CSI/PoolMath wonky with extreme values?

[Vyrolan]

Not sure if this is a problem with the calculator or an odd inflection in the CSI formula or what...

If you punch in certain values (which are close approximations of my pool currently unfortunately: PH = 7.2, TA = 110, CH = 675, CYA = 360, Salt = 0, Borate = 0, Temp = 83), then it shows a CSI value of -0.82...corrosion likely. That doesn't sound good. Well I figured the easiest adjustment is to increase PH since 7.2 is kind of low anyway and surely increasing PH would make corrosion less likely. However, if you keep those numbers but change the PH to 7.4, the CSI actually plummets down to -3.36.

Here's a screenshot of the values using current and goal to see them side by side:

I think the mega extreme CYA level and the very high CH level are just making the calculations wonky...if you increase PH further to 7.5, the CSI just goes NaN. Can anyone explain what's happening?

 

[JasonLion]

That is happening because CYA can not be 360 when TA is 110, it is simply impossible. CYA contributes to TA, and a CYA of 360, with nothing else in the water, means a TA of at least 120. As a result carbonate alkalinity goes negative (internal to the calculations) and things fall apart.

 

[Vyrolan]

That is happening because CYA can not be 360 when TA is 110, it is simply impossible. CYA contributes to TA, and a CYA of 360, with nothing else in the water, means a TA of at least 120. As a result carbonate alkalinity goes negative (internal to the calculations) and things fall apart.

Interesting. I need to redo my diluted CYA test at a couple of different dilution ratios to see if I can get some kind of convergence. My last test read somewhere between 70 and 80 with 1 part pool 4 part RO water sample, and that's where I was getting ~360.

Is there a CYA:TA ratio? I never see in any threads it being mentioned that adding CYA will increase TA...and the "effects of adding chemicals" on the calculator doesn't state such a change.

 

[chem geek]

It's not a ratio, but rather that CYA contributes to TA, though the amount of contribution is a function of pH as follows:

pH	CYA factor
7.0	0.231
7.1	0.252
7.2	0.271
7.3	0.289
7.4	0.305
7.5	0.319
7.6	0.331
7.7	0.342
7.8	0.351
7.9	0.358
8.0	0.364

So at usual pool pH the CYA contributes roughly 1/3rd its amount to TA. So if you are measuring a TA that is less than roughly 1/3rd the CYA level, then something in your measurements is very wrong.

 

[JasonLion]

You are close to plausible values. Given the gigantic error range on a CYA test with dilution, your CYA could easily be anywhere from 280 to 440 (if your testing was perfect, a wider range is also plausible with anything less than perfect test technique). Your TA measurement is consistent with CYA from 330 on down, but the TA is +-10, so CYA could actually be 360 if TA was actually 120.

Adding CYA doesn't really change the TA, but raising the PH back up to what it was before does. Once you have added enough base to raise the PH back up, the TA will have gone up by roughly 1/3 of the amount you raised the CYA level by. This isn't something you normally need to think about.

 

[Vyrolan]

It's not a ratio, but rather that CYA contributes to TA, though the amount of contribution is a function of pH as follows:
<snip />
So at usual pool pH the CYA contributes roughly 1/3rd its amount to TA. So if you are measuring a TA that is less than roughly 1/3rd the CYA level, then something in your measurements is very wrong.

Awesome...thanks for the info.

You are close to plausible values. Given the gigantic error range on a CYA test with dilution, your CYA could easily be anywhere from 280 to 440 (if your testing was perfect, a wider range is also plausible with anything less than perfect test technique). Your TA measurement is consistent with CYA from 330 on down, but the TA is +-10, so CYA could actually be 360 if TA was actually 120.

I was as careful and as accurate as I could be, but obviously I like the necessary tools to do it better than "kinda sorta not terrible". With lots of kids present over the holiday and some intentional draining and refilling, I wanted to test again anyway. So last night I made up two independent 1:3 dilutions and performed two tests each. I got 4 readings between 55 and 60, so I would say it is reasonable to believe I'm now in the 220 to 240 range. I had added some soda ash as well for PH so I retested TA as well and got 125. So at TA=125 and CYA=~230, things make sense and I'm actually around good CSI values. Now to replace 2/3 of the water to get CYA down around 80!

Thanks for all the explanations and background information...it's much appreciated.