PH and CSI relationship reversal

[JeanZ1]

Hello, typically when I reduce pH I see CSI also decrease (i.e. low Ph tends to push CSI lower and higher pHs push it higher). However, for some reason, on pool math with my current chemistries that relationship changes and the higher my pH is the lower the CSI. For example, today my pH tested at 8.2 which further reduced my CSI to -0.22 from -0.18 at a pH of 8.0. I've noticed this occurring since I've added borates to stabilize pH, which has not helped at all, still adding acid 2-3x per week. Can someone please explain this new relationship to me.

 

[JamesW]

The CSI is based on the adjusted Alkalinity aka carbonate alkalinity..

At higher pH, the correction factor is higher and that lowers the adjusted Alkalinity.

Adjusted TA = TA – (CYA X CYA C.F) – (Borate x Borate CF)

Borate C.F (correction factor) based on pH.

pH.......CF

7.2.....0.051

7.4.......0.0786

7.6......0.1248

7.8......0.1989

Cyanuric Acid correction factor based on pH.

pH........CF

7.0.......0.22

7.1.......0.24

7.2.......0.26

7.3.......0.28

7.4.......0.30

7.5.......0.32

7.6.......0.33

7.7.......0.34

7.8.......0.35

7.9.......0.36

For example, if the pH = 7.6, TA = 90, Borate = 50 and CYA = 70, the adjusted alkalinity is 90 - (70 x 0.33) – (50 x 0.1248) = 60.66.

 

[JamesW]

 

[JeanZ1]

Whew, thank you. It took me awhile to get thru it but if I understand the “cure” is to raise alk? I was thinking of doing this anyway since pH has not stabilized.

 

[JamesW]

Can you show a picture of your pH test and what your reading is?

 

[JeanZ1]

Hard to get a good pic but here are 2 that show it.

 

[JamesW]

I would call that 7.9 and leave it alone.

 

[JeanZ1]

Thanks!