Calculating GPM

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goldmaes

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Dec 5, 2018
110
Palm City, FL
I recently purchased a home with a perimeter overflow pool. I was using the Francis Formula to calculate the GPM of water that I need to push 1/16 inch of water over the edge. The problem I am encountering is that I have a rectangular raised perimeter overflow spa that sits inside the perimeter of the pool. My question is when calculating the linear feet of weir wall do I add the spa to the linear feet or is it concidered part of the pool and needs to be discounted. It is a difference by my calculations of having the pool at either 2200 RPM or 3100 RPM so it is quite a big difference. Any help would be greatly appreciated. Thanks
 
Eric,

I am pretty much math adverse, so don't have a clue as to how to calculate what you are looking for... :)

Why not just slowly adjust your pump speed, using your EasyTouch, until you can see, or measure, the water passing over the overflow?

Seems like it would be almost easier to do it than calculate it..

Thanks,

Jim R.
 
Jimrahbe said:
Eric,

I am pretty much math adverse, so don't have a clue as to how to calculate what you are looking for... :)

Why not just slowly adjust your pump speed, using your EasyTouch, until you can see, or measure, the water passing over the overflow?

Seems like it would be almost easier to do it than calculate it..

Thanks,

Jim R.
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Unfortunately that would be difficult to do because it would require capping off multiple drains and even if I could, I still would not know about the water going over the edge of the spa and back into the pool
 
Jimrahbe said:
Eric,

I am sure it is just me, but I don't understand what you are trying to accomplish.. :scratch:

Thanks,

Jim R.
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What I am trying to accomplish is to figure out what the minimum pump speed is to push the appropriate amount of water over the edge to properly skim the edge. The pool builder just wants to crank up the rpms which is not only wasting energy, but it ruins the effect of the pool as the pool no longer looks like glass as the water entering is at such a fast rate that it disturbs the perimeter overflow effect. I don't want to set it too low or otherwise it won't skim properly.
 
I would try what Jim suggests, step up your pump rpms and visually inspect your system until you achieve the desired effect. Why the the need to modify or block the drains? Let the system operate as designed and change only the pump speed then measure the overflow.
 
Oly said:
I would try what Jim suggests, step up your pump rpms and visually inspect your system until you achieve the desired effect. Why the the need to modify or block the drains? Let the system operate as designed and change only the pump speed then measure the overflow.
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I certainly understand, however let me give a few more details that I have left out. I suspect that when the pool builder retiled the entire pool, they didn't ensure it was perfectly level. As a result, I can tell from the gutters that more water is flowing over the ends of the pool then the sides. As a result each end of the pool skims at whatever the rpms are set at, however the same isn't for the sides. As a result, I am getting debris that should be flowing over the edge congregating at the edge and then stopping short. I need to calculate the appropriate GPMs to be able to prove that if the pool isn't skimming properly on the sides they need to redo their work to get it perfectly level. If it isn't skimming at the minimum level it was originally designed to do, then it is a significant problem where I have to now choose function (not skimming) over aesthetics (losing the still sheet of water effect of a perimeter overflow). I don't believe that I should have to do that with a $250k pool.
 
The only work around would be to increase your rpms for short periods or pulses to improve skimming then reducing the rpms that allow skimming only on the ends and still achieve the still water effect you want. Too bad about the level issue, perhaps if not to severe it can be corrected.

I'd like to see some pics of a $250K pool.
 
Oly said:
The only work around would be to increase your rpms for short periods or pulses to improve skimming then reducing the rpms that allow skimming only on the ends and still achieve the still water effect you want. Too bad about the level issue, perhaps if not to severe it can be corrected.

I'd like to see some pics of a $250K pool.
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Here are some pics. One is with the old tile, one is with the new.
 

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goldmaes said:
My question is when calculating the linear feet of weir wall do I add the spa to the linear feet or is it concidered part of the pool and needs to be discounted
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The issue here is that the pump produces one flow rate and it is split in the plumbing at one point so in order to do what you want, you would need to know the flow rate going to the pool vs the spa and then adjust each independently at the same time adjusting the total flow rate from the pump. Not an easy problem.

It is far easier to just use trial and error.
 
mas985 said:
The issue here is that the pump produces one flow rate and it is split in the plumbing at one point so in order to do what you want, you would need to know the flow rate going to the pool vs the spa and then adjust each independently at the same time adjusting the total flow rate from the pump. Not an easy problem.

It is far easier to just use trial and error.
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My thought is that the spa wouldn't matter as it is also a perimeter overflow design, so the amount of water being pumped in, equals the amount overflowing creating a zero sum for the spa. I am not so sure that is correct, but wanted other people's thoughts.
 
The spa spills into the pool on only three edges, correct? The fourth edge is on an exterior wall. The issue is that the spa steals some of the flow rate from the pool and if the flow rate over that 4th edge per foot is not the same as the pool, the formula does not work. So you will have to adjust the split in the flow rate between the pool and spa to ensure proportional flow over the edge.

Also what complicates the problem even more is that the weir is not completely level per your description. In order to use the Francis formula, you would have to take the average height variance and then add 1/16" to that to get the correct flow rate. Do you know how much out of plumb the edge is over the entire edge?

Again, it just seems easier to use trial and error.
 
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