Saturation Index

[chem geek]

Donnie,

Evan (waterbear) and I disagree somewhat on this subject which is why I referred you to this link in an earlier post so you can see the discussion about the LSI and its applicability. Though Professor Langelier may have originally developed his index for the scaling limit for boilers, the index with the more complicated TDS and temperature factors found in my spreadsheet is simply based on calcium carbonate saturation. It has nothing to do with boilers or closed vs. open systems. In an open system, things change, but if they change slowly then the much faster chemistry involved with saturation still applies.

Nevertheless, the index is over-touted since you can have your pool quite over-saturated with calcium carbonate and still not see scaling. The index value of +0.7 where some people start to see scaling (others don't see it until +1.0) is over-saturated by a factor of 5. At the other end, for corrosion which is really more about pitting and dissolving of plaster/grout surfaces, not metal corrosion, we haven't seen this reported on this forum so it also probably takes something like -1.0 (or at least -0.7) before there is a problem.

The "theory" of having a thin layer of calcium carbonate on metal surfaces to protect them against corrosion is also disputed by several sources. As waterbear points out, for metal corrosion it is the pH that is far more important (pH is also the biggest factor in the LSI) -- acidic environments are much more corrosive and I have rusty metal bar mounts to prove it where my Trichlor floater "parked" itself years ago! High chlorine levels, specifically hypochlorous acid levels, are also corrosive to metal so not using CYA would lead to much higher risk as has been found in some indoor pools (see this thread where an FC of 3-5 ppm with no CYA and with salt since an SWG was used).

So though you want your LSI or equivalent to be roughly near its ideal of 0, you don't have to be that close. It's simply not worth the extra time or bother to worry about it. In my own pool, I try to keep it slightly negative (closer to -0.2) since I have a gas-fired pool heater and in the heater the hotter temperature will be more likely to cause scale, but I don't worry if it's not exactly at my target.

Richard