Persistent algae

[RobbieH]

As algae is killed by the FC, the FC is converted to CC, which drives down FC and drives up CC.

That's what you said too.

CC is removed by the sun. If the CC isn't dropping though, that means there is still FC being converted to CC. A CC of .5 is just a good barometer that everything is done, but there has to be enough FC in the pool to kill algae and continue the process. So if FC is low, a low CC may not mean anything. And in conjunction with the OCLT it's even more of a sure bet that all is well.

 

[Freya]

i guess it depends on the testing computer but I have not heard of any computer results anyone here seems to think are reliable.

I did notice the article two comments. One was they only found ORP testing to be a reliable electronic measure of the safety of the water and that using melamine CYA reagent was one option for causing CYA to precipitate out of the pool.

The quote I identified with is:
"Resultant water conditions don’t appear to change for the worse until the demand threshold exceeds the greatly limited “insurance” ORP. Then the first thing to go is clarity. Second, if the conditions are right, comes severe and un-yielding algae. A distant third is unacceptable sanitation."

Severe and unyielding algae - perfect description for my pool!

 

[techguy]

I know the feeling. The persistent algae is what caused me to learn about CYA and the BBB methods. My water is cheap so it was was easier for me to dump 10K water and start over.

What I came away with was any CYA over 50, for a liquid chlorine pool, may be false economy when you add in the sanitation performance differences and the cost of the CYA.

Chemgeek may have more details.

 

[Freya]

Just a quick update. Been running for 4 weeks or so with crystal clear water. FC always above 4ppm but no higher than 8ppm. I find adding a little chlorine and often works well.

Thanks to all who offered advice but really the only important advice from Jason was to raise my FC.

 

[chem geek]

Excellent article! When he says "Establish levels of 5 to 12 ppm CYA, or a little more, for your outdoor automated pool. Allow 20 ppm or so to provide good retention in a manually treated pool.", does that really mean dropping CYA below 20ppm? Also, the graphs show diminishing returns with CYA above 25ppm. If this is right (and I understand it) then it sounds like at 50ppm CYA it is not surprising to find an algae problem as effectiveness is so low (and useless at under 3ppm FC as in my case).

His charts about the diminishing returns are not wholly accurate. It only seems that way in commercial/public pools with high bather-load because most of the chlorine is consumed oxidizing bather waste in such pools compared to that lost from sunlight so not much benefit is seen above 20 or 30 ppm CYA. However, residential pools have low bather-load and most of the chlorine loss is from sunlight where higher CYA levels even with proportionately higher FC levels has a lower chlorine loss. This is seen in the chart in this post to see the effect, but note that the post is referring to FC/CYA ratios used in saltwater chlorine generator (SWG) pools whereas for non-SWG a higher FC at each CYA level is required (roughly an FC that is 7.5% of the CYA level as shown in the Chlorine / CYA Chart).

As for the CDC, they are aware that CYA significantly moderates chlorine's strength, but neither they nor the PPOA fully understand the FC/CYA ratio relationship nor how FC with no CYA is over-chlorinating pools. Also, PPOA has an ORP bias, but ORP is in fact not any more reliable than looking at the active chlorine level (proportional to the FC/CYA ratio). We know more here about the chlorine/CYA relationship than anyone in the world, as sad as that may be. It's known science since at least 1974 when the equilibrium constants for chlorine bound to CYA were definitively determined in this paper. This is a situation where the chlorinated isocyanurate (Trichlor and Dichlor) manufacturers have misled for nearly 40 years saying that "only FC matters; CYA doesn't matter". It is not the only area of basic science where the industry has been incorrect -- the 10x rule for breakpoint chlorination of CC is wrong (it applies correctly to ammonia measured in ppm N units, but not to CC which is in ppm Cl₂ units and already has one chlorine as part of CC), the acid column or slug method of lowering TA is wrong, the need for CH in vinyl pools is wrong, etc. (you can see the thread Certified Pool Operator (CPO) training -- What is not taught for more examples).