help with adjusting CSI / pool calculator questions

[beezar]

I'm trying to pick a good spot for my spa in terms of balancing the rise in pH with a too negative CSI.

Numbers (TA was 170 so I was adding acid to bring it down):

pH 7.1
TA 40 (was over aggressive...)
CH 340
CYA 75
Borates 50
Temp 84 deg F

With these numbers, I get a CSI of -0.94 from the Pool Calculator. I was going to bring up my pH via aeration, but noticed that the CSI becomes less negative as I increase my pH to 7.5 (CSI = -0.79) but starts becoming more negative as I increase pH further from there (eg: pH 7.8 has a CSI of -1.05). And at pH of 7.9, it says I entered invalid data.

I thought with increasing pH, CSI should constantly increase, but obviously I'm missing something.

My real question is even with a TA of around 60 or 70 and a pH of around 7.6-7.7 with borates, I'm still getting a relatively rapid rise in pH. If I try to lower TA to 50, the CSI gets really negative (even with pH of 7.5, say which may occur after acid addition, CSI is -0.5) which makes me worried about my plaster spa.

I'm keeping my CYA higher to slow the FC drop in the hot texas weather (lose 3-4 ppm per day, ugh).

Any suggestions on what I should do? Thanks

 

[JasonLion]

You are running into a special case with the chemistry/math involved and getting invalid results. The trick is that TA couldn't be as low as 40 when the PH goes up given your other numbers. Try it with TA at 50 and you will get more realistic numbers.

TA is total alkalinity. In this situation that is composed of carbonate alkalinity, borate alkalinity, and cyanurate alkalinity. As the PH goes up, carbonate alkalinity would have to be negative to keep TA at 40, which is impossible, so strange things are happening in the calculations.

 

[chem geek]

The carbonate alkalinity with your numbers is still positive at 18, though that is very low. The Pool Calculator is calculating correctly. If you went too low in TA in The Pool Calculator (entering 20, for example) you'd get "NaN" since the carbonate alkalinity would be negative which is not possible.

Now as Jason says, entering in a higher pH without making other adjustments still has the saturation index be very low -- going to 7.7 in pH has the index go to -0.86. This is because the carbonate alkalinity gets even lower if the pH goes up from aeration alone, but it is unlikely to do that given the low carbonate alkalinity.

If you use about 50 ounces of 20 Mule Team Borax per 10,000 gallons to raise the pH to 7.5, then the TA would rise to 50 and your saturation index would be -0.5. If you instead use 26 ounces weight (23 ounces volume) of pH Up (sodium carbonate; Arm & Hammer Super Washing Soda) per 10,000 gallons, then the TA would rise to 58 ppm and the saturation index to -0.36. If the pH rises to 7.7 on its own, then you'd be at -0.24 and at that point you'd be in good shape. If you wanted to increase your CH you could, but it's not that necessary.