Downsides to salt pools

[Waterworks]

I don't think that Stainless Steel pools actually do 'exist' per se. As far as I know one of his companies builds SS tanks for agriculture or something and he had them build him this one as a one-of-a-kind thing.
He uses an off-line chlorinator, so the floating thing won't be a problem.

Brad

[chem geek]

Brad,

OK, so with an off-line chlorinator he won't have the problem of the floating dispenser, but given this guy's history of not monitoring his water chemistry (which may have gotten the chlorine to a high level) he could now be in the position of getting his pH very low in his pool if he uses Trichlor exclusively and doesn't check the pH once and a while and add pH Up (Sodium Carbonate or Washing Soda). If the pH gets too low, then he can end up corroding his stainless steel again. Given his history, it would be a good idea to keep the TA on the high side, at least 120 ppm, as long as he's using Trichlor. Also keep in mind that with the CYA from the Trichlor he will have to maintain higher Free Chlorine levels as the CYA level rises in order to have proper disinfection and to prevent algae. See Ben's Best Guess CYA chart as a rough guide for how much FC is needed at various levels of CYA.

By the way, if you can get contact info for the metallurgist, then you can give that to me (via E-mail or Private Message) and I'll contact him to see where he got his 500 ppm salt limit for this type of stainless steel.

Thanks,
Richard

[Poolsean]

There's a company in North Carolina, Bradford Industries, that makes these Stainless Steel pools and spas and cost into the Tens of thousands of dollars! Most of the ones they display and are on their brochures show most of the pool/spa being tiled. However they do have some very large custom built ALL Stainless Steel commercial projects.

Very unusual looking with all straight sides. We have one of these all stainless steel spas in Boca Raton Florida at what was formerly the Mission Bay Aquatic Complex. This is a non-salt chlorinated spa. They use sodium hypochlorite from a Stenner Chemical Feed Pump. Most of the corrosion appear at the welds.

Just the fact that this spa was already showing signs of corrosion was reason enough for me to not propose a salt system on it. I'm sure salt would have been blamed for any more damage.

[Waterworks]

This guy also has four pure copper spas from Diamond Spas at the same residence. AFAIK these spas are similar to Bradford spas' units. There is some tile and some copper exposed. He uses inline chlorinators for these units and has not complained at all about corrosion.

Brad

[chem geek]

That's interesting about the copper not showing signs of corrosion while the stainless steel did. I wonder if the chlorine levels are better maintained in his spa or if copper is more resistent to any potentially salt-amplified chlorine corrosion. Copper is more resistant than steel so perhaps even though chloride ion might interfere with any protective oxide layer that gets formed, that once removed the underlying steel in stainless steel corrodes faster than the underlying copper under any thin copper oxide protective layer (if any). This is all speculation on my part.

On a separate topic, I ran into this article about the damage to plaster from high levels of CYA. What is interesting is that the drop in CYA over time seems to indicate that even levels of CYA not much lower than 100 ppm can be detrimental to plaster. If you look at the line that starts with 200 ppm, it drops rather quickly down to around 130 ppm and then continues to drop more slowly to around 80-90 ppm. This may indicate that there is some plaster deterioration effect even at 80 ppm though it is clearly much slower. This would be another reason for SWG manufacturers to take a look at seeing if they can't figure out a way to operate more efficiently at lower CYA levels.

Richard

[Poolsean]

Richard,

It's not a matter of a salt system working efficiently at lower cya levels. Cya levels have nothing to do with efficiency. It has to do with retaining the chlorine in the water. Try the same with a bleach pool and low cya. The chlorine is quickly consumed by UV. Same thing with the chlorine produced by a salt system. UV.
We know, from trial and error more than anything, that the 60 -80 ppm range works. Since ORP controllers are affected by cya, 30 - 50 ppm is recommended.
But cya at lower than 25 ppm? You will drastically see an increase consumption of chlorine, salt generator or bleach, it doesn't matter the source.

[chem geek]

Sean,

If the SWG manufacturer recommendation for CYA is solely based on retaining chlorine in the water (which is independent of whether you use an SWG or use another source of chlorine), then why did Evan (waterbear) and others see such a difference in efficiency going from 60 to 80 ppm? Yes, more CYA has the chlorine last longer, but it has diminishing returns and the difference between 60 and 80 isn't that high yet he (and others) saw a large difference -- that is, he was able to turn down his SWG a large amount.

The half-life of chlorine is determined by the separate half-lives of unbound chlorine (hypochlorous acid and hypochlorite ion) with a half-life of around 35 minutes AND the half-life of bound chlorine to CYA (i.e. the chlorinated isocyanurates) with a half-life of around 8.4 hours (some sources say 6 hours). The net result is shown in this chart which shows that the bulk of the benefit from CYA occurs at relatively low levels of CYA. This chart is similar to and consistent with the infamous Kent Williams "Cyanurics - Benefactor or Bomb?" article. The half-life of chlorine is about 6 hours at 30 ppm CYA while at 80 ppm this only increases to a little over 7 hours. That's not a huge difference. The reason for the small change is that the vast majority of chlorine, even at 30 ppm CYA, is stored in the form of chlorinated isocyanurates (that is, bound to CYA) so adding more CYA only cuts down the remaining free, unbound and disinfecting chlorine that is already at such a small level that it doesn't contribute much to the total loss. At 30 ppm CYA the disinfecting chlorine level is at 1/30th the level it would be if there was no CYA or put another way, almost 97% of the chlorine is bound to CYA and only 3% is unbound and getting cut in half every half-hour. At 80 ppm CYA, almost 99% is bound to CYA and only 1% is unbound. Though 1% is certainly much smaller than 3%, it is still a small number in an absolute sense -- it's what happens to the 97% or 99% that drives the total chlorine loss more than anything else.

It was interesting that the rate constants for CYA combining with hypochlorous acid turned out to be about the same amount of time that water flows through an SWG cell when the CYA level was somewhere in the neighborhood of 50-60 ppm or so. That's what had me think that this was why the CYA recommendation was higher along with what Evan (waterbear) and others were seeing in terms of efficiency. Also, it seemed quite strange that most non-SWG CYA recommendations are 30-50 ppm while only some of the SWG manufacturers were saying to have it at 70-80 ppm with "80 ppm being ideal". Don't you find that strange?

Richard

[mas985]

Last year I did an experiment with my pool and SWG and got different results than what others have experienced. I started out at 30 ppm of CYA and gradually increased it to 60 ppm and saw no increase in production of chlorine at all. My residual chlorine started out at 1 ppm and ended up at 1 ppm with the same pump run time and SWG setting. I know this is at odds with what others have experienced but I was fairly careful to do this during the spring and space out the CYA addition since it can take up to a week to fully dissolve.

Since you can only measure the chlorine residual which is a combination of SWG production and CYA retention, you cannot be sure that the increase in chlorine residual that some people experience is due to higher production. It could be that the UV exposure is high enough that they are simply increasing the retention of chlorine and thereby increasing the residual.


One thing I have noticed is a huge difference in production with water temp. This fall when the water temp dropped to 60 degrees I noticed that my production almost doubled. This is probably a combination of water temp and UV exposure but this seemed to have a much larger impact than CYA level did.

Since I have very high CH fill water, I am planning a refill soon while the CH is at it's lowest. This gives me another opportunity to repeat the same experiment and see if I get different results this time.