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

[RavenNS]

Okay, let's see if I "get" this...
suppose I have cya 30 & I'd like my disinfectant to be at 6ppm...
I then add enough to make it 7.5ppm ...
(based on it being aprox. 25% less effective... yes?)

[chem geek]

Okay, let's see if I "get" this...
suppose I have cya 30 & I'd like my disinfectant to be at 6ppm...
I then add enough to make it 7.5ppm ...
(based on it being aprox. 25% less effective... yes?)

No, it's much worse that this. A CYA of 30 makes the FC chlorine amount 0.75*30=22.5 TIMES less effective. If you want your disinfectant (HOCl) to be at 6 ppm (which is WAY too high), you would need 22.5 * 6 = 135 ppm FC! The reality is that you need very, very little actual disinfecting chlorine to kill bugs and to oxidize simple organics (ammonia, urea, etc.). The "min/max" ranges in Ben's chart roughly correspond to disinfecting chlorine (HOCl) levels of 0.03 to 0.07 ppm. And yes, this means that the equivalent without CYA would mean maintaining FC levels of around 0.03 to 0.07 ppm, but obviously this is nearly impossible to do since it is so small and gets used up easily so instead you put in extra FC "in reserve", typically at least 1 ppm or 2 ppm. The "min/max" numbers in Ben's chart are at various CYA levels and are roughly the amount of FC needed to get to the 0.03 to 0.07 ppm disinfecting chlorine (HOCl) level. He figured all of this out through experience so his table doesn't exactly correspond, but it's pretty close (though the "shock" column doesn't correspond very well).

The German standard for chlorination is for 0.2 ppm chlorine, which I believe is without CYA (it had better be without!). Historically, before CYA was in widespread use, very low chlorine levels of 0.2 or 0.5 ppm and certainly not more than 1 ppm were the "standard" so long as such levels could be consistently maintained. Presumably these were indoor pools or somehow had their chlorine replenished at rapid rates since in sunlight about half of the chlorine can get consumed every half-hour.

Richard

[waterbear]

No, it's much worse that this. A CYA of 30 makes the FC chlorine amount 0.75*30=22.5 TIMES less effective. If you want your disinfectant (HOCl) to be at 6 ppm (which is WAY too high), you would need 22.5 * 6 = 135 ppm FC! The reality is that you need very, very little actual disinfecting chlorine to kill bugs and to oxidize simple organics (ammonia, urea, etc.). The "min/max" ranges in Ben's chart roughly correspond to disinfecting chlorine (HOCl) levels of 0.03 to 0.07 ppm. And yes, this means that the equivalent without CYA would mean maintaining FC levels of around 0.03 to 0.07 ppm, but obviously this is nearly impossible to do since it is so small and gets used up easily so instead you put in extra FC "in reserve", typically at least 1 ppm or 2 ppm. The "min/max" numbers in Ben's chart are at various CYA levels and are roughly the amount of FC needed to get to the 0.03 to 0.07 ppm disinfecting chlorine (HOCl) level. He figured all of this out through experience so his table doesn't exactly correspond, but it's pretty close (though the "shock" column doesn't correspond very well).

The German standard for chlorination is for 0.2 ppm chlorine, which I believe is without CYA (it had better be without!). Historically, before CYA was in widespread use, very low chlorine levels of 0.2 or 0.5 ppm and certainly not more than 1 ppm were the "standard" so long as such levels could be consistently maintained. Presumably these were indoor pools or somehow had their chlorine replenished at rapid rates since in sunlight about half of the chlorine can get consumed every half-hour.

Richard

Actually, the recommended FC level for hot tubs is usually in the range of 3-6 ppm.

[chem geek]

Actually, the recommended FC level for hot tubs is usually in the range of 3-6 ppm.

When I said "If you want your disinfectant (HOCl) to be at 6 ppm (which is WAY too high)" I was referring to the disinfecting chlorine, HOCl, not to total free chlorine, FC. Having 6 ppm of HOCl is, generally speaking, way too high. Having 6 ppm FC in the presence of CYA is not at all too high. I'm glad the recommended FC level for hot tubs is higher since with CYA the disinfecting chlorine would be significantly reduced.

Richard