Saturation Index (Langelier, etc.)

[chem geek]

More info from the NAtional Pool and Spa Institute - whats the feeling on the general content of this document?

http://www.theapsp.org/NR/rdonlyres/.../chapter_8.pdf

There is a heck of a lot of info in this document and you can find this forum's general feelings on all of these topics by reading through the stickies in each topic area.

The NPSI does not vary their Free Chlorine (FC) recommendations vs. the amount of CYA in pools as per Ben's Best Guess CYA chart and they are wrong not to do so.

The suggestion of lowering high TA water by adding acid is only partially correct since doing so efficiently also requires additional aeration (as I discussed in a post earlier referring to Ben's alkalinity reduction procedure).

As for their discussion of calcium hardness, they overemphasize its importance when you don't have plaster/gunite/grout exposed to the water. As I mentioned, the calcium carbonate is supposed to inhibit corrosion on metal surfaces, but keeping the pH above 7.0 is the best way to avoid metal corrosion. In theory, salt water pools should have more corrosion problems than non-salt water pools, but I haven't seen any reports of that on this forum. The problems may only come to play when the TDS gets exceptionaly high as with sea water where the conductivity increases quite a lot.

I already discussed the problems with the traditional Langelier Saturation Index, but remember that such problems are relatively small (a difference of about 0.1 with LSI too high at 120F). Their discussion of corrosion factors is reasonable and includes additional factors I didn't mention (like flow rates which causes erosion corrosion) and high halogen levels. By the way, remember that the disinfecting forms of both chlorine and bromine are oxidants and therefore can corrode metals by themselves (chlorine more so than bromine). This is more of an issue when CYA is not used (indoor or shaded hot tub, for example) since effective disinfecting chlorine concentrations are higher, but there has been no reporting on this forum about corrosion of metal surfaces except from the obvious problems of very low pH (including my own personal experience of using Tri-Chlor in a feeder that stayed too close to metal bars in our pool).

The discussion of pH and TA for spas is generally accurate and consistent with what we talk about on this forum regarding CO2 outgassing and how this raises pH and then adding acid ends up restoring pH but with lower TA so less buffering capacity.

Users of this forum tend to avoid the use of the "extra" products mentioned toward the end of the document except for borates (Borax). This is mostly to save money and simplify pool maintenance since most of these products are not necessary. The one primary exception is PolyQuat for prevention of algae especially when it is anticipated that sanitizer levels may drop too low (i.e. when on vacation). Generally, though, users only add a sanitizer and then just maintain pH and TA and CH.

Richard