Saturation Index (Langelier, etc.)

[DaveD]

Here is another one that is out there - Hamilton Index/:

http://www.southshoregunitepools.com..._balancing.htm

The Hamilton Index™ was developed by a pool technician for the pool and spa industry. This index was tested by field service and research over a period of 11 years. The index is so accurate, that in 20 years of field application, it has not needed to be modified.

Today, there are an estimated 400,000 pool and spas being run on the Hamilton Index™. Interestingly, in 1991, the National Pool and Spa Institute lowered the recommended total alkalinity towards the guide lines that the Hamilton Index™ has used for 25 years. But, of course, they are still calling it the Langelier Index.

Besides being very accurate, our 3-Step System in extremely easy.

Step 1: Test total hardess (not CH). That tells you where to put your total alkalinity - exactly!

Step 2: Put your total alkalinity where the chart tells you.

Step 3: Keep your pH at 7.8 to 8.2.



The chart is on the link I attached.

[DaveD]

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

[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

[chem geek]

Here is another one that is out there - Hamilton Index/:

http://www.southshoregunitepools.com..._balancing.htm

Well, I'm probably going to be showing my biases in what I am about to say and others can whack me over the head to give some balance. Let me begin by quoting something that is recommended on the Hamilton index page:

"To Lower Alkalinity - Add Acid in a 2-3 ft Circle Away From Return Lines And Skimmer"

As lots of people (including Ben) will tell you on this forum, this simply does not work and is not the effective way to lower alkalinity. The ONLY way to lower carbonate alkalinity (since other components of alkalinity such as that from CYA aren't the target for getting lowered) is to remove the carbonate, CO3(2-), and bicarbonate, HCO3(-), from the water and the way to do that is to enhance the outgassing of carbon dioxide, CO2, using Ben's Lowering Your Alkalinity procedure. It is the combination of low pH and aeration that leads to the faster outgassing of CO2 and the subsequent lowering of TA when you add acid to keep the pH down. Adding acid alone will simply lower BOTH the pH and the TA -- not just the TA alone.

The Hamilton recommendation is to never run the pH below 7.6 (with 7.8 to 8.2 as ideal). Running at higher pH does have some advantages and Ben talks about this at this thread where Ben tells us even more information about Jock Hamilton, the creator of the index. It mostly reduces the tendency for the pH to rise in pools due to the outgassing of CO2 since the pool (plaster/gunite ones, anyway) is intentionally out-of-equilibrium with the air to saturate the pool with calcium carbonate. At higher pH, less carbonate is needed so pH tends to be more stable. This reduces chemical (acid) additions and has other benefits that Ben mentions.

The Hamilton index uses Total Hardness that includes Magnesium, and not just Calcium Hardness. However, it is calcium carbonate that percipitates and is at saturation, not magnesium carbonate, so using Total Hardness will sometimes work and sometimes not depending on the ratio of Calcium to Magneisum in the water. Typical water has 4 times as much calcium (by weight) as magneisum, but this varies and the S.F. city and peninsula (i.e. Hetch Hetchy water) has a ratio of about 2.6 instead of 4. With the 4 to 1 ratio (by weight) the conversion calculation is to divide Total Hardness by 1.412 to get Calcium Hardness.

The Hamilton index would tend to produce scaling at the higher Total Hardness levels. At the extreme of 2000 ppm Total Hardness, if we assume this is about 1400 ppm Calcium Hardness, then the recommended 40 ppm TA with a pH of 8.0 yields a saturation index of +0.6 which might start to scale. In practice, most pools probably had far lower Total Hardness and it is possible that at the much higher Total Hardness levels that less of it is from calcium and more is from magneisum. In an extreme situation with a lot of magnesium relative to calcium, you could percipitate magnesium carbonate first, but that is not at all typical.

The bottom line is whether there was a problem with people using the Langelier index and finding that they were scaling or corroding their pools. Unless that was the case, then both indices in the typical ranges for pool owners produce similar results especially given the great leeway in the indices before problems are actually seen. What I did find is that the Langelier Saturation Index used by pool stores does not vary correctly by temperature or by TDS for reasons that I cannot figure out except that the index was "simplified" by someone at some point to use logarithms for everything including the temperature and TDS portions when technically those do not vary by logarithm (while calcium hardness and carbonate alkalinity DO). The full calculation for calcium carbonate saturation that is in my spreadsheet does track the Taylor watergram almost exactly except for the very highest temperatures (140F) where it is off by about 0.1 (assuming that Taylor is correct and I have no reason to believe that they are not).

Richard