Mark,
Great experimenting (again)! waterbear also asked about water shielding UV but in the range where most of the destruction occurs, mostly for hypochlorite ion around 270-320 nm (see this link) and the low absorption of water in this same range (see this link), it would seem from your experimental results that the protection from "shielding" is hugely significant and is a predominant factor and this shielding is from CYA and possibly also from the chlorinated CYA species. Deep water looks blue because most light except blue is absorbed and some of the blue gets scattered back.
Now it is possible that the other components of pool water, such as the carbonates, provide some shielding, but this link showed a modest shielding effect in waters with lower Dissolved Organic Carbon (DOC) of 0.53 ppm Carbon and that's organics, not carbonates (the water would already have far higher than 0.5 ppm carbon as carbonate just from dissolved carbon dioxide). The implied absorption coefficient is from 0.006 to 0.01 while the pure water number is around 0.0001 or 100 times less. So various organics in the water can clearly have a shielding affect (CYA itself is an organic, after all, but a very powerful one for shielding UV).
By the way, your experiment with different pH for SWG production is interesting and wasn't what I was asking for, but I'm glad you did it since that is also good to know and is a little unexpected. There is a variation in the reactions with pH, but the relatively high (for electrochemistry) voltages in the SWG would dwarf such differences and also explain why my original theory of CYA protection in the cell is not the gating item. Diffusion might be a more important factor. I suspect that there might be a difference in SWG efficiency with flow rate -- faster speed being more efficient, somewhat similar to what is seen with solar panels though the mechanism is different. (Yes, I know, I should just shut up with these experiment ideas...).
What I was really asking regarding pH was the chlorine lifetime where I believe that the chlorine will last longer at lower pH than at higher pH, all else equal, due to the difference in hypochlorous acid vs. hypochlorite ion concentrations (the former seems to be more resistant to breakdown from UV than the latter, based on the link I gave above).
If you could measure the chlorine drop in the spa and the bucket a little sooner -- before the bucket chlorine went too low too quickly, that would be better. Also (yes, another experiment), if you have a clean bucket you can use for tap water without CYA in it (though you could add a small amount of baking soda and calcium chloride to simulate a pool environment) and compare that with a similar sized bucket with your pool water that has CYA in it, then that would tell us the protection from CYA due to creating different chemical species as opposed to the shielding effect (it would actually be a little of a mix since the bucket has depth, but I can sort that out if I know the depth of the bucket).
Just so you know, I'm going on vacation for the next two weeks so may not be able to work on any calculations until I get back. I think we've got a good handle on what experiments we are looking for, however, and already have a strong indication that the shielding effect is a strong one second only to the species effect (i.e. the disinfecting chlorine concentration being much, much smaller in the presence of CYA). With no CYA, the half-life of chlorine (from one of the links above) seems to be around 10 minutes near the surface (in Florida direct noontime sun).
Richard
Great experimenting (again)! waterbear also asked about water shielding UV but in the range where most of the destruction occurs, mostly for hypochlorite ion around 270-320 nm (see this link) and the low absorption of water in this same range (see this link), it would seem from your experimental results that the protection from "shielding" is hugely significant and is a predominant factor and this shielding is from CYA and possibly also from the chlorinated CYA species. Deep water looks blue because most light except blue is absorbed and some of the blue gets scattered back.
Now it is possible that the other components of pool water, such as the carbonates, provide some shielding, but this link showed a modest shielding effect in waters with lower Dissolved Organic Carbon (DOC) of 0.53 ppm Carbon and that's organics, not carbonates (the water would already have far higher than 0.5 ppm carbon as carbonate just from dissolved carbon dioxide). The implied absorption coefficient is from 0.006 to 0.01 while the pure water number is around 0.0001 or 100 times less. So various organics in the water can clearly have a shielding affect (CYA itself is an organic, after all, but a very powerful one for shielding UV).
By the way, your experiment with different pH for SWG production is interesting and wasn't what I was asking for, but I'm glad you did it since that is also good to know and is a little unexpected. There is a variation in the reactions with pH, but the relatively high (for electrochemistry) voltages in the SWG would dwarf such differences and also explain why my original theory of CYA protection in the cell is not the gating item. Diffusion might be a more important factor. I suspect that there might be a difference in SWG efficiency with flow rate -- faster speed being more efficient, somewhat similar to what is seen with solar panels though the mechanism is different. (Yes, I know, I should just shut up with these experiment ideas...).
What I was really asking regarding pH was the chlorine lifetime where I believe that the chlorine will last longer at lower pH than at higher pH, all else equal, due to the difference in hypochlorous acid vs. hypochlorite ion concentrations (the former seems to be more resistant to breakdown from UV than the latter, based on the link I gave above).
If you could measure the chlorine drop in the spa and the bucket a little sooner -- before the bucket chlorine went too low too quickly, that would be better. Also (yes, another experiment), if you have a clean bucket you can use for tap water without CYA in it (though you could add a small amount of baking soda and calcium chloride to simulate a pool environment) and compare that with a similar sized bucket with your pool water that has CYA in it, then that would tell us the protection from CYA due to creating different chemical species as opposed to the shielding effect (it would actually be a little of a mix since the bucket has depth, but I can sort that out if I know the depth of the bucket).
Just so you know, I'm going on vacation for the next two weeks so may not be able to work on any calculations until I get back. I think we've got a good handle on what experiments we are looking for, however, and already have a strong indication that the shielding effect is a strong one second only to the species effect (i.e. the disinfecting chlorine concentration being much, much smaller in the presence of CYA). With no CYA, the half-life of chlorine (from one of the links above) seems to be around 10 minutes near the surface (in Florida direct noontime sun).
Richard