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What is the highest level of FC that is still safe for entering the tub?
According to the spa world, 5ppm. However, if you do not have sensitive skin, you can soak higher. The highest I've taken a plunge in is 10ppm with no irratation at all. I try not to soak higher than the suggested ppm as much as possible but in some cases I just couldn't wait.
I'm assuming that the EZ spa stuff is an enhanced shock from the sounds of things. The reason for the sky high FC readings is b/c of the mps in it. Sounds like a pain in the butt for daily post soak dosing as you would never be able to tell what you are bringing your FC to. I'd switch to a straight dichlor and a buffered mps for shocking purposes. It's much more user friendly.
EZ Spa is an oxidizer that somehow locks chlorine up for a week at a time. It also stabilizes pH so there must be buffers in it along with clarifiers. There is a dichlor that EZ Spa calls Boost that you also add once per week. Info is hard to come by on their site so it is hard to tell how they "lock in" chlorine but I don't think there is any magic here. As with anything else, if you are paying a premium for a product you have to decide on the value.
Thanks for the info Tony. Is there anything you don't know about the spa industry? 8-)