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Active Member


Saturday, May 18th 2013, 9:52pm

GPM vs. Precip Rate

OK, I've seen this formula several places now.

Precip Rate (in/hr) = (GPM*96.25)/area in feet, and the numbers seem to add up correctly to my feeble brain.(96.25=(60m/hr divided by 7.48g/cu ft) times 12in/ft)

That seems to work fine when I go to the Hunter PGP chart, use the red standard #5 nozzle performance data, 2.0GPM, 38' radius, and the precip in/hr is 0.27 in the chart. Using the formula I come up with 0.24 in/hr which is close enough for me.

Now go to the Rain Bird VAN15' black nozzle, at 30psi in the performance data, 3.7GPM, 15' radius, 1.58 in/hr in the chart, but if I use the formula, I get 0.50 in/hr. Where am I messing up? And what do those little rectangles and triangles in the chart mean?


Supreme Member

Posts: 5,287

Location: Metro NYC


Sunday, May 19th 2013, 6:58am

The squares and triangles are for square head spacing and triangular head spacing, so the precip rates given are for an array of heads, and not for only one head.


Active Member


Sunday, May 19th 2013, 1:08pm

I've decided to actually measure the precip rate in each zone. Since I've got 15VAN cranked down to 6-8' it's off the charts anyhow. That way I can set up my watering times correctly. Given what I've seen so far, I think I may combine some zones as long as I can supply the GPM.

I measured the psi on a couple of zones, one with 5 heads measures 45/25 (booster pump on/off) and a zone with 9 heads measures 47/18 psi (it's a bit lower in altitude). Of course some of the heads don't fully pop up at 18psi. I don't think the installers even thought this thing through. It took them all of a day and a half to install, but then another day or 2 to come back, pick up their trash, install the heads and set it up (which I had to redo because every time they came out they messed up the booster pump relay wiring, so when I wired up the relay right, water sprayed all over the house and up on the patio.) I made them install proper connectors on the pipes where they went into the boxes, but they got tired of coming out for that, and I ended up finishing things, it was just too much of a problem dealing and waiting on them. They buried the 120V wire for the booster pump about 4" below the top of the soil in the garden, and of course the first time I put a shovel in the ground I hit that. I had to buy one of those underground splice kits for the UF wire and I put the UF cable under one of the 1" pvc pipes.

I took an old irritrol 180 spray head they dropped, drilled and tapped a 3/8-24 hole in the center, put an air valve in it so now I replace a 180 spray head with it and can measure the psi with a tire gauge. I need to figure out how to do the PGP rotors. I can't find any Hunter 213600 anywhere. But psi is pretty irrelevant as long as the psi is up there. It may be up pretty high in the lower yard, some places are a good 15' below the well. I'll find out.

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