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Thread: Downsides to salt pools

  1. #51
    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: Downsides to salt pools

    A related topic to the post above is "what is the conductivity of an SWG salt pool compared to a non-salt pool?" The conductivity of 3000 ppm salt water (at 25C or 77F) is 5.7 milliSiemens per centimeter. This conductivity is approximately linear with the amount of salt so an SWG pool is approximately 3-6 times as conductive as a non-salt pool (assuming typical 500-1000 ppm TDS for a non-salt pool).

    In terms of what this conductivity means for resistance, that depends on the area of the metal that is in the water and the distance between them. The formula is C=G*(L/A) where C is specific conductivity in milliSiemens per centimeter, G is conductivity which is the inverse of resistance (R), L is the distance between the plates and A is the plate area. So, G=C*(A/L) and R/L = 1/(G*L) = 1/(C*A), but converting to Ohms (from the inverse of milliSiemens) gives R/L (ohms per centimeter) = 1000/(C*A) = 1000/(5.7*1) = 175 Ohms per centimeter.

    For 1 meter square area plates, we have 1000/(5.7*100*100) = 0.0175 Ohms per centimter or 1.75 Ohms per meter. So clearly the area of the exposed metal is a critical factor.

    The area of a light ring or a handrail (for the portion underwater) is probably around 200 square inches or 1300 square centimeters so that becomes 1000/(5.7*1300) = 0.135 Ohms per centimeter or 4.1 Ohms per foot. For comparison, a non-salt pool has a resistance (for the same amount of exposed metal) of around [EDIT] 12 to 24 [END-EDIT] Ohms per foot. In other words, an SWG salt-pool is more conductive (has lower resistance), but even a standard non-salt pool still conducts electricity quite a bit.

    One factor to consider for which I have no detailed information (yet) is how water flow affects conductivity. I would think that there would be rather high resistance to net ion charge current flowing "upstream" of water flow and rather low resistance to that current going with the water flow. So actual resistance may be lower than just calculated when taking into account water flow. If this is true, then metal objects closer to the "return-to-skimmer" and "return-to-drain" flows would be more likely to corrode. However, there are also differences in chemistry in such water from the SWG through the returns since the water is higher in chlorine level and is higher in pH (is more alkaline) so is therefore more corrosive to metal (due to higher chlorine) and may precipitate hydroxides of metal ions that may be in the water, but this is really no different in principle, and in fact is much milder, than what occurs if one adds bleach or chlorinating liquid to the pool.

    One thing that doesn't make a lot of sense is the graph shown in this post where it shows that the relative corrosion rate of steel in milligrams per square decimeter went up a very small amount (not even doubling) with even factors of 10 increases in salinity. Apparently, "normal" corrosion is not nearly dependent on the conductivity of the water and probably occurs directly as a chemical reaction rather than electrolytically so is probably most dependent on the concentration of the oxidizer (oxygen in air, dissolved oxygen in water, or disinfecting chlorine). Either that, or some localized corrosion has greater localized conductivity possibly due to some corrosion products. However, if there is a potential difference over a greater distance as with galvanic corrosion or electrolytic corrosion, then corrosion should be more directly and proportionately related (approximately linearly, in the range of TDS found in pools) to the salinity and specifically the conductivity of the water.

    I received some confirmation regarding the SWG electronics and what I said was essentially correct in terms of a transformer (to lower AC voltage) followed by a rectifier bridge (to convert AC to DC) plus some other circuitry designed to ensure isolation of the AC voltages. The bonding wire is a secondary safety ground protection so does not connect to the plate wires. So if there were a voltage/current leak as I described in the previous post, then it would not be intentional (i.e. it's not a faulty design).

    Another possibility again related to any voltage/current electrical leakage is if this leak went to "ground" which can be assumed to be a "sink" or "source" for electrons. The following shows what would happen in this case:
    Code:
                                                                                  < e-
                             < e-                               < e-      .........................
                  ______________________ DC Voltage from _________________|__                     :
               e- |                      Transformer/Diodes                 | ^                   :
               v  |                                                         | e-                  :
                  |                                                         |                     :
    Negative Plate releasing electrons                        Positive Plate absorbing electrons  :
    2H2O + 2e- --> H2(g) + 2OH-                               Cl- --> HOCl + H+ + 2e-             :
                                                                                                  :
                                        LESS         LESS           LESS LESS                     : ^
                 Cl-, OH- >              H+ + OH- --> H2O         < HOCl, H+                      : e-
                                                                                                  :
                                                                                                  :
                                           OH-                                                 Ground
                                            v
    In this case, notice that though there isn't direct electrolytic corrosion, there is a buildup of charge (in this example it's negative charge, but it could instead be positive charge if the opposite plate were connected to ground instead). Essentially, the pool acts like a capacitor. A voltage would be measured from the pool to the ground and to any metal which may be bonded. However, if the bonding wire is not fully connected to ground (moist soil), then the charge in the pool can build up. If metal in the pool were bonded and such bond were connected to a good ground, then the charge in the pool would likely be discharged and would not build up (though this depends on the rate of the charge buildup relative to the discharge rate).

    In the above example, the voltages are not usually sufficient to cause electrolytic corrosion and there is not an electron path to the pool metal, but if the bonding wire were connected to ground, then the Ground-to-Ground would be the electron path and corrosion would be possible just as with the example where a direct wire was connected (though the resistance through ground is clearly much higher than the direct wire example). Even with lower voltages, this still lowers the activation energy for corrosion so could still speed it up, but not as much as with electrolysis (remember that the oxidation potential of chlorine far exceeds what is needed to corrode iron so it is not a question of "if", but of "how quickly").

    Richard
    Last edited by chem geek; 03-31-2007 at 09:30 PM.

  2. #52
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    Default Re: High Recommended CYA Levels

    Quote Originally Posted by Gunslinger View Post
    Richard, your explanations of SWG chemistry are truly awesome, but I don't believe for NY second that the reason for the manufacturers' 70-80ppm CYA recommendation (I've actually seen 80-100ppm recommended) has anything to do with real-world chemistry. Rather, I suspect it has more to do with a slightly different interpretation of the acronym "CYA", rooted in the early days of the SWG evolution when reliable operation was not up to par with current devices. In a loss-of-power event, for example, high CYA levels would help to ensure that all your residual FC doesn't disappear on a sunny day before you've noticed something is amiss. I believe there is no longer any rationale justification for this recommendation other than my suspicion that the manufacturers have no interest in pursuing the matter and are content to leave it unchanged.
    Personally, I barely manage to keep my CYA level above 25ppm, and have experienced no consequences from that during the four seasons I have operated my Pool Pilot SWG -- although I do test my FC and Ph quite often.
    I want to apologize to you (and others) for very likely being wrong about the higher CYA improving SWG cell efficiency. The experiments Mark describes in the later posts in this thread have pretty much confirmed that SWG efficiency is not affected by CYA level (at least not from 45 to 80). There is some effect on efficiency from pH.

    I incorrectly assumed that the industry charts showing the half-life of chlorine vs. CYA level were correct and based on experiments, but apparently they are wrong and based on incorrect assumptions or too few data points. Mark's tests plus feedback from other users shows that higher CYA levels protect chlorine by more than accounted for by simply forming more stable compounds. The CYA itself blocks some of the UV and apparently does so over rather shallow depths. I'm reworking the theoretical model to account for this new data in this post.

    This is what science is all about. Finding real-world data and developing a model that explains it or changing (or expanding or replacing) an older model to account for this data. I'm sorry I didn't catch that or question the industry data earlier. Maybe now we will be able to make rational recommendations on CYA levels, including using higher CYA levels in very sunny areas, though keeping correspondingly higher FC levels, and no longer thinking that this is less efficient (i.e. uses more chlorine) because we now know that it probably doesn't.

    Richard
    Last edited by chem geek; 06-20-2007 at 09:31 PM.

  3. #53
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    Default Re: Downsides to salt pools

    well, something is going on. I have seen in my own pool and my customer's pools a definite increase in SWG efficiency when the CYA is at the higher level of the recommended range based on cell output percentage and FC levels for a given pump run time. In my own pool, for exampe, when my CYA falls to about 50 ppm (I get a lot of spashout when I run my water features and have a small pool) I have to turn my generator output up to about 30% but when I get the CYA back up to around 70-80 I run the output at about 8-15% to maintain the same FC level of 4 ppm. I have seen similar effects in customer's pools with some of them having to run their cells at close to 100 % when the CYA was in the 40 ppm range and once the level was increased to the higher end the cell was running below 50% to maintain the FC. I still think there is something more than just chlorine retention going on here since the efficiency of the systems seem to rise greatly as the CYA level is increased. Also, I find it interesting that some of the higher salt system like PoolX recommend higher CYA levels (80-100 ppm) along with higher salt levels . Not really sure why.
    Retired pool store and commercial pool maintenance guy.

  4. #54
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    Default Re: Downsides to salt pools

    If the ability to lower the SWG output at higher CYA levels is during the day when there is sunlight, then the CYA protection mechanism would account for that. It's not necessary for the SWG to be more efficient -- if chlorine usage dropped due to more protection from CYA (even at higher FC levels) so less breakdown from the sun, then the SWG output can be turned down.

    The reason I originally didn't think that CYA protection was the reason was that I trusted the industry chlorine half-life vs. CYA graph which did not account for the significant lowering of SWG on-time one can have at higher CYA levels. However, if this graph is wrong and CYA protection dramatically increases at higher levels, then that will result in the same ability to lower SWG output (level or time).

    You can do a similar experiment as Mark did by running the SWG at night and seeing the rate of increase in the FC level at different CYA levels -- essentially just repeat what you are seeing during the day, but do it at night instead (or during the day with an opaque cover that protects the pool from sunlight). If you find that the rate of FC increase doesn't change with CYA level, then there is no change in SWG efficiency with CYA and the reason for the ability to lower the SWG level is just due to a lower rate of chlorine usage. Definitely let us know if you come up with different results as that would indicate a more complex scenario -- possibly a dependency on specific model of SWG.

    Richard
    Last edited by chem geek; 06-21-2007 at 01:10 AM.

  5. #55
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    Thumbs up Re: Downsides to salt pools

    Didn't somebody give a little rant last year about the need for ~2x the cya for a salt pool ? I have been adding a lb or 2 extra cya when dosing the pools this year to get the cya to ~ 45- 60 ppm(I used to tie the cya to salt @ 1 lb : 100 lbs to get to 30-40ppm). Though I ranted about the extra cya last year, it hasn't caused any problems so far and I know our customers, they will add more salt when needed, but space the cya - so I've got them covered for a couple salt additions.

    Unscientific notes from the feild:
    I've been looking at all the salt pools I've opened this year and have seen NO sign of salt having degraded the coping or deck! The imprperly made 'stainless' screws are still rusting out (I've also seen it in some non-saline pools ) and the non stainless bolts on slides and diving boards are getting trashed. Although our oldest salt pool is only 4 seasons old, the aluminum coping and concrete decking seem to be holding up fine (of course there's still the whole 'try to get the stainless rails out of the aluminum cups' in the fall issue... )

    Take care all, and thanks for being here - Ted
    Luv & Luk, Ted

    Having done construction and service for 4 pool companies in 4 states starting in 1988, what I know about pools could fill a couple of books - what I don't know could fill libraries

  6. #56
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    Default Re: Downsides to salt pools

    Ted,

    Thanks for the update. I think we pretty much concluded a while ago that higher CYA levels in SWG pools were beneficial, allowing for lowering SWG on-time and having a lower rise in pH as a result (due to less aeration from the hydrogen gas bubbles). The recent discussion is as to WHY this was working. I had earlier come up with a theory about SWG efficiency improving, but it now is seeming like that was wrong and that instead the CYA is protecting chlorine more than predicted by the industry graph that seemed based on species concentration alone and not on additional "shielding" (absorption) from the CYA molecule itself.

    That's good news about the coping and deck being in good shape. What kind of coping and deck have you installed? Any limestone or is it all concrete or what? I believe that most of the complaints by the source in Texas (in my first post -- the blogger) were with limestone so maybe they are using particularly poor quality limestone in that area and maybe it would degrade anyway though maybe faster with higher salt levels. Also, be sure and check out the pool equipment such as the pool sweep and any metal parts in them. That seems to be another source of problem, but it would be good to see if there is a difference between salt and non-salt pools.

    As I've said before, most people love their SWG pools and find no problems with them. It would just be good to know exactly when the problems do occur and why and what can be done to mitigate them. About the only certainty is that using an SWG in a pool without CYA, such as an indoor pool, is a huge mistake. But that's somewhat predictable even from the SWG chlorine/salt study that showed high chlorine levels (with no CYA) causing corrosion. And even a non-SWG pool without CYA is far more corrosive so really the solution is to use some CYA in indoor pools, regardless of the presence of an SWG.

    Richard

  7. #57
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    Default Re: Downsides to salt pools

    This is our first season with a SWCG (QuikChlor) and I haven't noticed any corrosion or damage to our Shasta Deck (similar to Kool deck). The only metal present in our pool is the light bezel, and I've seen no evidence of corrosion there either.

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