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Thread: Saturation Index (Langelier, etc.)

  1. #21
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    Default Re: Saturation Index (Langelier, etc.)

    The spreadsheet is still accessible at the PoolEquations.zip link found near the end of the first post in this thread. The corrections or enhancements I made to the Langelier saturation index were not huge (to the temperature and TDS portions of his index), but it is just based on the solubility of calcium carbonate. The theory is that having calcium carbonate just "in balance", that is in full saturation, means that the plaster will not have a tendency to dissolve nor will it have a tendency to form additional scale. In practice, one needs to be well away from the "0.0" balance of the index to see problems. Some people don't notice issues until +0.7 while others don't see problems until +1.0 (on the positive side for scaling -- don't know for corrosion or pitting of plaster, but may also be this same order of magnitude).

    With regard to copper and other metal pipes, the theory is that having near-saturation calcium carbonate forms a thin film or other properties on the metal that helps to inhibit corrosion. Too much saturation will cause scale and too little prevents the film from forming. However, even without the thin film, corrosion of metal requires other factors including lower pH and dissolved oxygen and conductivity (ionic strength). The pool environment and the above neutral (> 7.0) pH usually mean that metal will not corrode. It is generally only when one uses Tri-Chlor tablets and doesn't carefully montor the pH that one risks getting the pH lowered. Storing chlorine in non-airtight containers in the pump house can also lead to corrosion of pumps and other metal.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    To clarify, my Dad's issue is with a fiberglass hot tub. He has white rings forming just above the water line and is concerned about the heater getting fouled. His tap water is 1000 Calcium (from a well). When he called the spa manufacturer, they recommended a sequestering (also know as chelating?) agent such as Leisure Time Spa Defender. Am requesting MSDS sheets for it. They also suggested the ultimate solution would be to use 50% salt softener water with 50% well. My suggestion to him was to lower his pH from 8 to 7.4 and his alkalinity from 80 to 60. He uses bromine. Will look at the calculator you mention for langlier. Doesn't temp have a major influence on the scaling indices? Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

    My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.

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    Default Re: Saturation Index (Langelier, etc.)

    To clarify futher - a pool heater is often gas fired into a heat exchanger. Differences in the tube or exchanger to water temp would most likely be in the 30F area as Ben suggests depending on the water flow rate over the heat exchanger. In an electric water heater, the heater is essentially a calrod like an electric stove and some have a sheath over the element made of S.S. Looking at a watlow info sheet on one of their heaters, the surface temp difference to the fluid can be as high as 100-200F. A very minor build up on the element sheath (same with a gas fired unit) can reduce transfer efficiency and cause higher gas or electric usage. For electric heaters, the buildup can be so severe as to cause the element to overheat and burn out. Since a pool heater exchanger material can be materials other than S.S. that do not lend themselves to being isolated and chemically cleaned with acid, it would seem to me to be important to keep the exchanger or elements clean - energy will only get more expensive!

    Below is a question about just such an issue:
    "Thank you for your replies. I am in New Zealand and investigating why we cannot get domestic water heater elements that last.We can get 304 s/s and Incoly sheathed elements (at a price) but as they are machine made the element leg diameter is rather small to give a good watts/sq.in.ratio. Corrosion of the sheath and/or resistance wire failure occures prematurely because of lime scale buildup which raises the surface and internal temp.of the leg.I also believe that current element design of two or more element legs in parallel causes a hot spot between the legs which encourages the deposit of lime in this area. It soon builds up into a solid mass and
    increases corrosion in this area.I am looking at a design for possible manufacture using a single tube of a material which will at least help solve the corrosion problem."

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    Default Re: Saturation Index (Langelier, etc.)

    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.

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    Default Re: Saturation Index (Langelier, etc.)

    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

  6. #26
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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by DaveD View Post
    To clarify, my Dad's issue is with a fiberglass hot tub.
    :
    Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

    My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.
    So with the fiberglass hot tub you don't need to worry about corrosion or pitting of plaster/gunite surfaces (unless you've got tile grout around a top edge or something like that). So you can tend to run your water in a "corrosive" state with regard to the normal indices. Just don't have it be too low in pH. And yes, having a SS heater means you are probably even less likely to corrode than copper (iron would be worse, of course).

    The problem, of course, is that your father's source water is so high in hardness to begin with. We've talked about this issue of high hardness source water in other threads and pretty much come to the conclusion that the most effective way to deal with this is to use a water softener (the type where water flows through an ion exchange resin or similar device) which will essentially exchange the calcium ions with sodium (or potassium).

    So assuming the source water is high in calcium hardness but isn't very high in TA (otherwise it would be very cloudy and percipitating calcium carbonate unless it's pH was also high), you could just run the hot tub water on the low side of TA (about 50) to compensate for the high CH. The main downside with the lower TA will be that the pH will be harder to control. Also, even with a TA of 50 and a CH of 1000, at hot tub temperatures you will still be in a scaling state though much less so. You could use some Borax (compensated with acid) for a borate buffer (see this thread for more info on this), but such a buffer tends to help prevent a rise in pH and isn't as good in preventing a drop in pH. All in all, I think getting the CH lowered from the source water is the way to go and using the water softener will let you do that.

    As for temperature, yes you are right that the scaling is a function of temperature so when a heater is used you want to take that into account. In practice, the heater isn't on all of the time so while some tendency towards scaling might occur while heating, it may have a tendency to dissolve such scale when not heating. Ben tends to run his pools slightly on the corrosive side for this reason (and possibly others) but in your specific case you can safely run more on the corrosive side without harm because the hot tub is fiberglass and has no plaster/gunite/grout.

    Your advice of running at lower pH (still above 7.0, however) and lower TA is good. It's just that with a hot tub it is much harder to control pH to begin with so with the lower TA and lower pH operating conditions it's trickier to prevent going too low. Fortunately, the bubbling action in a hot tub would tend to outgass CO2 and make the pH rise (though less so at lower pH and lower TA), but any chemical additions (including bromine) could affect the pH. You'll have to experiment to find that optimum balance of non-scaling, stability and control.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by DaveD View Post
    Since a pool heater exchanger material can be materials other than S.S. that do not lend themselves to being isolated and chemically cleaned with acid, it would seem to me to be important to keep the exchanger or elements clean - energy will only get more expensive!
    I agree, but the recommendations near a saturation level of calcium carbonate don't generally lead to scaling. The difference in index for a 30 degree rise in temperature is about 0.24 whereas scaling is usually seen only for much higher indices around 0.7 to 1.0 and higher (though Taylor and others talk about +/- 0.3 being a target range and others say +/-0.5). So when using a heater, yes it makes sense to keep the water a little more on the corrosive size of the "ideal" balance. That way, any scaling that occurs during heating could dissolve when cool. I believe the net feeling on this forum is that way too much attention and worry is paid to the indices -- not that they are useless, but rather that you have to be way out of whack before you get into trouble. Unfortunately, your father's situation had an index over +1.0 (pH 8.0, TA 80, CH 1000, Temp 104+) and that is into the much more clear area of scaling. Your lowering of the pH to 7.4 and the TA to 60 gets the index down to around +0.3 which still has a tendency to scale, but only somewhat. It probably won't add much to existing scale, but it won't dissolve such scale either.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by DaveD View Post
    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

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by DaveD View Post
    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
    Last edited by chem geek; 11-20-2006 at 02:41 PM.

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