SWCG with bromine?

[DavidLast]

Keeping the pH in the 7s is exactly what I'm concerned with. While also not running out of TA. And yeah, PoolMath does all those calculations automatically and it says I need to keep my TA above 50. But if I do that, my pH climbs even when I'm not using the jets. And when I said chlorine is acidic, I'm not talking about any particular product, I meant the free chlorine species that does the sanitizing, hydrochlorous acid. If I don't run the jets at all, just the circ pump for a week or so, and the salt cell is set too high so the chlorine level get too high, the water pH will fall, just from the increased chlorine concentration. Also, you said CYA helps keep chlorine effective even at higher pH levels, right? But I find this a bit confusing, because doesn't CYA also make chlorine less effective in general? So my FC levels have to be higher in general in order to keep things clean with more CYA, just like with higher pH. It seems that adding CYA is intended to buffer the water, so you trade some reactivity with having a larger reservoir of potentially available sanitizer stored in the water, right? So the levels don't move around as quickly? That's why higher pH isn't as big of a deal? Because the chlorine bank is bigger than what is immediately necessary by a healthy margin, so even at only 25% effective, it's still plenty.
And that works fine, I'm not saying that I can't keep the TA vs pH balls in the air, I'm just wondering if you might get an even more buffered and less pH sensitive behavior with a bromine sanitized tub. If CSI in the positive range is when you need to worry about scale formation, and that is heavily driven by CH, I'm just wondering if I can take advantage of my relatively soft tap water with a bromine tub and use lower sanitizer levels over a broader range of pH which could have the overall effect of reducing corrosion. It seems like that might be true, but I'm not enough of a chemist to figure it out for sure.

Cyanuric acid complex holds the chlorine within its matrix releasing a very small amount on demand. In other words as the chlorine level drops the cyanuric releases more from itself. The release amount is very small and will only add confusion I think.

I think overall the conflict is aeration driven. I think what may be happening is the TA is too low when not using aeration and consequently over a week the pH is seen to fall. However, when the aeration is activated, it is a considerable amount for the volume, leading to a pH climb., suggesting the TA is too high. I think there is a test you can do to prove this. Run the spa without jets on circulation and record the pH and TA as the pH falls. Then run the spa in the normal way with aeration etc for say 15 minutes and run the same tests and see if the TA moves relative to pH. Be interesting to see the result.

 

[mgtfp]

Also, you said CYA helps keep chlorine effective even at higher pH levels, right? But I find this a bit confusing, because doesn't CYA also make chlorine less effective in general? So my FC levels have to be higher in general in order to keep things clean with more CYA, just like with higher pH.

Most of the chlorine is bound to CYA, the vast majority, in fact. The bound chlorine is protected from UV, but is not effective as a sanitizer. This needs to be compensated by increasing the FC level according to the recommended FC/CYA Levels.

Unfortunately, the bound chlorine still shows up as FC, even though it's not really "free" anymore, it's just "free" in the sense of not being "combined" with nitrogen based organic compounds.

To give you a feeling about the amount of chlorine that's bound to CYA, here a few numbers: SLAM level FC, which is 40% of the CYA level, for example FC 20ppm for CYA 50ppm, is in terms of the concentration of the sanitizing chlorine species HOCl equivalent to FC 0.64ppm with no CYA (at pH 7.5). In both cases you have 0.32ppm of HOCl and 0.32ppm of OCl^-, but with CYA you also have 19.36ppm of chlorine chemically bound to CYA, which is a nice UV-protected chlorine reservoir.

Apart from protecting FC from UV, CYA also flattens the pH-dependency of the HOCl concentration. Without CYA, [HOCl] decreases by 50% from pH 7.5 to 8.0. With CYA, this gets reduced to only 15%.

So, yes, FC needs to get adjusted to the CYA level to account for the amount of FC that's bound to CYA, but then you have the benefit of a more or less negligible pH-dependency in the relevant pH-range.