Why do SWG need higher CYA?

[Amrie]

What I can find on this site is that the CYA doesn't "store" the chlorine fast enough as it's being produced, so high chlorine levels can shorten the life of the cell. I don't get it. The higher you raise CYA, the higher you raise chlorine in a ratio. So wouldn't the concentration of chlorine be the same passing through the cell? And if CYA is slow to store chlorine, why doesn't the gradual addition of chlorine from the swg make it easier to keep up with?

Running the SWG longer to keep chlorine higher would also shorten its life, right? Or maybe once you get levels up, the cell runs the same each day since the chlorine demand is consistent? But then I'm back to the question, how can the same ratio but higher levels of cya and chlorine be gentler on the salt cell?

 

[reggiehammond]

Higher CYA protects the FC from UV consumption. The reason to run higher CYA with a SWCG is actually to preserve the consumable life of the cell, because you can count on it producing consistently throughout each and every day. This is contrasted against a pool maintained with liquid chlorine which only gets FC once every day or so, which means the margin for error is greater.

High chlorine levels have no effect on the SWCG. Higher levels of CYA are more effective at protecting FC before it's needed for sanitation.

 

[DrewLG]

how can the same ratio but higher levels of cya and chlorine be gentler on the salt cell?

A higher level of CYA lessens the effect of the sun's UV on free chorine -- the chorine doesn't burn off as quickly, so there's less for the SWG to produce, so the SWG can work less hard.

A high level of CYA would provide the same benefit to non-SWG pools, too. But in those pools, chlorine isn't steadily and reliably added as it is with an SWG, so it's more likely that the FC concentration might sometimes dip below what's necessary to keep algae at bay -- and eradicating the algae will require a SLAM, which would be very difficult with higher CYA. So high CYA isn't recommended for those pools.

That's how I understand it, anyway.

 

[Newdude]

We are primarily teaching newbs here how to maintain their pool. They foul up or life simply happens and they need to slam around times. Ad said above, it's more difficult with higher FC targets. If it wasn't for that fact, we'd reccomend that liquid chlorine pools run a higher CYA as well.

If I was to give up my SWG and become a jug lugger, I'd keep the higher CYA aspect of it for the day to day benefits and just deal with the worse SLAM if a family emergency made the pool take a backseat.

 

[Jimrahbe]

Amrie,

SWCG's don't 'need' higher CYA to work.. They will work just fine with lower CYA... but, they work better with a higher CYA, because the CYA reduces the amount of chlorine your pool uses each day, and that means the cell has to make less chlorine each day.

And it is pretty telling and repeatable... I can easily tell when my CYA level has dropped below 60 ppm, as I have to increase my cell's output to keep up with the FC demand.. After I bring the CYA up to 70 or so, I can then reduce the cell's output and still maintain the same FC..

Thanks,

Jim R.

 

[ILSBJ]

Regarding the question of higher fCl needs at higher CYA levels:
You can achieve an initial increased level of fCl by using liquid chlorine, and then the lower load of the SWCG cell comes into play...

ILSBJ

 

[JamesW]

The primary benefit of a higher CYA level is that it allows you to keep a larger reserve of chlorine, which is a buffer for times that might deplete the FC like storms, heavy use, maybe the SWG goes bad or you forget to turn it back on after service etc.

For example, maybe you keep the CYA at 70 ppm and the FC at 7 ppm.

If you test every day, you should see a solid 7.0 all the time.

If something happens and the level goes down to 3 ppm, then you can address the problem before you get algae.

If you were keeping a 30 ppm CYA and a 3 ppm FC, then a 3 ppm loss would mean 0 FC and probably algae.

 

[JamesW]

The cell needs to run the same amount of time in any case.

For example, if the CYA was 30 ppm, the FC was 3 ppm, the SWG ran at 50% for 12 hours a day to maintain a steady 3 ppm and you increased the CYA to 70, the FC would rise to 7 ppm automatically without adjusting the SWG percentage or runtime.

In other words, maintaining a FC/CYA ratio of 10% requires the exact same SWG runtime because you have the same hypochlorite level and the same FC loss per day.

 

[Amrie]

The cell needs to run the same amount of time in any case.

For example, if the CYA was 30 ppm, the FC was 3 ppm, the SWG ran at 50% for 12 hours a day to maintain a steady 3 ppm and you increased the CYA to 70, the FC would rise to 7 ppm automatically without adjusting the SWG percentage or runtime.

In other words, maintaining a FC/CYA ratio of 10% requires the exact same SWG runtime because you have the same hypochlorite level and the same FC loss per day.

Are you sure? My understanding is that the ratio of fc to cya is human-controlled, not a chemical balance that self-corrects. Meaning, theoretically I could MAINTAIN the ratio with the same run time, but to jump from one level to another, I'd have to boost the chlorine with either liquid or longer runtime to get there, then go back to the original runtime to stay there.

 

[Amrie]

Not correcting you, genuine question!

 

[Amrie]

We are primarily teaching newbs here how to maintain their pool. They foul up or life simply happens and they need to slam around times. Ad said above, it's more difficult with higher FC targets. If it wasn't for that fact, we'd reccomend that liquid chlorine pools run a higher CYA as well.

If I was to give up my SWG and become a jug lugger, I'd keep the higher CYA aspect of it for the day to day benefits and just deal with the worse SLAM if a family emergency made the pool take a backseat.

Interesting. That makes sense. I don't know if it's my own assumptions or something in the way info is presented, but somehow I got the vibe that it was the other way around. I thought the ratio for liquid chlorine was the baseline and something about swg required higher, not that the swg ratio was ideal but the one for liquid chlorine was lower to allow for slamming.

I think part of my confusion is the chart linked on the CDC website, showing the effectiveness of different levels of cya at preserving chlorine in sun. It maxes out at 70ppm, and it's close to 100% even at 30ppm. So aiming for 70 or 80 seems like overkill. Is that just trying to give margin of error for rain and refills diluting it?

 

[ajw22]

Are you sure? My understanding is that the ratio of fc to cya is human-controlled, not a chemical balance that self-corrects. Meaning, theoretically I could MAINTAIN the ratio with the same run time, but to jump from one level to another, I'd have to boost the chlorine with either liquid or longer runtime to get there, then go back to the original runtime to stay there.

Please learn about active chlorine and reserve chlorine when bound to CYA by reading CYA - Further Reading

CYA lets you have reserve chlorine in the pool without chlorine harshness.

 

[Amrie]

Amrie,

SWCG's don't 'need' higher CYA to work.. They will work just fine with lower CYA... but, they work better with a higher CYA, because the CYA reduces the amount of chlorine your pool uses each day, and that means the cell has to make less chlorine each day.

And it is pretty telling and repeatable... I can easily tell when my CYA level has dropped below 60 ppm, as I have to increase my cell's output to keep up with the FC demand.. After I bring the CYA up to 70 or so, I can then reduce the cell's output and still maintain the same FC..

Thanks,

Jim R.

Thanks for your precision. "Need" wasn't quite the right word. I meant to ask why TFP *recommends* higher.

I am testing free chlorine daily and tracking how long I run the swg. I'm curious if I will have similar results to yours. I've been trying to keep cya at 30, but I just raised it a bit to see if I can run the swg less. Water quality is consistently fabulous, but if I could run the filter and swg less time each day, that would be nice. It's a balance, though, because I also don't want to swim in Monsanto/Bayer soup, lol.

I may also experiment with keeping it covered by day and uncovered at night. (It's hot where I live).

 

[Amrie]

Please learn about active chlorine and reserve chlorine when bound to CYA by reading CYA - Further Reading

CYA lets you have reserve chlorine in the pool without chlorine harshness.

Thanks, I've read that and understand it as well as I'm able based on my current level of contextual knowledge. There's a reason I originally posted this in "deeper dive"! I understand enough (I think) to have a good starting point and track and adjust what works for my pool. I'm just really curious about all the why's.

I get that if you bump the cya, and run the swg the same time, free chlorine will increase (unless you were already producing more than the sun could eat). But will it automatically increase to the very same ratio? If the ratio was 10% and I raise the cya, would it necessarily balance out at 10% again? I mean, that makes sense in a way. Maybe I'm overthinking it, based on a belief that real life is typically less linear than that!

 

[JoyfulNoise]

Interesting. That makes sense. I don't know if it's my own assumptions or something in the way info is presented, but somehow I got the vibe that it was the other way around. I thought the ratio for liquid chlorine was the baseline and something about swg required higher, not that the swg ratio was ideal but the one for liquid chlorine was lower to allow for slamming.

I think part of my confusion is the chart linked on the CDC website, showing the effectiveness of different levels of cya at preserving chlorine in sun. It maxes out at 70ppm, and it's close to 100% even at 30ppm. So aiming for 70 or 80 seems like overkill. Is that just trying to give margin of error for rain and refills diluting it?

Please post a link to that CDC chart.

I suspect that it is the standard S-curve for chlorine loss versus CYA concentration. What you have to realize is that curve is what you generate when you do the math on paper or when you measure the effect in beakers in a lab. That’s not real life. Also, that analysis only takes into account first order chemical reaction rates and UV loss. There is enough anecdotal evidence to show that the effect is not quite as perfect as those S-curves would suggest and there are probably other UV absorptions created by CYA that are concentration and depth dependent. So what we see isn’t a flat top to the S curve but one where higher levels of CYA tend to create a linear decrease in FC loss at levels above 50ppm. This is why higher CYA levels can be very effective with SWG’s - the loss is greatly reduced and so the generator can easily maintain higher levels.

I saw this in my own pool many years ago when I maintained a CYA level of 100ppm. I had daily FC loss rates that were lower than 1ppm even in the hottest parts of the summer. The danger is obvious though - if you accidentally get algae, a SLAM will be next to impossible and very costly to pull off trying to maintain an FC level of 40ppm until the algae is eradicated.

 

[JamesW]

Meaning, theoretically I could MAINTAIN the ratio with the same run time, but to jump from one level to another, I'd have to boost the chlorine with either liquid or longer runtime to get there, then go back to the original runtime to stay there.

If you add CYA, the FC loss drops and the FC begins to rise until it reaches a new equilibrium.

For example, if the FC is 3.0 at 30 ppm CYA, your FC/CYA is 10%.

If your gain rate is 0.1 ppm per hour and your loss rate is 0.1 ppm per hour, then they are in a dynamic equilibrium and the FC remains steady at 3.0 ppm.

If you add 30 ppm CYA, the new FC/CYA is 5% and the loss rate is about half (0.05 ppm per hour), so the FC rises until the FC/CYA is once again at 10% and the loss rate matches the gain rate.

 

[JamesW]

UV follows a Bell Curve, so losses also follow a Bell curve.

A SWG typically has a cycle, but it can generally be considered to be constant.

So, the gain and loss will never be 100% exactly equal all the time.

Your FC will also follow a bell curve, but it should remain relatively stable if it is high enough.

The FC curve will be the inverse of the UV curve.

As the losses reach a maximum, the FC reaches a minimum.

The FC maximum should be just before sunrise if the SWG runs 24/7.

The minimum FC should never go lower than the FC/CYA ratio of 7.5%.

 

[JamesW]

Please post a link to that CDC chart.

The CDC's Limit for Cyanuric Acid in Swimming Pools

Cyanuric acid (CYA) protects chlorine from sunlight, but the CDC has set a CYA limit of 15 ppm for public swimming pools with a fecal incident.

blog.orendatech.com

https://cmahc.org/documents/CMAHC_Ad_Hoc_Committee_Report_on_Stabilizer_Use._WAHC_2017-10-16_FINAL.pdf

 

[JoyfulNoise]

Those curves seem awfully optimistic. 20% loss at 2.5ppm initial FC would be a 0.5ppm loss per daily exposure. Pools lose a lot more than that in the day when you consider all other sources.

At the end of the day, UV losses are only one part of the issue. You have to account for all losses and that’s hard to do when a pool has junk and critters constantly blowing into it (I just fished out a dead mouse the other day). Keeping the CYA high to lower UV losses with an SWG just removes, or reduces, one of many sources of sanitizer demand in a pool. Thus it allows the SWG to have the margin it needs to make enough FC during a generation cycle without overburdening it. It’s more of an operations problem than a chemistry problem.

 

[ajw22]

I get that if you bump the cya, and run the swg the same time, free chlorine will increase (unless you were already producing more than the sun could eat). But will it automatically increase to the very same ratio? If the ratio was 10% and I raise the cya, would it necessarily balance out at 10% again?

No, because your SWG generation does not know about ratios. It just pumps out chlorine at a fixed rate.

My CYA is 50 and my pool needs 3PPM/day to maintain my FC at 6 which requires 40% SWG generation.

I increase my CYA to 70. Due to the increased UV protection my pool will only lose 2PPM/day. If I leave my 40% setting my FC will increase by 1PPM/day. Therefore I reduce my SWG % to 30% and 2PPM/day to maintain my FC level.

A SWG has no ability to maintain any ratio. It is up to you make make the necessary adjustments to maintain the desired FC/CYA ratio as your CYA or suns UV changes with the seasons and weather.