Carl,
In this post you say a couple of things I disagree with or perhaps am just unsure of or am misunderstanding.
1) Using a higher CYA of say 80 ppm may result in lower chlorine usage.
2) CYA should NEVER be used in an indoor pool.
Regarding the first point, I disagree because the benefit of CYA in protecting chlorine is a non-linear relationship of declining returns. The reason for this is that the chlorine bound to CYA (chorinated cyanurates) DOES degrade in sunlight. It just does so at a slower rate with a half-life of around 8.4 hours (though some sources say it's 6 hours). The half-life of unprotected (unbound) chlorine is 35 minutes (though some sources say that in a tube it's 11.6 minutes). These numbers are for direct noontime sunlight in summer (i.e. sun directly overhead).
Let's take a simplified example and assume that the chlorine bound to CYA is COMPLETELY protected from sunlight and lasts forever. The rough rule is that to get the same disinfecting chlorine concentration you need to keep the FC to CYA ratio constant so whereas at 30 ppm CYA you could have 3 ppm FC, at 90 ppm CYA you need 9 ppm FC. The amount of unbound chlorine in both cases is identical in this example so in this simplified example there is NO CHANGE in the amount of chlorine that is lost to sunlight. In other words, in this best-case you never use less chlorine with more CYA -- never (I'm not talking about SWG efficiency, but just chlorine lost to sunlight).
The reality, however, is that the chlorine bound to CYA IS lost to sunlight, albeit more slowly, and since the vast majority of the chlorine is in fact bound to the CYA it is this quantity that really determines what goes on. With three times the CYA and needing three times the FC you have roughly three times the amount of chlorine bound to CYA and three times the amount of chlorine lost to sunlight.
All of this is shown in this graph where you can see that the net overall effect from the two half-lives (of unbound and bound chlorine) is that at 30 ppm the half-life of 3 ppm FC chlorine is about 6 hours while at 90 ppm the half-life of 9 ppm FC chlorine is about 7.5 hours. So you only get a relatively small increase in the half-life of chlorine, but have three times as much to lose. The net result is that you lose FAR more chlorine at the higher CYA and FC levels. I show this net result in this graph where the 0.05 yellow line corresponds roughly to the mid-point of Ben's Min and Max columns. This clearly shows that the lowest total loss of chlorine is always at the lowest possible CYA level. In fact, with no CYA, the amount of chlorine required to produce 0.05 ppm of disinfecting chlorine is only 0.1 ppm FC. If there were somehow some way of putting 0.1 ppm FC into the pool everywhere and maintaining it, then that would lead to the lowest loss of chlorine since you would only be losing half, or 0.05 ppm FC, every 35 minutes. So over 8 hours, that's losing about 0.4 ppm FC total. Compare that to having 90 ppm CYA with 9 ppm FC where you lose half or 4.5 ppm FC in a little less than 8 hours. That's a HUGE difference!
Of course, there is no practical way to maintain 0.1 ppm FC everywhere in a pool at all times, even with an SWG blasting away. There can be localized demand for chlorine that overwhelms the ability of chlorine to diffuse from other areas of the pool fast enough. Also, you simply aren't going to be manually standing by your pool at all times adding chlorine to it continuously at a rate of around 0.05 ppm FC every half hour (0.1 ppm FC per hour).
This is what I mean when I say there is a tradeoff between using the lowest CYA possible to minimize chlorine breakdown from sunlight vs. the practical side of having enough chlorine buffer (capacity) in the pool to handle localized demand AND frequency of replenishment.
There is no question that Aylad may find that at a constant FC level that the chlorine lasts longer with more CYA, but there are two questions I would have. First, how much longer does the chlorine last, or put another way, what is the difference in loss? I doubt very much that it anywhere near a factor of 3 between 30 ppm and 90 ppm. I suspect that with, say, 5 ppm FC that at 30 ppm over 24 hours the end result is 2.5 ppm FC while with 90 ppm over 24 hours the end result is 3 ppm FC. The problem is that while at 30 ppm CYA one could keep 3 ppm FC; at 90 ppm CYA one needs to keep 9 ppm FC. In other words, to keep a minimum of 3 ppm FC and since half would get lost over a day, one has to start with 6 ppm FC (at 30 ppm CYA). At even double the CYA of 60 ppm, the half-life goes from 6 hours to 7 hours but the minimum FC level is around 6 ppm so now one needs to start with, perhaps, 11 ppm FC so one ends up with 6 ppm the next day. Part of what is happening is that Ben's chart is being interpreted at its extremes. The 30-50 ppm CYA says a Min of 3 ppm FC while the 60-90 ppm CYA says a Min of 5 ppm FC so it APPEARS that going from 30 ppm CYA at 3 ppm FC one can go to 80 or 90 ppm CYA at only 5 ppm FC and have the same disinfecting chlorine capability. At least from the chemistry, that is simply not the case. It's 30 ppm CYA at 3 ppm FC being equivalent to 90 ppm CYA at 9 ppm FC as seen in my chart (or accurately calculated in my spreadsheet).
So assuming half the FC is lost at 30 ppm CYA and somewhat less than half is lost at 80 ppm CYA, then the difference between Aylad going from 6 ppm FC to 3 ppm FC over a day to maintain the minimum of 3 ppm vs. going from perhaps 9 ppm FC to 5 ppm FC over that same day does seem to not be a huge difference, but even in this case it's a 3 ppm FC loss per day at 30 ppm CYA while a 4 ppm FC loss per day at 80 ppm CYA. Now if Aylad has some numbers for me that show these losses to be wrong, then we can see what is wrong about the model assumptions. But it would have to be a combination of two things: CYA would have to protect chlorine from sunlight much better than described (i.e. not 8.4 hours but far higher) AND the chlorine/CYA relationship would have to not be as linear as we think it is when CYA >> FC. I'm guessing that the issue here is more of a misinterpretation of Ben's chart due to its ranges.
One final point. In the above discussion I have only talked about the chlorine demand associated with breakdown from sunlight. In addition, there is a relatively fixed demand associated with typical algae and bather load (on average) and this is a constant to be added to the daily FC consumption. That doesn't change the end result of the analysis in making higher CYA better, but it does make the differences between different CYA levels smaller since there is an extra FC amount required. Note that I am NOT saying that 30 ppm CYA is the right number. This completely depends on the specifics of the total chlorine demand and the frequency of chlorine addition. I just doubt that 80 ppm CYA is the best level and this would be more apparent if the 9 ppm FC were the true target.
(CONTINUED BELOW REGARDING POINT 2)
Richard
Bookmarks