a decent one. Nevertheless, when I do this manifold test, I plan to turn the water source on very slowly so there is no surge or pegging action.

Quoted from "Drought Buster"

Am I crazy? First of all, thank you for all your replies. I really appreciate your knowledge and experience. I just came up with an idea and want to run it by you for your opinion. I'm probably not the first one to do this but I was thinking of making up a small manifold that I would attach to the PVB about a foot or two away. The manifold would allow for 4 attachments. 3 spots would be occupied by pgp rotors with nozzles that would give me my goal of 15 gpm output and the 4th spot would be the pressure gauge. This would allow me to read working pressure while putting out the 15 gpm. All that would be missing would be to account for pressure loss for the valve (planning on Hunter PGV about 1-2 psi loss), and the eventual length of the piping to the zone and the head (1-1/4 sched. 40 or 1-1/4 poly). I could then swap nozzles with lower or higher output to see what the optimum gpm would be to get that 35 - 40 psi working pressure all the while accounting for the additional loss from the valve and piping that are missing. Am I crazy? Your thoughts?


I like the test manifold idea. Just make sure your gauge will drop as the pressure drops. Like I said earlier, the cheaper gauges will stay pegged at the high pressure even though it may have dropped by 20psi and give you a false impression. If you run 15GPM and the gauge doesn't drop something isn't right.

Also, see if 1" poly will meet your requirements for the length of your lateral lines. The pipe is easier to find and the fittings more common (in my neck of the woods anyway). I don't think 1 1/4" will buy you enough PSI to be worthwhile. I kind of complicated my design and had my 1" poly lateral line for each zone tee off into 3/4" poly runs which then feed the sprinkler heads. There is a company that makes a neat 1" and 3/4" poly tee that connects directly to funny pipe. I probably could have use all 1" poly but it would have been overkill since my heads are using such a low GPM. 1 1/4" poly fittings are also more expensive than 1".

Depending upon exactly what you are trying to do, you might be putting the cart before the horse, or you might be doing a good piece of testing.

First of all, I assume this "manifold" (which most irrigation people associate with a manifold of valves for your various zones) isn't really a "manifold" of valves. (If it was, then you might be getting the cart before the horse because you're trying to set the number of zones before you've finished all the design work).

So when you say "manifold", I assume you really just mean a bunch of test pipes/fittings to allow you to run three PGPs at once and test the working pressure as they opperate (minus the pipe length pressure losses you can calculate afterwards with lookup tables).

If my assumptions are correct, then it sounds like a pretty good way to make some direct measurements. You will even be able to use your water meter and a watch (if no one is using any water in the house) to directly measure your flow rate as well as the pressure (basically, the bucket test without the bucket... use the meter to measure the number of gallons used).

As "Central Irrigation" alluded to (but you didn't specifically mention in your idea), you should include a zone valve of the type you intend to install between the PVB and the PGPs. That way, you will be including pressure losses through the valve in your measurements. Valves with flow control are recommended, because they will give you an additional measure of control at little extra expense. The flow control is basically just an extra valve that allows you to restrict the flow rate to the valve. When you start closing this flow control valve, you basically start make the valve input hole smaller partially cutting off the water flowing to the valve. By partially cutting off the water, you can simiulate lower static pressures (by basically increasing the pressure losses through the valve).

Hope that made some sense (I know it rambles on a bit), so let me know if it needs some clarification.

Not crazy at all. I've actually experimented in much the same way.



While running your test, you can close the valve slightly to simulate lower pressures.

Am I crazy? First of all, thank you for all your replies. I really appreciate your knowledge and experience. I just came up with an idea and want to run it by you for your opinion. I'm probably not the first one to do this but I was thinking of making up a small manifold that I would attach to the PVB about a foot or two away. The manifold would allow for 4 attachments. 3 spots would be occupied by pgp rotors with nozzles that would give me my goal of 15 gpm output and the 4th spot would be the pressure gauge. This would allow me to read working pressure while putting out the 15 gpm. All that would be missing would be to account for pressure loss for the valve (planning on Hunter PGV about 1-2 psi loss), and the eventual length of the piping to the zone and the head (1-1/4 sched. 40 or 1-1/4 poly). I could then swap nozzles with lower or higher output to see what the optimum gpm would be to get that 35 - 40 psi working pressure all the while accounting for the additional loss from the valve and piping that are missing. Am I crazy? Your thoughts?

Quoted from "Drought Buster"

So I bought a water pressure gauge and attached it to the outlet on the PVB and got a static pressure of about 50-52 psi depending on when I measure it. From there I plan on using 1-1/4 inch piping throughout, 1" valves, and pgp heads and a max of 15 gpm for any zone. You might recall from another thread that my 5 gal bucket test gave 25 gpm but I will only use 15 gpm. So to calculate my working pressure do I deduct pressure loss only from everything that is downstream from where I measured the static pressure (the outlet on the PVB)? or Do I need to also include the pressure loss from the water meter, 50 ft. of mainline, and the PVB that are upstream of where I measured the static pressure?

Your "bucket test" gave you the gpm with a "working pressure" of ZERO. That's why everyone has been saying you can't go by the bucket test.

As for pressure losses, you have to account for ALL pressure losses. To be perfectly accurate, you would even have to account for pressure losses in the pipes that run from the water tower to the water main running down your street. But that's a dynamic system where your pressure messurement (50-52 psi) varies as other water customers turn water on/off in there homes as well (so you can never get a TRUE static pressure).

So your best starting point is to make multiple static pressure reading at various times of the day and begin with your lowest reading. Even then you have to account for possible variables such as future pressure losses when more houses are built that use the same water mains, or if others have been running irrigation systems at the times you've been making your measurements.

So beginning with your lowest static pressure, you have to account for pressure losses in all the pipes (water main to meter, meter to backflow preventer, backflow preventer to valves, valves to sprinkler heads). Then you have to account for pressure losses through the devices (water meter, backflow preventer, valves). For pipes, you'll need pressure loss charts. For devices, you will have to look up manufacturers specs for your planned gpm (www.irrigationtutorials.com will give you some rough guestimates at what some of these might be, but if you try to use the guestimates, you have to add more fudge factors).

It's not unreasonable for pressure losses to be something like 3psi for the meter, 12 psi for the backflow preventer (if its an RPZ), 5 psi for the valves, and 5 psi for the pipes. If those numbers were accurate, that would leave you a working pressure of only 25 psi. With a PVB, that 12 psi is likely replaced with something on the order of 2-3 psi... and that would put your working pressure at about 35 psi. So it's not unreasonable that starting with a static pressure of 50 psi you only get about 35 psi to work with.

Quoted from "Drought Buster"

So If I take 50 static and deduct the meter, mainline, PVB, valves, laterals, etc... I'm down about 15 to 35? Darn, that sucks. Can anyone explain how I have a 5/8 inch meter, 50 psi static, and I can get 25 gpm out of the PVB? Doesn't seem right. Having a hard time accepting that. Seems that with a 5/8 inch meter and 50 static I should be only getting maybe 12 gpm. I wonder if the pressure gauge is just wrong and the pressure is actually higher. Will the PGP work properly at 35 psi?

Yeah, it sucks. I didn't want to accept it when I did my tests either, but it's true. You're getting 25GPM but you probably only have 5-10 PSI as the water exits the PVB, if that. If you hooked up a pressure gauge right next to the outlet of the PVB @ 25GPM you'd see very low pressure. If you had a valve on the outlet of the PVB and gradually closed it to reduce the amount of flow the pressure on the gauge would start to rise.

Oh and for what it's worth I have a cheaper hose bib gauge like you. If I open my hose bib fast enough (it fully opens with 1/4 turn) it will spike to 82PSI and stay there. If I slowly open the valve it clocks in at 51PSI. The gauge is cheap so it just sits at the highest reading it sees until all of the pressure is relieved.

You're absolutely correct. However different manufacturers perform differently. I would space a Toro 800 at 33' with your pressure, but a PGP could be different.

I think I remember reading in irrigation tutorials that head spacing should not exceed working pressure.

35 psi at the head is just fine.

When I design systems I try to shoot for 40psi, but only a long time professional would be able to tell the difference between 35psi and 40psi.



Also, like Pass1 stated, for every foot below where you tested your pressure from, you will gain 0.433 psi. So, if you have a head 10' below your pvb, you will have 4.33psi more at the head.



35 psi will perform just fine, just make sure and watch your head spacing. A PGP guy will comment and tell you the appropriate spacing based on your pressure.