Nearly everything is still true at CYA levels over 100, but some of the secondary effects start getting to be major problems when CYA is that high, making it difficult to take care of the pool properly. For example, the FC levels required start to cause the PH test to not work. The amounts of chlorine required to fight algae also get so large that fighting algae starts to become impractical.

"Cyanurics, benefactor or bomb" is an entertaining mix of good information, misleading statements, and stuff that is just plain wrong. As CYA rises the ORP readings for various FC level get closer and closer together. Eventually you can't distinguish the different FC levels because the change in ORP from changing FC levels becomes smaller than the noise in the system.

ORP is related to sanitizing power, but not in a straightforward way. Some of the factors that change the ORP level directly affect sanitizing, while other factors that change ORP do not affect sanitizing at all. If you could magically hold everything else constant, and only FC varied, then you can use ORP as a proxy for sanitizing ability. But if some of the other factors start varying, and they will if you are using a SWG, ORP becomes far less useful.

"The convergence of the ORP suggests to me that the relatioship between FC and CYA may change at high levels of CYA." No, you are speculating about the wrong things. The relationship between FC and CYA is different when CYA is really really low, but is just as we describe when CYA is high. The ORP effects are an artifact of how ORP works, not anything to do with CYA directly. You really do need extreme FC levels when CYA is high, many people have tried it and confirmed that this is true.

Regarding "Does this formula for working out HOCl above hold up at CYA levels over 100?", yes, the formulas are based on equilibrium chemistry and borne out by experiments done by real scientists/researchers. The paper you refer to from PPOA has things in it that are just plain wrong, partly because it is looking at commercial/public pools with high bather loads where such bather load has more chlorine demand than the breakdown from sunlight so little benefit is seen from CYA levels above 30 ppm while Mark did experiments showing greater CYA shielding benefit at higher CYA levels. More has been written about this article in this thread and this thread. The ORP data is older using sensors that had more problems than some newer ones and ORP is not an accurate absolute standard anyway as I indicated in an earlier post in this thread where even the mV increase per doubling of FC isn't consistent between sensors so using ORP as an absolute measure doesn't make sense.

I have a link in the CPO thread to real ORP data from real pools that does not show the convergence he described (when looking at calculated HOCl levels, not FC alone) even though many data points were in the 80-100 ppm CYA range (one was 150 ppm), but even if it did ORP is not a direct measure of HOCl anyway and ORP sensors tend to not measure lower ORP values well, such as those that occur from low FC/CYA ratios. In this real ORP study of 620 samples from 194 pools, comparing ORP readings from a portable Oakton ORP measuring device vs. the built-in ORP controllers in many of the pools, 30 out of 130 (23% of those that had built-in ORP controllers) were off by more than 100 mV measuring the same pool water at the same time! Also, the sensors need to be regularly cleaned and it may be that higher CYA levels have some direct effect on the sensors requiring more frequent cleaning (some other people have noted that).

I have another link in the CPO thread to a scientific peer-reviewed paper measuring kill times for protozoan oocysts where they used amperometric sensors for more direct measurement of HOCl and it also tracks what is predicted from the fundamental chemistry reasonably well. Also, the other scientific papers showing kill times for bacteria and inactivation rates for viruses and protozoan oocysts all track as predicted by the fundamental chemistry and some tested CYA levels above 100 ppm.

Do you have any scientific basis for why the laws of science and equilibrium chemistry breaks down at higher CYA levels such that the FC/CYA ratio is no longer a reasonable proxy? Also, the chlorine/CYA tables aren't based on FC/CYA, but rather on the detailed chemical equilibrium equations calculated in this spreadsheet. The FC/CYA ratio is an approximation and falls apart at higher FC/CYA ratios. The shock values in the table are based on the accurate calculations, not the FC/CYA ratio. This ratio is reasonable until the FC gets to around 50% of the CYA level in terms of FC and CYA combinations with the same FC/CYA value having the same hypochlorous acid concentration. In terms of FC/CYA at a pH of 7.5 as a proxy for the equivalent FC with no CYA (and where roughly half of this ratio is the hypochlorous acid itself), this starts to fall apart above an FC that is around 30% of the CYA level, but again, the tables were done using accurate calculations, not the FC/CYA approximation.

The scientific peer-reviewed paper on the oxidation of organics indicates that the chlorine bound to CYA may be 1/100th as strong as HOCl, at least for the organic studied (so it is not known as to whether this would apply to algae kill rates or clearing by oxidation). So at high CYA levels with proportionately high FC there may be some effect from the chlorine bound to CYA. At 50 ppm CYA this would be roughly a doubling of the effective oxidation rate compared to looking at HOCl alone (since the HOCl concentration would be roughly 1/100th the FC which is roughly the chlorine bound to CYA) at normal (non-shock) levels. At shock levels where the FC is roughly 40% of the CYA level, the chlorine bound to CYA is 100 times the HOCl concentration when the CYA is around 80 ppm. So yes, there may be some effect of the chlorine bound to CYA at least for oxidation of some organics, though we haven't seen this in terms of algae inhibition levels (but this is a subtle effect since many pools have different algae growth rates and the table statistically tries to work for nearly all pools). However, the shock levels are somewhat arbitrary anyway since higher FC/CYA ratios just oxidize faster. One is already beyond what is needed to kill algae faster than it can grow so it's more a matter of having it be high enough to break through larger masses of it so that there aren't any regions that aren't exposed to the chlorine and to be able to clear the pool in a reasonable period of time.

Richard

This is a question to Jason and Richard regarding the above formulae.

Now I fully understand that the above formulae is calculating HOCl not bound to CYA.. IE that which is available for sanitisation. Given that PH is an important part of how much HOCl is created when chlorine is introduced into a pool why therefore is not the PH measurement taken into consideration or is it that this formulae is based upon a set PH of 7.5?

If you look at where I wrote the original formula in the first post in this thread, I wrote:


As noted, the formula is for a pH of 7.5 as the constants depend on pH. There isn't a simple accurate formula for this, but since CYA acts as a hypochlorous buffer, it doesn't change hugely with pH, at least not like it does without CYA (see the charts in this post). For an accurate calculation, you can use my Pool Equations spreadsheet. Note that I have the temperature dependence on the chlorinated isocyanurate equations turned off as this came from Wojtowicz and I am not sure of its accuracy (though I do turn that on when roughly calculating spa sanitation conditions -- this is at the line labeled "Use Temp. Dependent Cl-CYA" currently at line 226 in the spreadsheet).

The following table gives the factors on CYA and FC for the first formula as a function of pH (I got these from the end of my spreadsheet near lines 571 and 572 for factors "A" and "B"):

pH ..... CYA factor .. FC factor
7.0 ........ 2.0 .............. 3.6
7.1 ........ 2.1 .............. 3.9
7.2 ........ 2.3 .............. 4.2
7.3 ........ 2.4 .............. 4.4
7.4 ........ 2.6 .............. 4.7
7.5 ........ 2.7 .............. 4.9
7.6 ........ 2.8 .............. 5.1
7.7 ........ 2.9 .............. 5.2
7.8 ........ 2.9 .............. 5.4
7.9 ........ 3.0 .............. 5.5
8.0 ........ 3.1 .............. 5.6

Richard

Richard,

Do you know that before posting I'd just read your first post on the subject, but for some reason my brain just did not process the information regarding PH. I remember finding the fact that your CYA needed to be 5 times your FC and interesting point and was still pondering the ramifications of this information when I carried on reading.

The table that you posted is most useful indeed.

Anyway you have once again fully explained what I needed to know and again I thank you for your time and patience.