Math question

[Wilkod]

I had a thought cross my mind while using pool math today. When we look at yhe csi index and it states "less than -0.6 is suggestive of problems" the less than reference is based on the number line correct? In other words -0.1 is greater than -0.6, right? Thanks and thanks for all the great info here!

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[Agent99]

-0.6 <= ideal <= 0.6

And 'yes' to your questions.

 

[Wilkod]

Thanks. I knew that but i couldnt get the question out of my head ! Thanks again

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[chiefwej]

The "ideal" CSI is zero. The allowable deviation from that is from +.6 to -.6. Any further away from zero and you are either building scale or in danger of damage to plaster and grout.

When I had some scale in my pool I purposely ran it at a negative CSI at or near the limit for a long time and the scale gradually disappeared. I also noticed a sizable increase in the CH level as it was slowly being dissolved off the pool surfaces.

 

[chem geek]

The "ideal" CSI is zero. The allowable deviation from that is from +.6 to -.6. Any further away from zero and you are either building scale or in danger of damage to plaster and grout.

When I had some scale in my pool I purposely ran it at a negative CSI at or near the limit for a long time and the scale gradually disappeared. I also noticed a sizable increase in the CH level as it was slowly being dissolved off the pool surfaces.

Yes, and the risk is that even a not-that-negative CSI over a longer period of time may dissolve plaster surfaces. Clearly, fresh scale dissolves more readily, but what we don't know is precisely how quickly the calcium carbonate in a plaster surface will dissolve if the CSI is only somewhat negative. Tests by onBalance seem to indicate that the process for well-made plaster is relatively slow (he saw CH increases in the -0.7 range), but the question is how slow and how far negative does such dissolving show up over many years, even a decade or more. The -0.6 has a calcium or carbonate level that is only 1/4th that of saturation. Even -0.3 has only half the level of saturation. Lower pH likely increases the rate of dissolving even at the same CSI level.

 

[chiefwej]

I think I'm safe. Once the scale was mostly gone, I've been keeping the CSI between zero and -0.15. Still on the negative side but only slightly so.

BTW: did you get a PM about adding a footnote and an additional page to Pool School re. CSI?

 

[chem geek]

Yes and some simpler changes are getting made first -- increasing the CH range slightly for plaster SWG pools and possibly removing the Target FC that just causes confusion since one just needs to add whatever is needed to stay above the minimum FC for their CYA level. Having the TA range for vinyl pools down to 50 ppm is a no-brainer, but putting an asterisk by the low TA range for plaster and fiberglass pools with a link to a page explaining what to do if the pH tends to rise (i.e. lower the TA, but raise other parameters to have the CSI not be too negative) will take longer as there is more text to write properly in that case. Feel free to write something to propose if you like. Anyone can contribute to this change. It should just be done in a way that is treated more like an exception -- use the Recommended Levels as a starting point and if you have the pH rising over time then you can consider more advanced approaches.