NSW Govt limits CYA to 50mg/L in public pools, requires min 3mg/L FC if CYA used

[Explorer]

Stabiliser/Sunscreen/Cyanuric acid/iso-cyanuric acid/cyanurate/CYA is limited by law in public pools to 50mg/L

• About 65% of free chlorine remains after 1 hour of strong sunlight on an outdoor swimming pool without cyanuric acid.
• At a concentration of 10 mg/L of cyanuric acid the loss of chlorine is reduced to about 12%.
• At a concentration of 20 mg/L of cyanuric acid the loss of chlorine is reduced to about 5%.
• At a concentration of 30 mg/L of cyanuric acid the loss of chlorine is reduced to about 3%.
• A concentration of 40 mg/L of cyanuric acid the loss of chlorine is reduced to about 2%.
• Further addition cyanuric acid does not produce any substantial benefit in reduced free chlorine destruction. This follows the law of diminishing returns.

If Stabiliser is used the minimum Free Chlorine required is 3mg/L.

See Stabiliser Cyanurate



Why does the Chart on this site seem to condone the use of higher levels?

Why do people sometimes recommend higher levels?

Given the likelihood of errors in testing for Stabiliser (have you ever taken two identical water samples to two pool shops to see how the results compare? - frightening!) it seems to me that no one should intentionally increase stabiliser above 40mg/L under any circumstances.

 

[JoyfulNoise]

Welcome to TFP!

Since you are new to TFP, I would recommend you read through the material in Pool School to better familiarize yourself with The TFPC Method that is taught here.

To specifically answer your question, see the second chart in this advanced pool water chemistry thread. It shows the the affect of CYA on FC half-life. The chart you link to in the URL shows chlorine loss in a 1-hour measurement which depends a lot on environmental conditions.

As for higher levels, TFP only recommends higher CYA levels (above 50ppm) for two situations - (1) if you live in a very hot & sunny climate where the pool is exposed to higher temperatures and more UV OR (2) your pool uses a salt water chlorine generator. In both those situations, the higher stabilizer level results in lower chlorine loss allowing the user to run their SWG's at a lower % ON time or, in the case of manual chlorination, allows the pool owner to have a longer interval between additions of FC.

In both of those cases and generally speaking, TFP always recommends a constant ratio of FC to CYA. So, if you operate your pool at a higher CYA level, then your target FC level must be higher so that your pool has the appropriate amount of active chlorine (HOCl or hypochlorous acid) in the water. It is the active chlorine level that ultimately matters for proper water sanitation. The absolute values of FC and CYA concentration, within reasonable limits, is inconsequential; only the ratio of FC to CYA matters.

Given the likelihood of errors in testing for Stabiliser (have you ever taken two identical water samples to two pool shops to see how the results compare? - frightening!) it seems to me that no one should intentionally increase stabiliser above 40mg/L under any circumstances.

This is why TFP always recommends that pool owners do their own water testing with one of the Recommended Test Kits

 

[chem geek]

Actually, the loss of chlorine in a pool in direct noontime sun in the summer is more like 50% per hour, though it's pH dependent where the loss is faster/higher at higher pH. It also depends on pool depth since the half-life near the surface of the water is around 35 minutes a pH 7.5. Also, the loss rates you are quoting are not correct. There is still lower loss of chlorine at higher CYA levels and if the FC/CYA ratio is 7.5% or somewhat higher, then typical chlorine loss at 50 ppm is on the order of 2 ppm per day. For a ratio of 5% FC/CYA as used in saltwater chlorine generator (SWG) pools, you can see the chart in this post where at 2.5 ppm FC with 50 ppm CYA the peak loss per hour is 0.16 ppm/hour while at 4 ppm FC with 80 ppm CYA it's 0.12 ppm/hour.

The active chlorine level is proportional to the FC/CYA ratio so 3 ppm FC with 30 ppm CYA has the same active chlorine as 5 ppm FC with 50 ppm CYA or 8 ppm FC with 80 ppm CYA. So one chooses the CYA level mostly on the basis of how much protection they need from sunlight, but even for indoor pools some smaller amount of CYA is desired to moderate chlorine's strength. The main reason to not go too high in CYA is NOT that chlorine is not as effective because you can simply proportionately raise the FC level to have it be exactly as effective. The reason is that if anything goes wrong and you need to SLAM the pool, it takes extraordinarily high levels of chlorine. This risk is lower in SWG pools because they are more automatically maintained so less likely to have the chlorine get too low. The same is true in pools using supplemental products to control algae.

Commercial/public pools are different than residential pools primarily in that the former have higher bather loads so more chlorine usage is for oxidizing bather waste and not loss from sunlight. So it doesn't make much sense to have higher CYA levels in such high bather load pools. Also, for Crypto remediation using chlorine, that takes an FC that is roughly 10 ppm more than the CYA level so becomes quite impractical when the CYA level is higher though there are other ways to kill Cyrpto after a fecal accident, namely use of chlorine dioxide that can be generated on-site using sodium chlorite added to a pool with chlorine and CYA already in it.

 

[Explorer]

Thanks Chem Geek
Are the chlorine loss rates you quote for 5% FC/CYA for a SWG pool with the pump/filter/chlorinator working? Is it a net loss after chlorine input from the automatic salt water chlorinator, or is it the loss without the chlorinator/pump/filter running?

 

[chem geek]

The daily loss % is without maintaining the chlorine level. The hourly loss % and ppm/hr are with maintaining the chlorine level (so filtration and SWG on) and are peak rates with direct noontime sun so is basically the output needed from the SWG. That's the worst-case so you know how to roughly size the chlorinator.