I have Wet Edge Luna Quartz plaster and my pool is about a year old. The pH has only recently stabilized and the sweet spot for my pH seems to be around 7.8. I still need to add some MA about once a week or so (which is way down from the amount and frequency of just a couple of months ago), and the pH will slowly rise to 7.8 and level off for several days.

My test results are as follows:

FC 3.5
CC 0
pH 7.8
CH 500
TA 70
CYA 70
Salt 3400
Borates 50

At 75 degrees CSI is -.12 which ought to be fine. But if I tried to keep pH at say 7.5 CSI goes down to -.34 (and at 7.2 is -.58) which could be problematic. Of course, I would like a lower CH but my fill water is 150 and with the continued drought it's hard to get it lower. Further, a partial drain and refill now would be illegal because of mandatory water rationing. If if wanted to keep my pH down to 7.5 it would require adding about 2-3 cups of MA every third day because my pH will rise until it reaches 7.8.

So my question is why does Wet Edge recommend pH of 7.2-7.6 without regard to CSI? Should I care at all if I keep my CSI slightly negative and not let pH go over 8.0? Heck, even at pH 8.0 CSI is -.02 which ought to be fine.

Its an industry norm and not really based on actual conditions.

Your chems are exactly where you want them.

Scott

Thanks. Peace of mind is nice.

Here's where I get up on my soapbox.........

CSI is not a necessary calculation for you to successfully manage your pool water. I have never calculated it for my pool nor do I ever intend to. In my opinion, it overly complicates things and one of the PRIME things we teach at TFP is to simplify as much as you can. If you keep your pool water within the guidelines suggested on the forum, you can safely assume you will never have a scaling issue.

Since your CH is slightly above the suggested guideline of 250-400, you can make a simple compensation by holding your TA and pH down towards the lower end of safe ranges (TA right where you have it and pH down around 7.4)

I suspect you wouldn't have a scaling issue at pH 7.8, either, if your pool stabilizes there and you keep a watchful eye for symptoms but lowering your pH simply adds a good measure of insurance.

Others here on the forum will not agree with my rejection of csi and I understand their viewpoint and I do not discourage anyone from using it if they like. However, I maintain that calculating CSI is virtually never necessary to have a balanced pool.

The only reason the higher pH would be a problem (assuming the reasonable CSI which you already have and it takes a rather high CSI before scaling is seen except in saltwater chlorine generator cells) is for metal staining. Unless you've got significant metals in your water, I wouldn't worry about it.