Pool Math calculation wrong?

riny

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Aug 20, 2020
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Jandy Truclear / Ei
I need a sanity check here! I'm using the Pool Math app on Android to calculate how much chlorine I need to reach SLAM level. To raise my FC from 1 to 16, using Bleach 10%, it says 207 oz. This means I'm looking for 20.7 oz of active ingredient (sodium hypochlorite), right?

Just for fun, I set the percentage to 100, expecting it to say 20.7 oz. But it doesn't. It says 49 oz. I tried a bunch of values and somewhere around 81% concentration, the "Add XXX oz" starts going up, rather than down, as the percentage goes up. Even at 80% it says 39 oz, which is 31.2 oz of NaClO. Then I checked 1% and got 2119 oz, which is 21.19 of NaClO. In other words... this calculation is all over the place.

Is this a bug in the app? Or does the app use a non-linear calculation because the volume of solution is somehow affected by the concentration?
 

mknauss

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Taking any of the calculation within the app outside of normal pool water chemistry ranges is full of error.

@JoyfulNoise will give you the chemistry reason.
 

Texas Splash

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My 2-cents until Matts (Joy) joins the conversation, 207 ounces is 1.5 gallons plus 2 cups. Don't try to separate everything on the ingredients label. I believe Poolmath takes care of all that anyways. Just keep it simple. If the chlorine is 10% strength overall, go with that in PoolMath. More to follow from our chemistry expert. :)
 

cledee

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My guess is that the app is set to calculate at 6%, 10% and 12%. Anything above 12% is set to 12% because there's nothing higher than 12% for Bleach/Liquid Chlorine
 

JoyfulNoise

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It's actually much more complicated on the backend because bleach changes it's units of measure when percentages change. 8.25% and lower is sold in units of weight percent (wt%) sodium hypochlorite. When you get to 10% liquid chlorine and higher, it is sold as Trade %. This post shows the various relationships -


Please don't put random values into PoolMath. As you can see, it creates spurious results. If you put in the normal label values (6%, 8.25%, 10%, 12.5%, etc), you will get correct answers. If you put in random percentage values, you will get garbage out.
 
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riny

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Ah thanks, that post explains it. I didn't actually start out to be random... I was looking for the exact amount of desired NaClO so I could compare prices. I figured that setting it to 100% would give me the non-diluted amount of NaClO following the simple ppm calculation (10,800 gallons, 15 ppm increase, just multiply and convert units). Turns out it's not that simple though. That thread is a fascinating read!
 

Leebo

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Ah thanks, that post explains it. I didn't actually start out to be random... I was looking for the exact amount of desired NaClO so I could compare prices. I figured that setting it to 100% would give me the non-diluted amount of NaClO following the simple ppm calculation (10,800 gallons, 15 ppm increase, just multiply and convert units). Turns out it's not that simple though. That thread is a fascinating read!
By any chance did you see the "Bleach Price Calculator" in the menu of PoolMath? If I'm reading your post correctly that tool may help you out?
 
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riny

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Jandy Truclear / Ei
By any chance did you see the "Bleach Price Calculator" in the menu of PoolMath?

I did but that just seems to use the simple linear calculation. It even says in the app that it works by "standardizing the volume to 100% chlorine concentration." But this doesn't match the non-linear, density-weighted formula used by the FC calculator.

What you really want to calculate is not "price for X amount of NaClO" but "price to raise my pool's FC by X ppm" and the bleach price calculator doesn't go that far. For that, you need to use the FC calculator to give you the volume of each chlorine source (6% bleach vs. 10% chlorinating liquid, in my case) and then use that to figure out the price. The volume needed of the 6% solution is 162.8% of the volume needed of 10% solution, not the expected 166.7%.
 
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