Pool Math inconsistencies?

Tbefi1

Member
Mar 30, 2018
10
Austin / Texas
I have a 30,000 gal plaster pool with an SWG. Chemistry is: FC=8, CC=.5, TA=70, PH=8, CH=325, CYA=62, Temp=71F, salt=3400, borate=0

Two questions regarding pool math. I am using the pool math website on a MacBook with safari.

1) My PH is 8. My goal is set to 7.5. The detail column says to add 30 oz of 31.45% muriatic acid. Yet if I go to the bottom of the page under 'Effect of adding chemicals', it says that adding 30 oz of 31.45% muriatic acid will lower my ph by .24, resulting in a PH of 7.76. Which is correct and why do they differ?

2) With the above chemistry, pool math says my CSI is 0. I also checked the csi using the cardboard wheel which came with my Jandy SWG and it says csi is +.3. The wheel only uses PH, TA, CH and temp but this still seems like a big difference. To sanity check, I set FC to 2, salt to 2500, CYA to 0 in pool math and the calculated csi went to +.22. I am trusting the pool math answer but wanted to get an expert view on why the wheel seems so inaccurate? I also checked the pool math app on IOS and it agreed with the website.

Thanks!!
Tony
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
20,477
Northern NJ
1) My PH is 8. My goal is set to 7.5. The detail column says to add 30 oz of 31.45% muriatic acid. Yet if I go to the bottom of the page under 'Effect of adding chemicals', it says that adding 30 oz of 31.45% muriatic acid will lower my ph by .24, resulting in a PH of 7.76. Which is correct and why do they differ?
The detail column considers your TA level while the effects of adding does not. The detail column considering pH and TA is a more accurate calculation.


2) With the above chemistry, pool math says my CSI is 0. I also checked the csi using the cardboard wheel which came with my Jandy SWG and it says csi is +.3. The wheel only uses PH, TA, CH and temp but this still seems like a big difference. To sanity check, I set FC to 2, salt to 2500, CYA to 0 in pool math and the calculated csi went to +.22. I am trusting the pool math answer but wanted to get an expert view on why the wheel seems so inaccurate? I also checked the pool math app on IOS and it agreed with the website.
TFP uses CSI. The cardboard wheel calculates LSI.



What test kit are you using that gives you a CYA of 62?
 

Tbefi1

Member
Mar 30, 2018
10
Austin / Texas
Thanks for the quick reply! I am using the tf-100 test kit, I estimated 62 on cya since the black dot disappears when the fluid is slightly above 60. I guess I am being too precise. Btw I generally keep cya around 70 so I plan to add some.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
20,477
Northern NJ
We always round CYA up. If the dot disappears between 70 and 60 line call it 70 and move on. The CYA test is not that precise. And the lines are logarithmic, not linear so you can't eyeball values between the lines.
 

mgtfp

Bronze Supporter
Mar 5, 2020
492
Melbourne, Australia
2) With the above chemistry, pool math says my CSI is 0. I also checked the csi using the cardboard wheel which came with my Jandy SWG and it says csi is +.3. The wheel only uses PH, TA, CH and temp but this still seems like a big difference. To sanity check, I set FC to 2, salt to 2500, CYA to 0 in pool math and the calculated csi went to +.22. I am trusting the pool math answer but wanted to get an expert view on why the wheel seems so inaccurate? I also checked the pool math app on IOS and it agreed with the website.
I suspect the main difference is that your wheel doesn't consider CYA. CYA contributes to the TA (Total Alkalinity) reading. But for the CSI (or LSI) calculation only the Carbonate Alkalinity (CA) part of TA is relevant. Most pool stores do this correction, often without telling their customers, and still call this result TA instead of CA. The wheel has probably been designed to be used with pool store test results.

PoolMath uses TA as input and does the correction to CA in its CSI algorithm.

You saw the difference when you set CYA to 0 in PoolMath. The remaining difference between 0.22 and 0.3 is the difference between CSI and LSI - LSI is usually about 0.08 to 0.1 (I just edited that, had a 0 missing there...) bigger than CSI.

This pool store procedure of giving their customers CA in the test results but still call it TA really bugs me. TA is still an important value, it tells you the buffering capability of the water (CYA is also acting as a buffer - that's why it shows in the TA test). CA is important when judging Carbonate depending effects like CSI or CO2 outgassing.

When using your wheel with your test kit results, you need to subtract about 1/3 of your CYA reading from your TA reading to get an estimation for CA to be used with your wheel to calculate LSI (not CSI!).
 
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