Why higher CYA levels in SWG pool?

[Helter]

I've seen a lot of mention that a salt pool wants a higher CYA level of 70ish, but can't find any explanation of *why*. Is there a writeup somewhere that someone can point me to? It's a bit confusing to me, because it seems like CYA protects the chlorine, but also requires me to have more chlorine, so why wouldn't I just keep the levels of both lower? If CYA sequesters chlorine and makes it unavailable... that doesn't seem like something I'd want.
Does it matter if my pool is in the woods and gets fairly little direct sunlight, but lot's of leaves bugs and dirt? I think I only get about 2 hours of full direct sun on the pool a day (which is nice, but also chilly).

 

[mknauss]

You are welcome to maintain a lower CYA level if you wish in your pool. The TFP recommended levels are based on data as presented in Pool Water Chemistry and anecdotal data from 1000's of pool owners that find maintaining slightly higher CYA level allows them to generate less FC each day to maintain the target FC levels.

We live in dramatically different climate zones, so your results will likely be different than mine. But if my CYA drops below 70 ppm in the swim season (April - October) my FC consumption each day jumps by 25-50%. Yours may not.

Be sure your FC does not fall below 7.5% of your CYA at any time and you should be fine.

 

[mgtfp]

It's a trade-off: Higher CYA provides better protection from UV, but also requires higher chlorine levels, which makes a SLAM at CYA 80 quite challenging.

With a well dialled in SWG, it's unlikely to drop below min levels, so you'll likely never have to SLAM, and it's worth taking the benefits of less chlorine use at higher CYA. This translates ultimately into a longer lifetime of your SWG, as you have to create less chlorine each day.

With liquid chlorine it's more likely to miss adding chlorine from time to time, and the higher likelihood of having to SLAM outweighs the benefits of higher CYA. Since SLAMs are more likely to happen, you want to keep CYA at a slammable level.

 

[mgtfp]

If CYA sequesters chlorine and makes it unavailable...

It doesn't make it unavailable, it keeps it in reserve, the chlorine bound to CYA is sometimes also called "reservoir chlorine". The unbound, or "active chlorine", more precisely HOCl, is immediately available to do its job. Then it gets turned into chloride (salt), and reservoir chlorine gets released following the equilibrium equations and turned into active chlorine.

Like that, you always have a nice buffer of chlorine, without actually having uncomfortably high levels of active chlorine, to survive a hot, sunny day with lots of bathers, some of them too busy to make it to the bathroom in time.

 

[Helter]

You are welcome to maintain a lower CYA level if you wish in your pool. The TFP recommended levels are based on data as presented in Pool Water Chemistry and anecdotal data from 1000's of pool owners that find maintaining slightly higher CYA level allows them to generate less FC each day to maintain the target FC levels.

We live in dramatically different climate zones, so your results will likely be different than mine. But if my CYA drops below 70 ppm in the swim season (April - October) my FC consumption each day jumps by 25-50%. Yours may not.

Be sure your FC does not fall below 7.5% of your CYA at any time and you should be fine.

Thanks! That makes some more sense to me.

 

[Helter]

It doesn't make it unavailable, it keeps it in reserve, the chlorine bound to CYA is sometimes also called "reservoir chlorine". The unbound, or "active chlorine", more precisely HOCl, is immediately available to do its job. Then it gets turned into chloride (salt), and reservoir chlorine gets released following the equilibrium equations and turned into active chlorine.

Like that, you always have a nice buffer of chlorine, without actually having uncomfortably high levels of active chlorine, to survive a hot, sunny day with lots of bathers, some of them too busy to make it to the bathroom in time.

Ok, this was a piece of the puzzle I was missing. I didn't understand that the bound chlorine could still be accessed/used, I thought it was just "lost" effectively.

 

[Orion7319]

My pool only gets full sun for about 2 hours a day. My CYA is 50. My cell is generating 1.5ppm per day. Some days my chlorine levels go up. Most days they stay the same. My water temps don’t usually get above 85 and I run the SWG 24/7 and I keep the chlorine on the high end (I like to try and maintain my chlorine at 8) Last year I was only running my SWG 8 hours a day and my CYA was 60. I would get algae sometimes after a big rain event. I haven’t had a speck of algae this year, and I’ve had more big rain events then last summer. I do know that if my pool was getting full sun all day, and my water temps were getting hot, I would need to run the CYA higher. I also know that my pool is the exception to the rule. Most pools aren’t partially to fully shaded most of the day.

 

[SoDel]

I’ve seen a few different theories on why lower FC / CYA ratio and high CYA level works ok with SWG pools and likely they’re all correct in some fashion and work together. I have an additional theory that the SWG itself and the chlorine with the water flowing down the return pipe acts as a primary sanitizer for all of the water passing through it, and a lesser residual FC in the pool is thus adequate to control pathogens and algae in a residential pool. Seems as plausible as the other theories and like I said, all the various theories appear valid and may stack together to actually make it work with lower FC.

 

[Helter]

I’ve seen a few different theories on why lower FC / CYA ratio and high CYA level works ok with SWG pools and likely they’re all correct in some fashion and work together. I have an additional theory that the SWG itself and the chlorine with the water flowing down the return pipe acts as a primary sanitizer for all of the water passing through it, and a lesser residual FC in the pool is thus adequate to control pathogens and algae in a residential pool. Seems as plausible as the other theories and like I said, all the various theories appear valid and may stack together to actually make it work with lower FC.

That's actually how I thought SWGs worked until I got here. I thought the chlorine sanitized the water in the pipe, then any remaining chlorine recombined with the sodium (somehow?) to create salt again.

 

[ajw22]

I’ve seen a few different theories on why lower FC / CYA ratio and high CYA level works ok with SWG pools and likely they’re all correct in some fashion and work together.

The LC chart assumes you only add chlorine cone a day. Therefore you need to have sufficient chlorine in the pool to not drop below the minimum between chlorine additions.

The SWG chart assumes that a small amount of chlorine is generated slowly over most of the day. The pool FC never sees a big dip in FC as there is some constant adding of chlorine.

The only place where the SWG chart can break down is if a very small pool ahs a very oversized SWG and all it's chlorine needs are generated in an hour or two of pump running. We have seen some folks do this and they need to run a higher FC level more like the liquid chlorine pools to allow for more of a FC dip between when the SWG runs.

I have an additional theory that the SWG itself and the chlorine with the water flowing down the return pipe acts as a primary sanitizer for all of the water passing through it, and a lesser residual FC in the pool is thus adequate to control pathogens and algae in a residential pool.

Your theory could be valid if you could have 100% turnover of water passing through the cell daily to sanitize it. In fact 1 turnover would only pass 63% of the water through the cell daily. Read Turnover of Pool Water - Further Reading

 

[SoDel]

The LC chart assumes you only add chlorine cone a day. Therefore you need to have sufficient chlorine in the pool to not drop below the minimum between chlorine additions.

The SWG chart assumes that a small amount of chlorine is generated slowly over most of the day. The pool FC never sees a big dip in FC as there is some constant adding of chlorine.

The only place where the SWG chart can break down is if a very small pool ahs a very oversized SWG and all it's chlorine needs are generated in an hour or two of pump running. We have seen some folks do this and they need to run a higher FC level more like the liquid chlorine pools to allow for more of a FC dip between when the SWG runs.



Your theory could be valid if you could have 100% turnover of water passing through the cell daily to sanitize it. In fact 1 turnover would only pass 63% of the water through the cell daily. Read Turnover of Pool Water - Further Reading

The whole chart thing is partly based on experience which makes it a circular argument. It works because we’ve observed it to work and that’s why it works.

For your second observation, no primary sanitizer is any different in terms of not ever really reaching 100% of the water yet are still used effectively in many situations, along with a chlorine residual for secondary sanitation. That’s sort of the concept of a primary sanitizer. 63% of water having had everything in it well killed and oxidized in the SWG cell will substantially reduce the load for the residual sanitizer, which I think pretty well validates, if not proves, the theory.

 

[SoDel]

That's actually how I thought SWGs worked until I got here. I thought the chlorine sanitized the water in the pipe, then any remaining chlorine recombined with the sodium (somehow?) to create salt again.

It does do that, but it also generates the necessary extra chlorine residual in the water. An experiment would be to use a cell that runs 100% of the time 24/7 but does not generate enough chlorine to produce a residual and see what happens in the pool (but don’t swim in it). I’m not willing to try that in my pool and see what happens in case the result is bad lol.

 

[Orion7319]

It does do that, but it also generates the necessary extra chlorine residual in the water. An experiment would be to use a cell that runs 100% of the time 24/7 but does not generate enough chlorine to produce a residual and see what happens in the pool (but don’t swim in it). I’m not willing to try that in my pool and see what happens in case the result is bad lol.

Hummmm I think we’re going to need a bigger pool…
Yep a MUCH larger pool! Or a lake perhaps…

 

[SoDel]

Hummmm I think we’re going to need a bigger pool…View attachment 437883
Yep a MUCH larger pool! Or a lake perhaps…View attachment 437885

Or a smaller cell. If you are willing to do this experiment on your pool, I am willing to round up an old cell and perform some “modifications.”

 

[Orion7319]

Or a smaller cell. If you are willing to do this experiment on your pool, I am willing to round up an old cell and perform some “modifications.”

Those calculations I ran were the smallest cell listed in pool math Your the one who’s supposed to run all the experiments! I’m just the experimental critic…

 

[SoDel]

Those calculations I ran were the smallest cell listed in pool math Your the one who’s supposed to run all the experiments! I’m just the experimental critic…

I thought of a better way to do it. A separate power supply to a stock cell which can limit and allow simple adjustment of the average current. Chlorine output is related to cell current. Scouring global sources of expensive and sophisticated lab equipment, I found something that will work lol.

https://www.amazon.com/RioRand-Controller-Efficiency-Generating-Protection/dp/B007TH4EN6

 

[Orion7319]

I thought of a better way to do it. A separate power supply to a stock cell which can limit and allow simple adjustment of the average current. Chlorine output is related to cell current. Scouring global sources of expensive and sophisticated lab equipment, I found something that will work lol.

https://www.amazon.com/RioRand-Controller-Efficiency-Generating-Protection/dp/B007TH4EN6

I don’t think that the salt cells use variable current at all. They just run 100% for X amount of time. I don’t think you can modify them either like that…. So sayeth the experimental critic…

 

[mgtfp]

I thought the chlorine sanitized the water in the pipe, then any remaining chlorine recombined with the sodium (somehow?) to create salt again.

It never recombines with sodium. NaCl dissolves as Na⁺ and Cl^- in water. An SWG only cares about the Cl^-, it doesn't matter where it comes from. You could turn a pool that has been run on liquid chlorine for a couple of years into a SWG-pool simply but using all the Cl^- residues from all the added chlorine without additional salt. When we refer to "salt", we actually just mean Cl^-, that's also what the Taylor salt test actually tests for.

A SWG turns chloride (Cl^-) into chlorine (Cl₂), which dissolves as HOCl in water (and OCl^- and all the chlorinated Cyanurates). When HOCl kills something, it gets turned back into Cl^-.

 

[SoDel]

I don’t think that the salt cells use variable current at all. They just run 100% for X amount of time. I don’t think you can modify them either like that…. So sayeth the experimental critic…

You are right that almost all of them work that way but the reason is that non-regulated supply is the cheapest, and the controller just turns it on and off, over very long time periods (like many minutes on vs many minutes off). There is a complex relationship for optimum chlorine production between voltage, current (determined by voltage as a function of electrolyte conductivity via a simple Ohm’s law calculation) and cell on time. E.g. Optimization of free chlorine, electric and current efficiency in an electrochemical reactor for water disinfection purposes by RSM with very pretty 3d surface graphs.

Can you take the NIH paper and my word for it that chlorine production varies with current (within limits, of course) or how can I convince you? Don’t get over~confused or distracted by how most things work when why they work that way is driven by mass production cost and simplicity. We don’t have those constraints.

I can very easily limit current to a stock cell and reduce its chlorine production. How long it will last and how much generally undesirable H and O2 and desirable Cl(g) produced versus energy used are separate questions that are irrelevant to the experiment.