Why a higher CYA for an SWCG pool?

[phalcon51]

I was reviewing the recommended levels in Pool School and I've got a couple of questions:

In a plaster pool the chart calls for a CYA level of 30-50 ppm when using bleach, but 70-80 when using a SWCG, yet the recommended chlorine levels are higher for bleach (3-7 ppm) and lower for SWCG (3-5 ppm). The shock level is the same for both given an identical CYA level. If CYA binds usable chlorine, requiring a higher level for it to be effective, shouldn't it be the other way around? Is SWCG chlorine somehow different than liquid bleach chlorine? Can someone explain this to me?

Also, the TA level for the bleach-fed pool is 70-90+ whereas for the SWCG pool it's 60-80 with both types calling for the same pH levels, 7.5-7.8. What's the reason for the different TA levels, and is pH related to TA?

Thanks for any enlightenment,

Gary

 

[chem geek]

The SWCG provides a more consistent chlorine dosing throughout the day and also provides some superchlorination with very high active chlorine levels in part of the cell where the chlorine is generated (it's not only high in FC, but at low pH as well, unlike bleach). This combination seems to allow for a lower active chlorine level target to prevent algae growth compared to manually dosed pools.

The higher CYA level for the SWCG is to help reduce the rate of pH rise by lowering the amount of chlorine lost to UV from sunlight. This lets one turn down the SWCG on-time percentage and that lowers the amount of hydrogen gas bubbles that aerate the water as well as the amount of chlorine gas that may remain undissolved and outgas. Both of these effects would make the pH rise. Though one can also have a higher CYA level in a manually dosed pool, especially in a very sunny area (e.g. Arizona), it's always a bit risky to do (harder to shock and the higher FC levels tend to make the pH test less reliable) and isn't as necessary since the pH tends to not rise as much in such pools.

The lower TA level for the SWCG is to lower the amount of carbon dioxide outgassing that leads to a rise in pH. This can also help in manually dosed pools, but isn't usually as necessary unless there is a lot of aeration, such as from waterfalls, spillovers, etc.

 

[JasonLion]

Or to put things in practical terms, higher CYA levels are more prone to problems, but SWGs are less prone to problems with the same issues. That means that your can raise the CYA level with a SWG and not have any more problems than you would have had with lower CYA and without a SWG. Meanwhile, higher CYA levels increase the lifetime of the SWG and reduce the PH drift which is often an issue with a SWG.

 

[wetwilly]

I've always wondered the same thing. Thanks for the enlightenment - this forum is great! Much more wisdom here than my local pool store.

 

[phalcon51]

Thanks for the info. Very enlightening!

Gary