# Thread: Calculating flow

1. ## Calculating flow

I would like to calculate the flow of my system, and as far as I can tell I can get this two different ways. The first is to install a flow meter on the return pipe to measure the actual flow in GPM. The other is to use a vacuum gauge on the the pump (perhaps replace the basket drain plug with the gauge), and and use the pressure pump from the filter. Then use the pump's head curve to determine the flow in GPM.

Now the flow meters tend to be a bit expensive, so I was thinking of just buying a vacuum gauge, but would like to know how accurate the results would be if I did this. I don't need to know the flow down to the last GPM, but if I could get an accuracy of about +/- 5GPM, that should work good. So how accurate would the flow rate be using the vacuum and pressure gauges along with the pumps head curve?

2. ## Re: Calculating flow

I don't know diddly about measuring pool flow rates, so you should take this as just one man's opinion.

But in general instrument terms (something I do know about) you would be asking the question in the wrong way. For example: is your current flow 5GPM or 100 GPM? A +/-5GPM error (if achievable) would vary between virtually worthless and improving up to a five percent value. Said another way, measurements of this sort are phrased in terms of accuracy over a certain range, not the measured values themselves.

In any event the accuracy of your measurement would depend on the accuracy of your specific gauges and the accuracy of the published head curves when compared to your exact pump (OEM tolerances, new vice used, etc.).

3. ## Re: Calculating flow

Point taken.

What I have is a UV system, which requires that the flow rate doesn't exceed 57GPM for an effective kill rate with the UV bulb. So I'm wondering if my rate is say 70GPM, or 50GPM, at least it gives me a ball park figure to work with.

4. ## Re: Calculating flow

A flow gauge costs around \$70 and is really easy to use. A vacuum gauge only costs \$20, but requires calculations. The flow gauge will be more precise, but either approach gives enough precision to be useful.

With a 2 HP pump, the odds are very very high that you are exceeding 57 GPM. Unless your plumbing is especially long or convoluted, you can often get 57 GPM with a 3/4 HP pump.

5. ## Re: Calculating flow

That's my hunch, the flow rate is way higher than 57GPM. The pump has two suction lines, one 2" from the skimmer, and one 1 1/2" from a suction line about 6" below the water line. Both these go into one 2" line going into the pump. The return lines are a 2" from the pump, splitting into four 1 1/2" returns to the pool (they split just after the heater). The equipment pad is about 25 feet from the pool, and about 5 feet above pool water line.

I have been calling for the last couple of days, and here in Canada, the flow meters are hard to come by, and the cheapest Blue-White I can get shipped here from the States is about \$85. I just called a plumbing store near me, and they have a 0-30 in Mg vacuum gauge 1/4" thread for \$15. So I think I'll get the vacuum gauge, and go from there.

Once again, thanks everyone at TFP for your help...

6. ## Re: Calculating flow

Based upon the information you gave, I get about 110 GPM @ 48 ft of head, filter pressure at 11 PSI and suction at 13" hg.

What is your filter pressure?

7. ## Re: Calculating flow

I am about to install the suction gauge (hope it fits the basket drain plug, I got the 1/4"NPT).

The filter pressure is about 8-9psi.

8. ## Re: Calculating flow

Well the vacuum gauge went on no bother....and it was reading about 8.5" Mg, and the pressure gauge is reading about 8 PSI, for a total of 28' head (if I did the calculation right). Using the Jandy chart, it shows about 160GPM...WOW that's a lot of water!

Well after looking at the pump again, I have made a big mistake....it's a 1HP, not a 2HP....

So the calculations based on a 1HP is about 80-90 GPM, a bit more reasonable...

9. ## Re: Calculating flow

SHPM is max rated and SHPF is full rated. Sounds like you have an uprated pump? SHPM 1.0 at 28' of head is about 92 GPM. SHPF 1.0 at 28' of head is about 122 GPM.

10. ## Re: Calculating flow

Originally Posted by mas985
SHPM is max rated and SHPF is full rated. Sounds like you have an uprated pump? SHPM 1.0 at 28' of head is about 92 GPM. SHPF 1.0 at 28' of head is about 122 GPM.
I actually just went out and double checked, and it is the SHPM one.

Thanks for everyone's help

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