Thanks, Jim. I think I'm just surprised by the much faster decay rate. Besides my wife and I, we have a 11 and 13 year old using it randomly, the FC hasn't been too predictable for me yet like I'm used to with pool measurements.
It's a small volume and a lot of waste, so it's a lot different then a pool. For example, your pool is 8600 gallons. Assuming it has the same FC level as the spa, there is 33 times more available chlorine in the pool for breaking down waste. On top of this, a pool is usually much cooler, whereas in a spa you are usually sweating to some extend, and the sweat is waste that needs to be broken down. So this double whammy of much less available chlorine and way more waste means that the FC levels drop
very quickly with usage, whereas the pool will be a lot more stable.
My experience is even if the FC level is at the mid to upper end of target for our CYA, and my wife and I soak for an hour or so, by the next day I may have 0 FC left in the spa. What I do, and what I think nearly all spas need, is to add some amount of chlorine immediately on leaving the spa, so that the residual bather waste can be broken down without crashing the FC down to 0.
Depending on how long we soak and what the FC level was before we got in, I may add 2-4 oz of 8.25% bleach immediately after getting out of the spa. Usually that is closer to 4 oz. My personal goal is to add enough on getting out that when I test the next day I'm at or above the minimum FC level. After some trial and error you should find an appropriate amount of chlorine to add on getting out of the spa based on your particular soak lengths and temps.
I would suggest the following, based on my experience:
- Test FC every time before entering spa, ensure it is at least at the minimum end of the target range before getting in.
- If you are going to soak for an especially long period of time (like 1+ hours), you may want this at the mid to upper target range before getting in.
- Get in, enjoy your soak.
- On getting out, add X oz of bleach based on soak length and experience.
- Test FC the next day. If it is below the minimum, boost it up to the "SLAM" level for your CYA, and add another oz or two to the amount you added in Step #3 the next time you use the spa.
After a couple of soak cycles, you should get a feel for how much to add so that the next day the FC is at or above the minimum level.
Hope this helps!
P.S. Some bather waste takes time to break down, as much as a day or two, maybe even three days after the last use. While the FC will drop rapidly for the first half a day to a day after use, this drop should level off with time. Within a few days of no usage, assuming the FC level is maintained in the target range, you should a very low drop in FC usage, like a couple ppm/day or so, assuming no UV/ozone systems since those attack and destroy FC.