Low CSI - how serious/time-critical?

[PaulR]

I have brought up low CSI a couple times in threads recently and been (gently!) corrected. I don't mind that, as an Advanced Beginner I kind of expect it from time to time. But it leaves me wondering what the story is. I guess I'm just sensitive to CSI as I have a plaster pool myself and have had balance problems. I don't want to add plaster patching to my list of grumpily attained skills.

Really I'm not sure how much of a problem a low CSI really is. If all your numbers are at the low end of commonly suggested ranges, especially with pH down toward 7.2, then your CSI could easily be in the neighborhood of -.6 and that's not so good. But is it not so good in a this-week sense, or a this-month sense? (Or a this-year sense??) If the former then I feel it ought to be mentioned, if the latter then yes in a green pool you clear the algae first and see where you are afterward.

There are a few Deep End threads about CSI that I found.
- this is a mind-numbing discussion of negative CSI but concentrates on metal corrosion not calcium leaching.
- this is basically a disagreement about what the don't-care limits are.
- this is by chem geek discussing aspects of CSI, and among other things he notes: "That would mean that for dissolving of pool plaster, pH is the most important factor. It is only for scaling where the TA and CH play a significant role."

But... how fast does all this happen? Days, weeks, months, years...
--paulr

 

[Guest]

the problem with just about every saturation index is, while they can pretty accurately predict scaling propensity of water they really do not predict how so called 'aggressive' water will affect plaster and have even less predictive ability when it comes to metal corrosion.
What we do know about damage to plaster is that low pH for extended periods of time are a culprit. as is exposure to extremely soft water for extended periods. However, just because water is 'agressive' does not necessarily mean that plaster damage will follow.
I agree that pH is the most important factor to look at. In fact, I have stated this many times when I have said that SI is not really a valid measurement as commonly applied to pools.
Low pH can cause plaster damage much faster than low CH but even low pH is not going to be instantaneous unless you are filling a pool with pure muriatic acid!
Low pH will damage a copper heat exchanger much faster than it will damage plaster in most, if not all, cases.

 

[JasonLion]

There aren't a lot of data points available. No drain calcium scale treatments typically bring the CSI down well below -1.0 for weeks, or sometimes months, and even then they often don't work (ie have no visible effect).

 

[Guest]

In other words, if you keep your pH within range most of the time and you don't have a pool with extremely soft water most of the time then don't lose any sleep over it. There are a lot of things that can have an effect on plaster besides SI, for example high sulfate contentration in the water from long term use of dry acid and/or MPS is suspected of causing plaster damage. It's a complicated subject that even the plaster industry cannot agree on.