After accumulating multiple pieces of conflicting evidence, I think it's about time to discuss and investigate the mechanism of how CYA protects chlorine from sunlight. The starting point for the theory, that I'm starting to think is only partially correct and needs to be enhanced, is that the chlorine that is in the form of hypochlorous acid or hypochlorite ion breaks down in direct noontime sunlight with a half-life of around 35 minutes while chlorine that is attached to Cyanuric Acid (CYA), also known as chlorinated isocyanurates, breaks down from the sun with a half-life of around 8.4 hours.

This graph shows the net result. The conclusion from this graph is that a little CYA provides a lot of protection of chlorine and that there are diminishing returns for using high CYA levels. There are two pieces of evidence that are in conflict with this theory:

1) Some users, most notably Janet (user name Aylad), report that in their non-SWG pools using high levels of CYA shows dramatic improvement in chlorine's staying power. In Janet's case, with a CYA below 60 she found that the FC would go from 7-8 to 2-3 in one day (5 ppm FC per day) while with a CYA of 80-90 the FC would go from 8-9 and take 3 days to go below 5 (about 1.2 ppm FC per day). That is a huge improvement that is wholly inconsistent with the graph.

2) Several users of SWG pools have found that raising the CYA to higher levels, especially approaching 70-80 that some manufacturers recommend, has a dramatic increase in FC levels at the same SWG output. Though one theory is that the CYA makes the SWG cell more efficient by combining with the generated chlorine in the cell "hiding" it from the plates in terms of equilibrium (thus making the generation proceed more quickly), an alternative explanation proposed by some is that the higher CYA levels simply protect the chlorine from destruction from sunlight at a rate faster than the baseline theory outlined at the start of this post.

I've been thinking of mechanisms that might explain the above data and that could be added to the theory to make it predict more accurately. One possibility is that CYA itself is able to absorb UV radiation and possibly re-radiate it as non-UV radiation at lower energy, with the rest of the energy becoming kinetic (i.e. heat or temperature increase). This link shows that indeed CYA does absorb UV at the pH found in pools, though it absorbs even more in more basic/alkaline solutions.

If one adds direct CYA absorption and essentially shielding of UV from lower depths of the pool, then the "CYA shielding chlorine" description would in fact be accurate for this mechanism (while it is not accurate to describe the chlorinated isocyanurates which do not "shield" chlorine but are distinctly different molecules with different absorption rates and affect disinfecting chlorine levels). The net effect of this new mechanism would be to have higher CYA levels reduce chlorine loss at a greater rate than shown in the graph I linked to at the top of the post.

So how can we prove that this new mechanism exists (or is likely) and explains what is being reported in (1) and (2) above? Let's start with the easier of the two, namely the second item of whether CYA improves SWG cell efficiency. This can readily be determined by comparing SWG FC output at different CYA levels, BUT with no sunlight shining on the pool (i.e. either at night or with an opaque cover or with an indoor pool). To the degree that CYA increases the SWG cell output to generate higher FC levels, then this leads credence to the efficiency theory; if not, then the protection from degradation from sunlight is more likely.

As for whether CYA "shields" chlorine through absorption of UV (clearly it does absorb some UV, but the question is more one of whether this is a significant mechanism in quantity), this should be a function of the depth of the pool. The presence of higher concentrations of CYA essentially lower the density of UV radiation reaching lower depths in a pool. So this protective effect of CYA should show up more in deeper pools where a significant fraction of the water is at greater depths and should be less effective in shallower pools. The chlorinated isocyanurates, on the other hand, do not have this same effect since they do in fact degrade (the chlorine attached to them degrades to chloride ion) and are in fact less likely to interact with sunlight in this way so light is more likely to continue to lower depths (i.e. it doesn't act as a shield and even if it does, it's a much smaller concentration than unbound CYA itself).

The experiment would be harder and would require measuring the difference in the destruction of chlorine in waters of different depths at varying CYA levels. The half-life of the chlorinated isocyanurate would be the dominant factor in the shallowest basin of water while CYA's "shielding" effect would be a greater factor in the deepest basin of water.

Richard