Chlorine/Cyanuric Acid Ratio Recommendations for SWG Pools

[Tom ONeill]

Thank you "newdude". I hadn't seen this new chart guidance for SWCG pools. An improvement.

 

[JoyfulNoise]

Tom - you also operate and maintain semi-public pools (HOA). You have to be much lore concerned with pathogens than most residential owners. The only people that swim in my pool are families members that I am already exposed to. So it makes little difference if my CT kill rate is 30secs, 1min, or 12 hours. Believe me, my kids primary schools are a much greater source of infection than anything brewing in my pool.

Also, remember too that TFP recommends AT LEAST 60ppm for an SWG pool with 70-90ppm as the preferred target. We know for a fact that there are UV shielding effects at higher CYA levels that are not accounted for in the basic photolysis equations. So when you run at higher CYA, the FC lasts a lot longer and you never have to worry about running below the minimum.

And, for a data point , I ran my outdoor pool uncovered several years ago at an FC/CYA ratio of 0.025-0.03 with 100ppm CYA without a single issue all season long. So even though there are recommendations out there for absolute minimums and desirable kill times, what you can do in reality is far more flexible.

As the old saying goes, “You do you!”, because at the end of the day your pool care needs are going to be very different than others.

 

[Tom ONeill]

Thank you JoyfulNoise for your input. It's very interesting that you can run consistently well at "FC/CYA ratio of 0.025-0.03 with 100ppm CYA". At what pH are you generally running?

As an aside: I was recently talking with a very senior pool technician here in Tucson who said they had one client that insisted they maintain that client's 100,000 gallon pool at 7.2 pH. A major headache for them, as the system uses 12.5% Sodium Hypochlorite on an ORP sensor controlled peristaltic pump and a pH sensor controlled Muriatic Acid peristaltic pump to hold the pH that low. Not surprisingly they use large volumes of Muriatic and also have to manually add a lot of Sodium Bicarbonate to maintain a reasonable alkalinity.

I was just reading a TFP article titled "CYA Chlorine Relationship - Further Reading", updated 5/26/24. There was a statement, "In general pools chlorinated with tablets or liquid chlorine should have CYA up to 40ppm. Pools chlorinated using a SWG can have CYA up to 70-80ppm."
Can you point me to any source documents supporting the SWG - CYA 70-80 ppm aspect? I have yet to find anything on the web from a research paper supporting that. Other than TFP, Google is all I have to find any sources. Everything I have found so far is anecdotal information.

However, my personal thought is that a higher CYA/FC ratio has a much larger "reservoir" of chlorine that is only released as chlorine is used up. Therefore, it can hold a steadier HOCL level across the day and not drop as rapidly as would occur with a much smaller "reservoir" CYA/FC ratio. And I can see where that may be more useful in a situation where the SWG might not be able to keep up with the immediate loss and then has to play catch-up later and/or overnight, and maybe not be able to "get there" at all.
p.s., I would think an ORP controlled SWG could really be a challenge, as the controller could be trying to push the SWG beyond its maximum production rate per hour attempting to counter peak chlorine loss at mid-day.

The flip-side is that a higher CYA/FC ratio should be equally applicable, at least in this respect, to liquid chlorine use. SWG and liquid chlorine don't introduce additional CYA or calcium to the pool. Concerns about pH effect of the chlorine source? SWG produces the direct equivalent of NaOCl and appears to have a pH of 10.7-8, while liquid chlorine can be over 13.

The only downside I can see would be that a high CYA/FC ratio will make it more difficult to SLAM the pool.

In our pool operations, we don't have ORP controllers. We use measured continuous chlorine feed across 24 hours/day (our pools run 24/7) to match daily chlorine consumption. Presently (at 6/19/24), we can start at sunrise with a .08 FC/CYA ratio and bottom out at .05 around 5:00 PM, and then the continuous feed recovers through the evening and overnight. Nowhere near that sort of operating profile in spring, fall and winter months.

Thanks

 

[Newdude]

Can you point me to any source documents supporting the SWG - CYA 70-80 ppm aspect?

We are our own science here. With thousands of data points, that's where we ended up.

The only downside I can see would be that a high CYA/FC ratio will make it more difficult to SLAM the pool.

Bingo. For newbs there's a learning curve with expensive consequences if they goof. For the rest of us, As much as we care for our pools, we are all but one family emergency from the pool being thrown on the back burner.

 

[wayner]

How do you also account for issues like a solar cover being on or off? Should I be adjusting my SWCG for when my cover is off?

 

[Newdude]

How do you also account for issues like a solar cover being on or off?

You effectively have two pools and need to know how they both respond at any point in the season. Then adjust your FC accordingly for tomorrow. Your choice if you use the SWG to cover the loss when open, or just spike the FC with LC if you're going to have an all day weekend swim.

Or the FC was high enough either way and it doesn't matter.

 

[wayner]

Thanks - I have so far gone with the latter strategy - my CYA is about 50 and FC 6. I have been meaning to get the CYA up a bit higher but haven't been to the pool store in a while since I added the last dose of CYA.

 

[Newdude]

I have so far gone with the latter strategy

The best part about being free and clear of min is that it doesn't matter how you do it.

You can even go back and forth based on convienence at that time.

I'm too lazy to fiddle with any of it so it's #teamrunhot for me. Doesn't matter who or what, my FC is goooooooooood today.

 

[wayner]

I agree. I normally keep the cover on during the week as we typically only swim on weekends but I left it off this week since I won't need it to keep the water at 84. And it is nice to go for a spontaneous dip when it is this hot.

 

[JoyfulNoise]

Thank you JoyfulNoise for your input. It's very interesting that you can run consistently well at "FC/CYA ratio of 0.025-0.03 with 100ppm CYA". At what pH are you generally running?

As an aside: I was recently talking with a very senior pool technician here in Tucson who said they had one client that insisted they maintain that client's 100,000 gallon pool at 7.2 pH. A major headache for them, as the system uses 12.5% Sodium Hypochlorite on an ORP sensor controlled peristaltic pump and a pH sensor controlled Muriatic Acid peristaltic pump to hold the pH that low. Not surprisingly they use large volumes of Muriatic and also have to manually add a lot of Sodium Bicarbonate to maintain a reasonable alkalinity.

I was just reading a TFP article titled "CYA Chlorine Relationship - Further Reading", updated 5/26/24. There was a statement, "In general pools chlorinated with tablets or liquid chlorine should have CYA up to 40ppm. Pools chlorinated using a SWG can have CYA up to 70-80ppm."
Can you point me to any source documents supporting the SWG - CYA 70-80 ppm aspect? I have yet to find anything on the web from a research paper supporting that. Other than TFP, Google is all I have to find any sources. Everything I have found so far is anecdotal information.

However, my personal thought is that a higher CYA/FC ratio has a much larger "reservoir" of chlorine that is only released as chlorine is used up. Therefore, it can hold a steadier HOCL level across the day and not drop as rapidly as would occur with a much smaller "reservoir" CYA/FC ratio. And I can see where that may be more useful in a situation where the SWG might not be able to keep up with the immediate loss and then has to play catch-up later and/or overnight, and maybe not be able to "get there" at all.
p.s., I would think an ORP controlled SWG could really be a challenge, as the controller could be trying to push the SWG beyond its maximum production rate per hour attempting to counter peak chlorine loss at mid-day.

The flip-side is that a higher CYA/FC ratio should be equally applicable, at least in this respect, to liquid chlorine use. SWG and liquid chlorine don't introduce additional CYA or calcium to the pool. Concerns about pH effect of the chlorine source? SWG produces the direct equivalent of NaOCl and appears to have a pH of 10.7-8, while liquid chlorine can be over 13.

The only downside I can see would be that a high CYA/FC ratio will make it more difficult to SLAM the pool.

In our pool operations, we don't have ORP controllers. We use measured continuous chlorine feed across 24 hours/day (our pools run 24/7) to match daily chlorine consumption. Presently (at 6/19/24), we can start at sunrise with a .08 FC/CYA ratio and bottom out at .05 around 5:00 PM, and then the continuous feed recovers through the evening and overnight. Nowhere near that sort of operating profile in spring, fall and winter months.

Thanks

Mark (@mas985 ) did an experiment many years ago that showed there was more screening going on than could be described by the standard chlorine loss equations. Perhaps he can link to that thread ... it's an old one but a good one.

As @newdude alluded to, there isn't a lot of science to point to on the matter of higher CYA levels for SWG's, but we have tons of anecdotal evidence that would point a true scientist in the direction of studying the phenomenon. That said, no one has ever really done any work to tease out the mechanisms because there is either no profit in it or anyone interested in doing so simply doesn't have the time, money, or technical resources to do the work required. Obviously "the plural of anecdote is not data" applies here but those that run SWGs in hot and high UV climates like ours see a big benefit to running their CYA at higher levels. I, for one, know exactly when I need to increase my CYA every season without doing any reagent testing ... I will see a distinct INCREASE in FC demand to the point where I have to add more than a 20% increase to my SWG to keep my FC up. When I see that happen, I measure my CYA and, sure enough, it is below 60ppm. As soon as I bump my CYA up to 80ppm, and without changing the SWG output, my FC will go through the roof by the next day. It is a very dramatic and measurable effect.

The thing to keep in mind about chlorine chemistry is that there is a significant difference in manual chlorination versus electrochlorination. With manual additions of LC, you are adding a high pH solution of sodium hypochlorite into the pool. It takes time for that to disperse and the hypochlorite achieves equilibrium with hypochlorous acid as the solution mixes and the pH drops down to normal pool levels. Free cyanuric acid will react with those chlorine species and buffer (hold) the chlorine atom in reserve. Similarly, in an SWG, the electrolysis of brine creates chlorine gas which then dissolves into solution and undergoes a hydrolysis reaction where a water molecule is consumed and a molecule of hypochlorous acid and hydrogen are created. Because of the equilibrium constants involved, hypochlorous acid is the dominant species at the anode and using brine at pool water pH values. There is significant flow into and through an SWG cell such that the bulk of the water remains near the pool's pH but the water nearest the cathode will be slightly higher pH due to hydroxide formation. So for an SWG the predominant active chlorine species is hypochlorous acid which will then eventually come into equilibrium with hypochlorite as it mixes into the pool water. Further equilibrium will happen with free CYA to take up the chlorine into reserve.

So while the end result of these chlorine additions are the same, the routes they take to get there are somewhat different. This is where any good scientific experiment would begin and it would be a fruitful area of discovery to tease out how different levels of CYA affect the different species of chlorine and how they are created. The equilibrium reactions between chlorine and CYA should be fast but we have no idea how those reactions rates are modified when an electrolysis cell is used. It's one thing to mix chemicals up in bulk solutions and do standard tests on them to measure equilibrium constants, but it's an entirely different exercise to construct flow cells for electrolysis and then look at chlorine creation and reactions in real time.

 

[mas985]

Here are the threads for the topic:

CYA Testing Update

I thought I would give those who are interested an update on some testing that I have been doing regarding CYA levels and chlorine. First a summary of the testing that I did in this thread. - Running my spa with the SWG at CYA levels from 45-80 ppm showed no change in chlorine production. -...

www.troublefreepool.com

CYA vs UV Chlorine Loss Test Update

Many years ago, I did a test of chlorine loss vs CYA level and published the results here: https://www.troublefreepool.com/threads/cya-testing-update.2984/ There were a few issues with the test that I wanted to resolve with some additional testing but never got around to doing it...until now...

www.troublefreepool.com

 

[wayner]

@JoyfulNoise - Would the testing that you call for also have to take into the account the local climate? Would the conclusions possibly be different for you in Tucson, vs me in Toronto?

 

[JoyfulNoise]

@JoyfulNoise - Would the testing that you call for also have to take into the account the local climate? Would the conclusions possibly be different for you in Tucson, vs me in Toronto?

There’s certainly lots of variables in outdoor pools. The fact that my pool (and most in Tucson) never close (they just go unused) definitely would affect pool care as well as the more extreme UV here than what you see way up north. Even with careful experimentation you’re only going to get a slightly better prediction of where to start and then refining will take into account local conditions.

 

[Tom ONeill]

"wayner",
The rate constants for photolysis of exposed HOCL/OCl- are affected only by the intensity of the UV radiation applied (received). UV radiation is a fairly fixed fraction of total solar intensity, and the amount received is also affected by atmospheric screening and altitude over sea level. The effect on photolytic chlorine loss is directly proportional to UV and total solar intensity, so your latitude, north or south of the equator, time of year, time of day and atmospheric conditions, will affect the rate of chlorine loss from UV photolysis. You will definitely lose significantly less chlorine per hour of daylight in Toronto than in Tucson, AZ.