Yep - you’re right. I trapped myself by thinking that FC was only active chlorine. Two questions that I have. One is what is the mechanism that lowers active chlorine as CYA ppm increases. Another is a practical one. I see the recommended FC/CYA ratio increases with an SWG. Is this still the case with a hot tub? I have almost no sunlight to deal with, my UV and ozone are off, and I’ve been running about 30 ppm of CYA. Tnx.
CYA not considered in Pool Math?
- Thread starter DougJack
- Start date
- Jun 24, 2021
- 15,880
- Pool Size
- 29000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- CircuPool RJ-60 Plus
Let's use HOCl instead of "active" chlorine to avoid confusion. At the same FC level, adding CYA will bind more of the FC. See red arrow:
The assumption being made with SWG is that the SWG is continually making chlorine, and the FC is fairly flat throughout the day...matching production to demand.
With liquid chlorine, you typically dose once a day. The higher FC target for LC is to consider that the FC may end up lower after a day without any chlorine being added. The higher target allows that daily daily demand/fluctuation.
For now, use the liquid chlorine chart for the hot tub without SWG. If you find your daily demand is low, you may be able to use the SWG chart. In either case, never get close to minimums. If you have to dose HIGHER than target, so you are WITHIN target range when you test, you should do that.
Last edited:
- Sep 23, 2015
- 2,012
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Astral Viron V25
Yes, I would be happy to go with that although it should be noted that it is HOCl that is released from the chlorinated cyanurates but the chlorinated cyanurates are not that “active”.
I’ve been thinking about and thought the same as @PoolStored from above. I’ve never really been interested in spars but I believe there is a need the shock or slam a spa after use, particularly in the event of heavy use. This management strategy would be easier with a CyA in the lower scale
Last edited:
- Jun 24, 2021
- 15,880
- Pool Size
- 29000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- CircuPool RJ-60 Plus
I fixed it, was just typing fast.
The reason I made the distinction is we are throwing around terms like "Free Available Chlorine," "Active Chlorine," and "Free Active Chlorine," without distinction/definition. HOCl clarifies the point.
I understand the whole concept now - thanks. Just for closure, I started down this path because I was having to use a lot more liquid chlorine than Pool Math suggested, not knowing that liquid chlorine has a pretty short shelf life. I just finished going through the measurement process with the jug I was using. I did dilutions to measure at a target of 5 ppm if the 12.5% jug rating was currently accurate. With several measurements it was between 2 and 2.5. Pretty decent closure and I learned a lot a lot more deeply if I hadn't gone through this process. Thanks all!
Besides the weak bleach, the rest of the problem is that it takes a while to use the gallon of bleach in a hot tub. I get 2.9 FC from a jug of 10% in my pool. By the time i fugure out a jug is a dud, im already onto the next bottle. 
Either double dose it until you use it up, or delegate that bottle for cleaning patio furniture (etc). Each spring i use last years leftover gallons to get the winter mildew off my vinyl siding. If I still have some left, it gets donated to the laundry room where it's basically clorox strength at that point.
Either double dose it until you use it up, or delegate that bottle for cleaning patio furniture (etc). Each spring i use last years leftover gallons to get the winter mildew off my vinyl siding. If I still have some left, it gets donated to the laundry room where it's basically clorox strength at that point.
I understand! I switched to the SWG and now I rarely if ever use it. Patio furniture is a good idea - and cleaning cutting boards.