Pool Math calculation wrong?

[riny]

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]

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]

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]

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]

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 -

Bleach concentration per unit volume and other calculations

In the process of validating the math for my bleach automation system, and I ended up down a rabbit hole learning about various methods of how bleach concentrations are characterized. Correct me if I'm wrong here, but, I think it's most common for US companies to specify the concentration as...

www.troublefreepool.com

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.

 

[riny]

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]

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?

 

[riny]

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%.