Reverse calculate pool size

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D

Dent

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May 15, 2016
10
Sarnia, ON Canada
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.
 
Dent said:
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.
Click to expand...


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?
 
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.
 
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.
 
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.
 
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.
 
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.
 
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