I BBBed my hot tub , but it won't stay

[chem geek]

Personally, and especially in a hot tub, I would use the smallest amount of CYA you can get away with that will give you sufficient chlorine buffer and protection from sunlight (UV) so that you won't run out before adding more while not being so much that it cuts down chlorine's effectiveness too much. That probably means 30 or less, which makes it hard to measure (unfortunately). The main difference between the hot tub and the pool is that the bug that causes "hot tub itch" thrives in the hotter temperatures and is a much harder to kill bug than E.coli which is a moderate-to-kill bug (most others are easy-to-kill). The standard set for pool disinfection (650 mV) kills easy bugs in less than a second and E.coli in seconds to a few minutes (depending on whose data I look at), but the Pseudomonas are a hard-to-kill bug.

I don't want this thrown into The China Shop so instead I refer you to this post where I presented my results a while back. The bottom line is that CYA significantly reduces chlorine's effectiveness because it binds up the chlorine into a form (technically one of several combinations of CYA plus chlorine called chlorinated isocyanurates) that while significantly protected from breakdown from sunlight (UV), has minimal disinfecting and oxidizing ability. Think of it like this: CYA converts chlorine into a form that is better protected from sunlight, but is not "active" for disinfection. It is like being in reserve and can be converted to the active form as the active form gets used up, but it is not itself an active form. Chlorine "effectiveness" depends on how much chlorine is in the active form and doesn't matter on how much is in "reserve".

At a pH of 7.5, CYA reduces chlorine's effectiveness by a factor that is 75% of the ppm of CYA. So, 30 ppm CYA reduces chlorine's effectiveness by a factor of 0.75 * 30 = 22.5 so that chlorine in 30 ppm CYA is only 1/22nd as powerful as with no CYA. This is because there is only 1/22nd the amount of the disinfecting and oxidizing form of chlorine (HOCl) present since most of it is bound up with CYA and not effective. This rule of thumb only applies when the amount of CYA is much higher (about 3 times or more) than the chlorine level. Here is a set of graphs that shows how little disinfecting chlorine there is with 30 ppm CYA. Let me know how this can be presented in a way that doesn't scare people off. It's important information to know, but needs to be made less technical.

Richard