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Thread: Confirming pool size?

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    Gooserider's Avatar
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    Confirming pool size?

    In the interest of making better plans for my future work with our pool, I'd like to get confirmation of the size of the pool... I had thought it was 16K gallons, but looking at some of our old pool store chemistry reports (from one of those dreaded computer programs) they were saying 17K. I think this is what the pool store had estimated based on a sketch I gave them.

    I have also done a drawing of the pool in QCAD, which is a semi-open source, low cost CAD drafting program that is supposed to be mostly Autocad compatible. I have attached a .PNG export of the drawing, unfortunately the forum won't let me attach the actual .dxf file that QCAD uses. (If anyone would like the .dxf file, send me a PM w/ an E-mail and I can get it to you directly)[attachment=0:22izq9g8]cad-layout.png[/attachment:22izq9g8] I think the drawing is pretty accurate, but I may redo it this summer to be sure...

    The surface area numbers are calculated by QCAD using it's built in "polygon area calculator" and tracing around the perimeter of the drawing.

    I've tried to calculate the pool volume using those surface area numbers, and I get answers ranging from 17,870 to 25,391 gallons, without allowing anything for the steps, or the fact that the bottom and sides of the pool, plus the dropoff are radiused a lot.

    BTW, there is a bit of ambiguity in the Pool Calculator - it says
    If the depth varies, use the average depth of the pool. A good approximation is the shallow end depth plus the deep end depth divided by two.
    Does this mean "(shallow end depth plus the deep end depth) divided by two" or "shallow end depth plus the (deep end depth divided by two)" - it makes a difference...

    I got the 17,870 number by assuming an average 39" depth for the shallow end, and calculating the volume of the shallow end, plus the volume of the deep end; and adding the two numbers. Making a wild guess at what I loose to the volume of the steps, plus the curves on the bottom, makes the 17K figure from the pool store sound about right - can anyone confirm or point to where I am making a mistake?

    FWIW, I currently have a 1hp pump, and a 36sq ft DE filter rated at 72gpm max flow, does this seem like an appropriate size equipment setup for the pool?

    Gooserider
    Attached Images Attached Images
    Free-form Inground gunnite pool, Estimated 16-17K gallons. New Pentair TR60 ClearPro 24" sand filter and Compupool CPSC-48 SWG, Hayward SP1607X10 Pump w/ 1HP motor, 1.5" plumbing, Polaris Pressure cleaner w/ booster pump. pool is more than 25 yrs old, less than 35. Not painted, deteriorating tile surround. I am paraplegic, get in/out of pool w/ S.R. SMITH PAL portable pool lift (significantly modified)

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    mas985's Avatar
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    Re: Confirming pool size?

    Average depth is (shallow end + deep end) / 2.

    But that is an estimate. It is pretty hard to get real accuracy since you really don't know the volume of the stairs and curves. Or do you?

    You may want to upsize the filter since with a 1 HP pump, you could get to 90 GPM with a good plumbing design. Figure on 2 GPM per sq-ft as a maximum flow rate but design for 1.5 GPM per sq-ft. So 60 sq-ft might be more appropriate.

    Also, I would upsize the pipe as well. For each skimmer and main drain pair, run at least a 2" line from pool to pad. For each pair of returns, run at least a 2" line pool to pad.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Confirming pool size?

    Quote Originally Posted by mas985
    Average depth is (shallow end + deep end) / 2.
    That works best when the shallow and deep ends are around the same size. In your drawing, that looks close enough. (In my pool the shallow end is probably 3x the area of the deep end, so this averaging trick doesn't work.)
    --paulr
    BBB "Intermediate Swimmer"
    IG plaster pool 18.5K gal, Hayward Pro-Grid DE filter, 3/4 HP Hydramax II; Polaris 380, 3/4 HP booster
    AG spa 325 gal, probably Sundance of some kind
    Water testing instructions on one page

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    Gooserider's Avatar
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    Re: Confirming pool size?

    Well Mas, I don't have any real idea of the volume on the stairs and curves. I would say the perimiter is probably good to within an inch or so (and now that I have one of those laser tapes I should be able to get it to within about an eighth this summer if I redo the drawing) but I don't have a good way to do the steps and the curves.

    On the steps, the number of treads, and the points where they hit the walls are about right, but I'm guessing at the shape. The curves I'd guess have a 6 or 8" radius in the shallow end, with maybe a 2-3 foot radius in the deep end - I can stand with my head out of the water in the deep end if I put my feet right where the wall starts curving in...

    Not allowing for the stairs and curves, using the "Average depth is (shallow end + deep end) / 2" formula, I get between 18,186 and 19,398 gallons depending on what I plug in for the shallow end - if I average the shallow end first (30-48" range) I get the larger number, if I just use 30" I get the smaller one. If I take the average of the shallow end, 259 sqft x 3.25' deep, plus the deep end volume (174 sqft x 8' deep) I come up with 17,870 gallons, which may be a better figure since there is a pretty significant difference in the sizes of the deep and shallow ends. It then doesn't seem unreasonable to round it down to 17K considering the stairs and curves are all going to subtract from the initial number....

    Thanks for the advice on the plumbing - it would take MAJOR demo to replace most of the pool to pad piping as it's under the concrete pool deck - but I could probably shorten it by 10-12 feet in each direction, and upsize all the on-pad plumbing. If you remember the pic in my first thread, the machinery used to be under the back porch 5 or 6 feet before I moved it out to where it could be accessible and built that little shelter around it. When I moved it, I just put an elbow on where the pipes came out of the ground and ran it back out to the new location. Under the porch is just dirt, so I could presumably dig up that much of the pipes, shorten them and get rid of a couple elbows. Not ideal but better than it is...

    The filter size comment is interesting as well - my current filter is 36 sqft, w/ a 72 gpm max flow - if my 1hp pump might put 90gpm into it (or the equivalent pressure) would that possibly account for the premature grid failures I've been having? It is worth noting that downstream of the filter is not just the return eyeballs, but also the booster pump for the Polaris, so at times essentially the entire pressure head of the system is being dropped across the filter. As I've said elsewhere, I usually see about 8psi on a fresh charge of DE, but have seen as high as 25-30 if things got badly clogged before I noticed... In addition I've seen the filter is usually pretty well caked up when I take it apart in the fall, even though I had been backwashing it for several hours as I used it to pump the pool down for closing...

    Gooserider
    Free-form Inground gunnite pool, Estimated 16-17K gallons. New Pentair TR60 ClearPro 24" sand filter and Compupool CPSC-48 SWG, Hayward SP1607X10 Pump w/ 1HP motor, 1.5" plumbing, Polaris Pressure cleaner w/ booster pump. pool is more than 25 yrs old, less than 35. Not painted, deteriorating tile surround. I am paraplegic, get in/out of pool w/ S.R. SMITH PAL portable pool lift (significantly modified)

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