Downsides to salt pools

[chem geek]

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

[waste]

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

[chem geek]

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