Poolmath shows weird CSI at very high pH

[Katodude]

So now that the water is cold and my TA has dropped to 50, I am keeping my pH higher. Like 8.1, 8.2, 8.3. It seems like the higher the pH the lower the CSI which I believe is an error.

 

[mknauss]

Water temperature effects CSI also.

 

[Katodude]

I am aware that temp affects CSI.

Enter these numbers:

FC: 10
TA: 50
CH: 500
CYA: 60
Temp: 76
Salt: 3400
Bor: 30

Enter a pH of 8, then change it to 8.2, then change it to 8.3 and 8.4. The CSI starts dropping the higher the pH. While changing no other factors.

 

[JamesW]

You have multiple components that comprise the “Total Alkalinity”. The total alkalinity primarily consists of carbonate, bicarbonate, borate and cyanurate. Only the carbonate contributes to the CSI.

As you change the pH, the amounts of the components of the total alkalinity change. Since you are creating more borate and cyanurate, you have less carbonate. If you remove the borate or cyanurate, the CSI goes back up.

Cyanuric acid and boric acid both have a higher pKa than carbonic acid.

 

[Katodude]

So just to confirm, all other things remaining the same once pH goes above around 8.2 the CSI starts to drop?

 

[JamesW]

So just to confirm, all other things remaining the same once pH goes above around 8.2 the CSI starts to drop?

It depends on the specifics. You have to determine the amounts of carbonate, cyanurate, borate etc. at each pH and each TA combination.

 

[mgtfp]

The formula to calculate CSI looks in a simplified form like this:

CSI = pH + f(CH) + f(CA) + f(T) - f(TDS)

where "f(...)" means "function of ...". We don't really need to know how these "functions" actually look like, we can just let PoolMath do the work, but it helps to at least understand this general structure of the CSI formula.

Carbonate Alkalinity (CA) is not a parameter that is being directly measured, it has to be calculated from parameters accessible to pool test kits, therefore the CA-part of the formula gets re-parametrised as

f(CA) = f(TA, CYA, Bor, pH).

So, pH affects CSI in two ways. First, there is a direct influence via the first term in the formula, i.e. increase pH by e.g. 0.1, and CSI will increase by 0.1. But there is also an indirect influence via the Carbonate Alkalinity. Usually, esp. without Borates in the water, this indirect pH influence on CSI is much smaller than the direct influence, and CSI just keeps increasing with rising pH. But with rising pH, more and more of the TA will actually be made up from CYA-Alkalinity and Borates-Alkalinity. At typical pool-pH, the CYA-Alkalinity has only a weak dependency from pH, but the Borates-Alkalinity is strongly pH-dependent. Without Borates in the water, you will never see this "negative" influence of pH on CSI, but with Borates you can.

Some examples based on your numbers (calculated with chem geek's PoolEquations spreadsheet):

TA=50, CYA=60, Bor=0, pH=7.8 --> CA=28.0, CYA-Alk=21.9, Bor-Alk=0
TA=50, CYA=60, Bor=0, pH=8.0 --> CA=27.4, CYA-Alk=22.4, Bor-Alk=0
TA=50, CYA=60, Bor=0, pH=8.2 --> CA=26.8, CYA-Alk=22.8, Bor-Alk=0
TA=50, CYA=60, Bor=0, pH=8.4 --> CA=26.3, CYA-Alk=23.1, Bor-Alk=0

TA=50, CYA=60, Bor=30, pH=7.8 --> CA=21.8, CYA-Alk=21.8, Bor-Alk=6.2
TA=50, CYA=60, Bor=30, pH=8.0 --> CA=17.9, CYA-Alk=22.4, Bor-Alk=9.5
TA=50, CYA=60, Bor=30, pH=8.2 --> CA=12.4, CYA-Alk=22.8, Bor-Alk=14.5
TA=50, CYA=60, Bor=30, pH=8.4 --> CA= 4.8, CYA-Alk=23.1, Bor-Alk=21.6

You can see that with without Borates in the water, CA remains pretty stable. But with 30ppm Borates, Borates-Alkalinity starts to rise and Carbonate Alkalinity starts to crash at higher pH (which causes the drop in CSI that you noticed in PoolMath).

But you should actually never see this happen in reality, because with these water parameters, the CO₂ dissolved in your pool water is in equilibrium with the atmospheric CO₂ at a pH slightly above 7.8, so it shouldn't rise above about 7.8 by outgassing. pH might temporarily get higher, e.g. when adding a lot of bleach to get to SLAM-level (which will get down again when the chlorination cycle is completed and chlorine back to normal), but this will also temporarily increase TA (remember, the only TA-neutral process to increase pH is by aeration) which will prevent CA from crashing.