HouTex said:
I agree. During the first 4 months or so after construction I religiously followed the CSI and always stayed within the "balanced" range. Yet when my CH went over 600 and my CSI never approached .4 I still developed some discoloration of my plaster. I suppose you could say that CSI works when you are within the recommended ranges, but then what's the point of having a separate scale to tell you that the pool is balanced when it's already apparent? Or perhaps the rule is that CSI works unless you have high CH?
That isn't my recollection. According to your posts
here,
here and
here, your pH was 8.0 to 8.2 for perhaps one every four days. The CH varies from 440 to 675 (and later to 800). You do mention in
this post about the CSI being balanced but in
this post you mention how the pH gets up to 7.8 to 8.2 in a few days.
With fill water that has 200+ TA and CH of 150 (is that from the pool store or your own testing?), it is a challenge to keep your pH from rising. Using numbers from
this post (and using temp of 89 from
this post), the CSI will vary from around -0.3 at a pH of 7.2 to +0.4 at a pH of 8.0 and +0.6 at a pH of 8.2. When your CH was at 800 the index would have been +0.2 higher.
The problem isn't with the CSI but with the definition of "balanced" being the broad +/- 0.6 in the pool calculator. The industry usually uses +/-0.3, but in reality the number just tells you what is possible, not what will happen with any certainty. In practice, we usually don't see scaling in pools until the index gets to around +0.7 but I've seen reports of scaling in spas at around +0.3 to +0.4. Your pool finish is different than others and given that it is newer and was apparently still curing, the pH at its surface will be higher than in the bulk pool water so scaling will be much more likely at lower bulk water CSI than in a pool where the plaster is more fully cured. This same principle explains why there can still be calcium flaking in a saltwater chlorine generator cell when the bulk pool water has a zero or negative CSI -- the pH at the cathode (hydrogen gas generation plate) is much higher than in the bulk pool water. However, even under these conditions, the lower the CSI the less scale there will be (there is also another measurement called the Calcium Carbonate Precipitation Index to determine the amount of scale possible, but that's another subject that I briefly mentioned in one of your threads
here).
As for ranges vs. CSI, I suppose one could consider your pH getting to 8.0 and above to be out of range and just attribute your scaling to that for simplicity, but if your CH or TA were much lower then that would not have been a problem (except for potential metal staining). Also, for some pools and most certainly for spas one sometimes sees the TA get lowered significantly below 70 ppm, even to 40-50 ppm, so in that case how can one go by standard ranges and think everything is OK? If one gets that low in TA and has a plaster surface, then the CH should be raised significantly and perhaps a higher pH target of 7.7 to 7.8 be used. One could only figure that out based on the CSI, not based on looking at ranges alone.
