FC/CYA for swg

[mkflyfish]

So this information from the CYA sticky above explains why the FC recommendation is different between liquid and swg pools -

“3Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.”

But why is the Recommended ideal level of CYA higher for SWG than Liquid Chlorine ? I can’t seem to find this answer.

My IC40 manual recommends FC 2-4ppm and CYA of 30-50ppm. Do other swg recommend higher CYA?

Thanks,
Michael

 

[Texas Splash]

But why is the Recommended ideal level of CYA higher for SWG than Liquid Chlorine ?

Welcome to TFP! Basically, with SWGs you have a lower FC level being produced & maintained continuously through the 24 hr period as opposed to the one-time liquid dumping. That along with the production of chlorine gas (versus liquid) has shown that a higher CYA protects the chlorine gas better and helps to maintain a more consistent/reliable FC level. The generic 30-50 recommended range is still provided by many in the industry and assumes pool owners will use chlorine tabs that are not only very acidic, but also use stabilizer in each puck. There fore that level increases exponentially requiring the pool to be drained in a short period of time. Today, there are more manufactures of pool products (i.e. Circupool) who recognize the need for a tailored FC-CYA relationship as noted on the FC/CYA Levels, that how recommend a CYA of 70-80 for those very same reasons.

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[Newdude]

Hey Michael and Welcome !!!

My IC40 manual recommends FC 2-4ppm and CYA of 30-50ppm. Do other swg recommend higher CYA?

We have an incredible member base of real world users that report back and has shaped us over time. We start with the recommendation of 30/70, but have found many people in hot climates need to add 10 or 20 more regardless of which chlorine they are adding. (Liquid/gas).

But why is the Recommended ideal level of CYA higher for SWG than Liquid Chlorine ? I can’t seem to find this answer

Let's use your pool with 14k gallons. If you dose with liquid chlorine that's about 1/3 gallon added in one shot. It immediately raises the FC and will slowly drift down over the course of the day. Think how long that would take to burn off 1/3 gallon over 10 hours of sunshine. One drop every few seconds. It's nice and slow.

With a SWG, it is producing chlorine for the period it is set to run (8-24 hours) and would be adding a similar drop frequency as it burned off. So we raise the CYA to help protect those drops added (in gas form).

If you are producing during daylight, (the CYA/FC chart is assuming you do), you can run a lower FC and remain basically constant while staying above minimum FC, hence the reason for the lower #s on the chart.

 

[trivetman]

Newdude - I am having trouble with that logic because the cya/fc ratio is higher for both the minimum and target levels when using swg vs lc. The spike/decline pattern when dosing using lc would lead to a higher recommended target FC level (target being the high point reached immediately after lc addition) in order to stay above the same minimum fc level over the course of the day. BUT, the minimum recommended levels are also different between swg and lc, implying that the reason is something other than adjusting for the spike/decline pattern of lc dosing.

Ive also searched this site for an explanation I fully understand and haven’t found it. Tx says above it may be based in the difference between gas and liquid form of chlorine….to which I just shrug and say well ok if you say so. Ive gathered the differences in recommended levels is really evidence based without a complete understanding. I. E. Nobody really fully understands the reasons but it just seems to work better maintaining the different ratio when using swg

 

[Newdude]

the cya/fc ratio is higher for both the minimum and target levels when using swg vs lc

From what I've read about it (and what I recall ). They were able to adjust the SWG side as low as they could go, because of the scheduled dosing that happened automatically. Consistency was key here. When manual dosing, people tend to get a phonecall from the school nurse and be 2 or 4 hours late here and there. The min was left slightly higher for a little wiggle room.

@Donldson is great with these. Let's get his take on it. Hang tight.

 

[Bperry]

I. E. Nobody really fully understands the reasons but it just seems to work better maintaining the different ratio when using swg

I never assumed it was because it worked better, more that “you can get away with” a lower minimum FC based on experience/experimentation.

The issue is that achieving the “minimum” is not a binary event that triggers a chemical reaction in the water that convinces all the algae starts running for the hills to get away from the chlorine. That’s the reason there’s a range of recommended FC levels at each CYA amount. As you approach the minimum FC level, it’s “more likely” you’ll be inviting algae to move in. Some pools do better than others. You really want to stay well above the minimum.

 

[Newdude]

And true minimum is not a garaunteed algae bloom. It's just where it may begin. My many friends who hit it daily had a swamp within the first few weeks of the season. Maybe it was the 6th time, or the 23rd time. But it will get you. You may get lucky once or twice. The LC side of the chart allows some wiggle room when it's pouring rain today and you don't get out there at your perfect adding time, or you got busy. The SWG side will just add no matter what when you tell it to everyday.

 

[trivetman]

I never assumed it was because it worked better, more that “you can get away with” a lower minimum FC based on experience/experimentation.

That’s really what I meant by works better. Works more efficiently might be a better choice of words.

You really want to stay well above the minimum.

Thats not a question. I’ve learned to follow advice on this site whether I fully understand it or not

 

[deerocks]

Hi Michael,
I'm a pool novice. My 1st pool ever owned was installed last year, 13,500 gallon above ground. I'm running a SWG on 50%, my CYA ranges from 60-70 and Chlorine 4-6. My last test was CL 6 and CYA almost 70. I use the Taylor test kit. We are in upstate NY so last weekend 90 degrees, this weekend in the 70's . We have a solar cover and keep it on the pool covered when not using it. Therefore, the CL doesn't evaporate. I had a pristine pool all last summer, as well as in May at opening. I'm not worried about CYA because this isn't a hot climate.

 

[mkflyfish]

Welcome to TFP!

Thanks, and thank you for the reply.

I’ve generally had the CYA around 50 or so, and FC from 3-8, and not had many past issues.

This year though, I’ve struggled with Cloudy water. I think I’m on the right track, but if I continue to have issues, I”ll post something in a new thread.

Thanks everyone -

Michael

 

[SoDel]

I think maybe some discussion of sanitization vs oxidation and ORP might add something to this mystery? I (think) high levels of CYA reduce ORP, possibly below that needed to actually oxidize anything in a reasonable time, and also increase needed sanitization contact time. In a residential pool with low bather load, likely not a problem? Also, there are other ways of eliminating non-living organics besides brute force oxidation with chlorine so also not an issue?

I target CYA at 30. Just my preference despite using SWG for sanitation.