Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 9 of 9

Thread: Reverse calculate pool size

  1. Back To Top    #1

    Join Date
    May 2016
    Location
    Sarnia, ON Canada
    Posts
    10

    Reverse calculate pool size

    Hi all, I just switched to salt. And like a dummy I added too much based on an assumption of pool size I have been using for several years. So if I know I added 240 kg of salt and achieved 4300 ppm. Does anyone know the actual math so I could reverse calculate the size of my pool?

    Thanks. This would help with all my future math needed.

  2. Back To Top    #2
    Mod Squad woodyp's Avatar
    Join Date
    Apr 2010
    Location
    East Texas
    Posts
    9,004

    Re: Reverse calculate pool size

    It would be much easier using a fas-dpd chlorine test in one our recommended test kits and pool math to get a good handle on your volume.. Welcome to the forum.
    16x32x52" Steel Cornelius Miramar AGP Vinyl liner 13,100 gal. Buried 2 ft.
    2 Speed Hayward Power-Flo Matrix 85 g.p.m. 22" 250lb. sand filter hard plumbed
    Pool Rover Jr., Pool Blaster Max, Diver Dave TF100 Test Kit/Speed Stir
    Margaritaville Frozen Concoction Maker, Liqour Chiller, & Drink Mixer & Party Tub----Collect 'um all!

  3. Back To Top    #3
    JoyfulNoise's Avatar
    Join Date
    May 2015
    Location
    Tucson, AZ
    Posts
    11,625

    Re: Reverse calculate pool size

    You could try this - A Chemical Way of Calculating Pool Volume but it uses a lot of TA reagent.
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

  4. Back To Top    #4

    Join Date
    Jul 2013
    Location
    Lelystad, The Netherlands
    Posts
    60

    Re: Reverse calculate pool size

    Quote Originally Posted by Dent View Post
    Hi all, I just switched to salt. And like a dummy I added too much based on an assumption of pool size I have been using for several years. So if I know I added 240 kg of salt and achieved 4300 ppm. Does anyone know the actual math so I could reverse calculate the size of my pool?

    Thanks. This would help with all my future math needed.

    240 (kg salt) divided by 0.0043 (kg of salt per liter) equals = 55813.95348837209
    Your pool has roughly 55813 liters water.

    If you use 55813 liter in the poolcalculator with 0 salt and a target of 4300 ppm the result is to add 241 kg

    I hope this anwers your quoestion?
    40000 liter in ground vinyl pool with SWG : http://www.troublefreepool.com/threa...he-Netherlands

  5. Back To Top    #5
    Mod Squad Bama Rambler's Avatar
    Join Date
    Jun 2009
    Location
    SouthWest Alabama
    Posts
    22,347

    Re: Reverse calculate pool size

    Salt isn't usually a good way to figure the volume because you first need to know what the beginning level is and second the test isn't very precise. It's common to start out with 1500 to 2000 ppm of salt in pool water. All forms of chlorine add salt, so it's never zero.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit

  6. Back To Top    #6
    needsajet's Avatar
    Join Date
    Jan 2016
    Location
    Sydney, NSW, Australia
    Posts
    3,557

    Re: Reverse calculate pool size

    Welcome to TFP! I'm no expert; I'm just a newbie, but I learned a bit after working on this. I haven't tried the TA test because I have nowhere near enough reagent and thereagents are a bit of bother to get down here.

    There are a couple of things I can think of that you'd have to consider before relying on the straight reverse calculation based on your salt addition. First would be salt content before adding the pool salt. You would need to test before adding the pool salt, and then net this off the 4300 before doing the described calculation (unless you've already done this). You mentioned that you've switched, so it’s likely there was salt in the pool already, as opposed to a new pool.

    Next would be the error level for your test or perhaps the number coming out of your SWC controller. For example, the Taylor K-1766 is +/- 10%, so the calculated number would become a range of 90% to 110% of the calculated number. If the number is from your SWC controller, others here can tell you what error level to expect. If the number is from a pool store or a poorly calibrated EC pen, well, fuggedaboutit.

    Only if the 4300 ppm is the net increase in salt, and not the total salt, then your volume estimate (mathematically, including 10% error) would be 13,000 to 16,000 US gallons (50,000 to 61,000 litres). I've rounded off because two significant figures would be the most you could reasonably expect from the approach.

    Good point above about FAS-DPD for this purpose, because test accuracy can be quite high by using the 25 ml sample in a Taylor K2006 kit. I defer to others here, but I believe that if you use CalHypo for the test addition, the chlorine strength is fairly predictable. Chlorinating liquid varies from its stated percentage, so you would need to first carefully dilute the chlorinating liquid and test it, and then add from the same source jug to a clean pool after the sun is off the pool, wait a little while for mixing, and then test the pool water. So you would have two sources for error.

    If your pool happens to need more calcium, hardness increaser worked well for me. It's a known purity, and the calcium hardness test can be done to a relatively high level of accuracy. Plus, you don't need to test for CH very often, so using up the reagent is not a big deal. I knew my pool volume from the original fill and water meter readings and then tested against that benchmark when I raised calcium. The calculation based on the calcium added was off by 8% vs. the meter reading, call it +/- 10%, which I think is about the best you can expect from such an approach.

    My take is that the best way to do this, by far, is to use Pool Math on this site, over a period of time and multiple chemical additions. Pool Math is amazing and incredibly handy. Write down what Pool Math suggests will happen from a chemical addition, do the chemical addition, then test and compare. After that and subsequent chem additions, keep adjusting your pool volume on the Pool Math web page until the variation between the predicted change and the actual change gets down to +/- 10%. The variation will always be there because of measurement and testing errors, but if you're both undershooting andovershooting, and by a relatively small amount, you've got your volume close enough for pool maintenance purposes. Pool math is correct mathematically, subject to any caveats mentioned on the webpage, such as pH effects. So you can rely on the fact that the numbers are correct, subject to the user entering the correct pool volume.


    When you get a chance, enter your pool details in your signature, including what test kit you’re using. If you haven’t done so already, get the TF100 or K2006 test kit. If you stay out of the pool stores, you’ll pay it off in no time at all, even with the crazy Canadian prices for Taylor kits.
    12k IG salt; glass beads in plaster; K-2006C, K-1766, CCL, and Aussie 4in1 (HTH); Pentair Eco800 1.2HP VS; Zodiac SWC 1.3 lb/day (25 g/hr); 25" filter recycled glass; OKU solar panels; 1/2 HP solar pump; Rebel (Warrior) pool cleaner; FlowViz; prior pool AG 10k | Read Before Posting to get the best possible advice | ... and this helped me a lot!: TFPC for Beginners

  7. Back To Top    #7

    Join Date
    Jun 2014
    Location
    Roseville, CA
    Posts
    341

    Re: Reverse calculate pool size

    From personal experience is it way too easy to over salt your pool. As you probably know, what matters is what the salt reading on your SWG is. That is what determines if it runs or not.

    The best way, IMO to "back calculate" might be to use the PoolMath calculator if you borate your pool with 20 Mule Team Borax. If you start at the correct PH, and then return to that same PH, you should be able to get pretty close based on the amount of X% strength acid you use. Municipal water can have a small amount of Boron in it.

    Plan B would be to rent a meter if you ever empty and refill it.
    SWG: CircuPool (May 2014) SI30+Plus; 3000ppm seems ideal;
    My Ideal Specs:
    FC 5; PH 7.4-7.6; TA 90; CH 350; CYA 75; = SI ~ 0.19
    14,000 gal, in ground, plaster, Cartridge filter, Borates
    TAYLOR K - 2006

  8. Back To Top    #8
    Mod Squad Bama Rambler's Avatar
    Join Date
    Jun 2009
    Location
    SouthWest Alabama
    Posts
    22,347

    Re: Reverse calculate pool size

    You can get a pretty good idea of volume by using pH, FC & TA adjustments, and keeping track of it over time. And adjustment or two may not be very close, but dosing and recording the change and then adjusting the pool size in Pool Math will get you pretty close.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit

  9. Back To Top    #9

    Join Date
    Apr 2016
    Location
    South East USA
    Posts
    77

    Re: Reverse calculate pool size

    Hi,

    The 4300 indicated may just be a temporary 'too-high' reading, especially if you added a bunch of salt and checked at your SWG reading very shortly afterwards.

    I have seen it recommended (on this site) to run the pumps for at least 24hrs with the SWG off after adding a lot of salt to make sue the salt if fully dissolved before turning on your SWG. Brushing will help.

    After that salt should be fully dissolved and the readig on your device may be more accurrate.

    Further, salt test kits are reputed to be more accurate that device readings.

    ft.
    • ~22,000 gallons, freeform, Pebbletec, beach entry • AquaRite SWCG v1.33 • Hayward Tristar SP325EE pump for main draw & skimmer • Hayward sand filter S310S • 1.5 hp booster pump • Paramount PCC2000 in-floor cleaner, skimmer, debris containment canister • Waterfall • Pentair Intellibrite 5g LED light in Pentair stainless wet niche • Jandy Lite2 heater: removed • Auto-fill: deactivated •

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •