help understanding CYA level in SWGC pools vs non

[camueller]

I've been running with the TFP method since opening my pool this summer. I get what I'm supposed to do for my setup, and have been following faithfully. But I always wonder why, when SWGC is just another method to add chlorine besides dumping in bleach, would the CYA levels vary?

From what I can find searching on this topic, SWGC manufacturers usually recommend a specific CYA range. I've also read that they have to maintain higher CYA to protect the FC that is generated since it is generated in such small amounts. Which all makes sense, but does not explain why a target you'd target 5FC with 70ppm CYA when a non SWG pool would target that with 40ppm CYA.

If the issue is that with SWG you are running closer to a minimum, but getting away with it because you are constantly adding chlorine, then I should be able to get away with the same recommendations by using my stenner to constantly add chlorine throughout the day, rather than dumping a bunch in once or twice per day.

The CYA/chlorine chart page just says that "Most SWG pools appear to prevent algae at lower FC levels" but gives no indication as to why. If it's the salt, then the salt I added for "feel" should allow me to run with lower FC levels too...but the fact that EVERYONE told me to run my pool as normal when I did the salt thing made me think that must not be right.

So, can someone tell me more definitively why a SWG pool can maintain lower FC levels at higher CYA levels?

 

[JoyfulNoise]

SWG pools use a 5% minimum for the FC/CYA ratio. For liquid chlorine dosed pools (bleach or liquid chlorine, manual or Stenner), the ratio is 7.5%.

The recommendation is based on experience. The idea is that the SWG cell produces a much smaller dose of chlorine added to the water over longer periods of time than the other methods. The inside of an SWG cell, when it is running, is an incredible harsh environment - high FC, high pH and the formation of both oxygen and hydrogen gas. Therefore many have postulated that the water that does travel through the cell is being super-chlorinated/shocked well above any levels present in manual or automated bleach dosing. A survey is currently being run by TFP Guide needsajet to see how Stenner users dose their pools.

The reason for the higher CYA recommendations is solely to save on SWG cell life. When CYA levels are high (I run my pool at 80ppm CYA), there is a non-linear protective effect on the FC such that the daily loss rate to UV is a lot lower at higher CYA than what the simple chemistry of chlorine buffering would predict. The belief is that the CYA itself (whether bound with a chlorine or not), absorbs some of the UV light in the pool and radiates it back at a lower wavelength, thereby shielding the FC from UV photolysis. So if one lowers the daily loss rate to UV, then one has to run their SWG less frequently. All SWG cells have finite lives, around 10,000 hours or so, thus anything you can do to shorten run times saves you from having to replace a cell more frequently.

You could always try to lower your FC/CYA ratio to 5% and see if that works for you. The FC/CYA ratio is not some kind of hard scientific rule derived from first-principles. It's a phenomenological quantity derived from experience and statistics. Some pools, for whatever reason (mine included), can operate at low FC/CYA ratios and not go cloudy or green or otherwise develop any sort of problem. Some people have pools that will turn into swamps or get bad algae outbreaks the minute their FC drops below the minimum guidelines. You have to determine for yourself what your pool's behavior is.

 

[camueller]

ahh, ok. Thanks for that thorough explanation. I did not realize the salt cell was a harsh environment and almost doing double duty to sanitize the water. That definitely helps me get it.

And I guess it is true I could always try things a little lower and see how they work out...but honestly, I wasn't so much worried about my levels being higher as much as just itching to understand what the difference was.

Thanks again!

 

[needsajet]

As Matt mentioned, the harsh environment inside the cell is only a theory, with no evidence, so don't hang your hat on it.

Consistent chlorine dosing is more likely to be the driving factor which leads to real pool success, which allows TFPC to include the recommendation, and thereby conserve SWG cell life as Matt explained. There's other possibilities as to why SWG pools have success with a lower minimum. For example, it's possible that a higher % of SWG owners also use borates to dampen pH rise, and benefit from the algaestatic properties of borates. Another hypothesis is that there's a synergy effect between FC, CYA and the salt, but there's not enough non-SWG pools with 3000-4000 ppm salt to see any trend. So for me, I just think of "SWG 5%" as a phenomena with plenty of empirical evidence to back it up. I have to be comfortable not entirely knowing why, but knowing it works.

And I can raise my CYA because the SWG makes me a bit more sure I won't need to SLAM. Thus I gain the benefit of the larger FC reserve, and perhaps the higher CYA is beneficial through physical screening or changing of UV light that reduces FC loss. There's a fair number of non-SWG pools here using higher CYA for the same reasons.

We're really curious about chlorinating liquid dosing being similar enough to SWG to allow a lower FC/CYA ratio. So if you were able to set your Stenner to dose relatively frequently through the day, and try FC/CYA at 5% as the low end of your range, that would be great. Especially good to try in a new clean pool. Not that I'm recommending it, but if a few liquid dosers try it, then we might, over a few years, be able to add a tool to the toolbox for future pool owners.

In addition to what Matt mentioned above, manufacturers have used the 70/80 ppm CYA in their recommendations suggesting they know that something works better. I've read some complicated stuff about the cell that I barely understood. One of Chem Geek's explanations covers the topic, but I'm unable to find it again at the moment, but one thing that stuck with me was that physically longer cells would be helpful for the same reasons. I was really scratching my head by the time I got to that part!

But the key thing is that TFPC recommended levels work on lots of pools, so we can follow those any day of the week and have safe, clean, sparkly pools. Experts here have considered what levels take an owner straight down the middle of the lane and not too close to the ditch or the oncoming traffic, for average conditions and level of attention.

But for my own pool, I've learned enough and done enough testing to be a bit edgy, including TA less than recommended minimum and pH above recommended maximum. In the same sense, if you want to in your own pool, you can use 80 ppm CYA when you're comfortable you won't need to SLAM, and gain the benefit of higher CYA. But if someone asks me for the best levels, I'll recommend TFPC levels because of the expertise and wisdom of all the experts here who have seen many pools go sideways when people get too edgy. Hopefully that makes sense.

 

[camueller]

that makes perfect sense. Thank you both for taking the time to explain and help me make as best sense of this as I can.

having invested in my stenner setup already, I don't see myself getting a SWGC anytime soon. But I do think I may experiment a little. At last measure (though I did just get 5+ inches of rain and the pool overflowed, also pumped out a lot) my salt level was 2800ish, my borate 40ppm ish, and CYA 50. My pool does not see a big bather load, and doesn't really get dirty much. The previous owner ran with a super high CYA level and often forgot to refill the chlorine canister in the old Frog XL system. With all that said, I suspect I can get away with a lower FC level for my CYA, but only I can figure that out.

Thanks again! I really appreciate you all talking this out with me.

 

[Vickery]

On the "hostile environment of the swg cell" concept, does the actual electrical field generated around the plates work to destroy the algae and bacterial DNA, thus cleansing at least the portion of algae and bacteria that passes through it? Cleansing through electrocution?

 

[Holydoc]

Thanks all for participating in this thread. It is indeed very interesting to hear the WHY behind the HOW. It is also refreshing to hear that sometimes we have just not figured out the WHY yet, but from experience have figured out the HOW. As Camueller and Needsajet experiment more with different scenarios, maybe we can continue to expand on the HOW of maintaining our pool. It sure would be quite the study for a Chemist/Engineer in search of their doctorate to explain better the WHY.

Maybe RIO was right when they said that pool chemistry is not an exact science....YET.

 

[lightingguy]

There is nothing in-exact about the chemistry. Since pools exist in such a wide variety of environments - averages are constructed that work for the vast majority of users. The chemistry never changes - it's all the other aspects for the water that vary widely.

What happened in RIO - and what was said about it afterwards - was just stupidity. Any member of this website who understands the basics would have done a perfectly fine job of managing those pools.