New owner trying to understand relationship between CYA and Chlorine

[dwizum]

Just bought a house with an inground pool, my first ever pool ownership. It had a failed liner, that has been replaced and it has been running great for about a week. I started it with liquid chlorine and powdered CYA and have been adjusting TA and pH with Muriatic acid, as the TA was super high due to hard fill water.

No real issues and I've been able to understand and follow the advice I've found here and elsewhere to test and keep parameters in check. My question is much more about theory than practice, the pool is running fine - I just don't understand fully the relationship between CYA and Chlorine.

I read that CYA is like sunscreen for chlorine and that makes sense. If your CYA is higher, the chlorine dissipates more slowly. However, I also read that at higher CYA levels, you need more chlorine for the same sanitation effect. How can that be? In terms of balancing chlorine over an entire season, don't those two trends cancel each other? More CYA means your chlorine lasts longer - but it also means you need more chlorine to begin with?

Are there any resources where I can get a more in depth understanding of how this works? i.e. the actual chemistry mechanisms at work? Are the typical target CYA levels simply chosen to balance these factors out? If you were more concerned about adding chlorine less often, versus adding less in each dose, would you skew the CYA target higher or lower accordingly?

 

[Msch99]

Scope out "The Deep End" in the forum. https://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart
and a few other threads there.

Made my head hurt!

Have fun!
Mike

 

[ka8yiu]

This may be the article you are looking for. I hope the link works:

New Thinking: Chlorine/Cyanuric Acid In Balance - AQUA Magazine

This article has been posted a few times. I found it an interesting read.

 

[dwizum]

This may be the article you are looking for. I hope the link works:

New Thinking: Chlorine/Cyanuric Acid In Balance - AQUA Magazine

This article has been posted a few times. I found it an interesting read.

Thanks. That article was perfect. The comments were interesting. Seems like some people with salt systems are finding that a slightly higher CYA level means they can generate less chlorine.

I would be interested to see if anyone using liquid chlorine has done similar quantified comparisons. For instance. If you set CYA at 80 vs 40, can you get away with a smaller chlorine dose?

 

[JimMarshall]

Thanks. That article was perfect. The comments were interesting. Seems like some people with salt systems are finding that a slightly higher CYA level means they can generate less chlorine.

I would be interested to see if anyone using liquid chlorine has done similar quantified comparisons. For instance. If you set CYA at 80 vs 40, can you get away with a smaller chlorine dose?

With the higher CYA, using Liquid Chlorine, you MUST target the higher FC level this is not negotiable. It can be confusing that SWCG users can target lower levels than LC users, but......
The truth of the matter is, SWCG users can target lower chlorine levels because the generator is continuously adding chlorine to replenish that which was consumed by the sun and organics.

Take a SWG pool with a CYA of 70 and a cell set up to keep the chlorine at 5.... the chlorine is always 5. If over the course of an hour, the pool is consuming 0.2 FC, over that same hour the generator produces enough chlorine gas to add 0.2 FC to the pool.... so the level remains constant at 5, which you’ll see is the recommended minimum for a non swg pool with a CYA of 70. In this pool, you target a FC of say 8, and then you use 3? FC during the day before making your once daily chlorine addition. Since you’re onky adding chlorine once, you need to stay at higher levels to make sure you don’t go below minimum before you make your next addition.

If you were continually checking and redoing your pool by hand hourly all day every day, or continuously dosing with a very low volume stenner pump all day long , you’d safely be able to follow the lower SWCG levels with LC usage.

 

[kchinger]

With the higher CYA, using Liquid Chlorine, you MUST target the higher FC level this is not negotiable. It can be confusing that SWCG users can target lower levels than LC users, but......
The truth of the matter is, SWCG users can target lower chlorine levels because the generator is continuously adding chlorine to replenish that which was consumed by the sun and organics.

Take a SWG pool with a CYA of 70 and a cell set up to keep the chlorine at 5.... the chlorine is always 5. If over the course of an hour, the pool is consuming 0.2 FC, over that same hour the generator produces enough chlorine gas to add 0.2 FC to the pool.... so the level remains constant at 5, which you’ll see is the recommended minimum for a non swg pool with a CYA of 70. In this pool, you target a FC of say 8, and then you use 3? FC during the day before making your once daily chlorine addition. Since you’re onky adding chlorine once, you need to stay at higher levels to make sure you don’t go below minimum before you make your next addition.

If you were continually checking and redoing your pool by hand hourly all day every day, or continuously dosing with a very low volume stenner pump all day long , you’d safely be able to follow the lower SWCG levels with LC usage.

I think I read there's also the possibility that the higher chlorine levels present in the SWG itself while it's generating, which theoretically all of the water is cycling through at some point over a couple days, helps to allow a lower overall level in the pool. I'm not sure how valid this is, but it makes sense on its face. A stenner pump would also get a similar effect I would think.