Now that I have the book in front of me, I see the full quote and context of the statement. The statement is in the section on "ORP and Free Chlorine Probes with Cyanuric Acid" after talking about the ORP probe sensing only the unbound chlorine when cyanuric acid is present. There are plenty of incorrect statements made, but I'll get to that in a moment. Then he writes the following:
For cyanuric acid to work we need a certain amount in the water (preferably 8-10 times the free available chlorine) to protect the chlorine from UV light. This is enough to have 95% of the chlorine attached at any one instant.
So the context here is about having enough CYA in the water to bind to enough chlorine to protect most of it from sunlight. There is no discussion about this being the amount needed to prevent algae growth. In fact, this entire section has a lot of incorrect information as he talks about the chlorine bound to CYA as being active:
When both free chlorine and cyanuric acid are dissolved in the water, part of the chlorine is attached to the nitrogen atoms in the cyanuric acid. The ORP probe cannot sense the active chlorine attached to the cyanuric acid therefore, this part of the active chlorine does not contribute to the ORP reading. But the free chlorine test reagent (DPD, syringaldazine or TMB) does sense the chlorine attached to the cyanuric acid because it is so weakly attached.
:
Chlorine that is attached to cyanuric acid is still active chlorine. The proof is that when you do a free chlorine test with cyanuric acid in the water, the chlorine shows up as free, not combined.
:
If the attached and unattached chlorine reacts with the free chlorine test reagent, it will also react with anything that is oxidizable (bacteria, alage, swimmer waste, etc.).
This is, of course, completely wrong logic since the free chlorine test just measures the amount of chlorine that is potentially available to become active, that is it includes the chlorine in reserve (bound to CYA and technically includes hypochlorite ion as well), but this is NOT the same thing as the concentration of truly active chlorine (hypochlorous acid) that determines reaction rates.
I've used an analogy of front-line soldiers as active chlorine while those in reserve are inactive. It doesn't matter how many soldiers you have in reserve nor how quickly they replace a fallen soldier on the front-line, the rate of killing is solely determined by the number of soldiers on the front line. His logic regarding the further reacting of the DPD dye is false because that reaction is very fast so obscures the fact that the rate of that reaction is determined only by the amount of active chlorine unbound to CYA (specifically, hypochlorous acid). If there were no CYA in the water, the reaction would occur orders-of-magnitude faster, but for the DPD test the difference between taking 1 second vs. taking 0.05 seconds doesn't matter. However, for killing pathogens that take longer to kill, it does make a difference as it does for the rate of killing algae. In a later section on page 119 under "Cyanuric Acid and ORP Readings" he writes:
If the cyanuric acid was really slowing down chlorine's oxidizing ability we would expect the reaction of chlorine in the water sample to take longer to react with the reagent or to not react with it at all, indicating it was combined. But it doesn't.
He again doesn't realize that in fact the reaction IS slowed down, but not by enough to notice. The half-life of chlorine bound to CYA producing more hypochlorous acid is (for the fastest reaction path) 0.25 seconds so assuming the reaction of the dye with chlorine is very fast, when CYA is present the chlorine test will be essentially complete in a matter of seconds, regardless of how high the CYA level (until it got to a point where the active chlorine concentration was so low that it was very slow to react with the dye -- that could be a huge CYA level before that happens).
I see that in the section "The Proper Amount of Cyanuric Acid" on page 117 he writes:
Experience has found that it takes 10 ppm of cyanuric acid in pool water to protect up to 1.5 ppm chlorine in the water. Any amount of chlorine in excess of 1.5 ppm in the water when you have 10 ppm of cyanuric acid is not protected. It is then subject to sunlight destruction at the rate of 90-95 percent in 2 hours ... Depending on water conditions, it will take about 8-10 times as much cyanuric acid as free chlorine to be effective.
This simply isn't true. There isn't some sort of magic cutoff for this protection. The rate of breakdown of chlorine is a function of how much is bound to CYA vs. how much is unbound and it is simply not true about the "excess" amount not being protected. The amount that is unbound is proportional to the FC/CYA ratio so he does have that relationship understood correctly, but it doesn't mean more chlorine isn't protected. At 3 ppm in the water with 10 ppm CYA there is NOT 1.5 ppm unprotected -- there's somewhat more than 2.5 times as much unprotected (0.33 ppm vs. 0.13). And even that's not quite true since the chlorine bound to CYA breaks down as well, just not as quickly. And then there's the CYA shielding of lower depths that he is ignoring (though we only discovered that ourselves rather late).
Now that I have the book in front of me I see all of the notes I wrote in it about the things that were wrong. To his credit, Bob Lowry did understand the bound vs. unbound chlorine/CYA relationship, but he gets a lot wrong thinking the bound chlorine was active and thinking that CYA protecting chlorine from sunlight had hard limits rather than being driven by proportions determined by equilibrium chemistry.
So the 8-10 times of CYA to FC has nothing to do with algae prevention and using his logic that the bound chlorine is active it is the FC alone that should determine algae prevention. So he really didn't come up with the appropriate chlorine/CYA chart for algae prevention though he accidentally came up with something in the ballpark (though in a backwards sort of way) by thinking of CYA protecting FC from sunlight.
Anyway, the Pool Chlorination Facts book is 7 years old and Bob Lowry is now updating books he has written for
IPSSA (the Basic and Intermediate Training Manuals on Water Chemistry), but I haven't had a chance to thoroughly review those yet.
So, again, as far as I know, Ben Powell is the first to determine the chlorine/CYA relationship with respect to levels needed for algae prevention. His
Best Guess CYA chart doesn't exactly match constant FC/CYA ratios (or more specifically, calculated hypochlorous acid concentrations), though for the Min and Max it's reasonably close above 30 ppm CYA as shown here though shock levels don't track as well. His chart was created mostly from his experience with real pools.
) 
:grrrr: :evil 
