SWCG Percentages: Do they Have a direct linear effect on FC?

panamax53

Gold Supporter
LifeTime Supporter
Dec 10, 2015
86
Doral, FL
I pose this question due to an anomaly I am observing with FC consumption and SWCG outputs. Recently we have had sunny dry days without much rain here in So FL. A good time to dial in my SWCG or so I thought.
My salt water pool is balanced TA: 70, CYA 72 - 76, CH 425, Salt 3600. pH a very stead 7.7 . CSI -0.9 - -0.12. The swcg has a tiny bit of scale on it. The water is clear, filters are clean and there is no sign of algae.

Currently I am running the SWCG for 16.5 hrs per day. It produces 0.2666 ppm per hr. at 100%. It is a Jandy Apure 35. Total FC potential production at 16.5hrs * 0.2666 = 4.399

At a 60% setting total daily production is 2.639 ppm
At 40% 1.759 ppm

Two days ago the FC reading was 8.5 at 8 a.m. with the SWCG running at 60% 8.5 + 2.639 = 11.1639 minus the following morning's FC reading of 9.5 = A loss of FC of 1.6639 ppm over 24 hours
Reviewing those numbers, I say to myself I should safely be able to switch the SWCG output to 40% because that will be 1.759 ppm, an overage of 0.1 ppm, right? I switched over to 40% output and here is what happened.
Instead of the expected reading the following morning I found I did not maintain the FC level of 9.5 as expected, but that it had dropped to 8.5 for a total loss of 2.6639 ppm over 24 hours.

Can two minutes less of run time every ten minutes create a disproportionate drop in FC?

Geeks to the rescue, Please!
 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
23,128
Bedford, TX
53,

It is just not that perfect...

Every day is different.. Some days cloudy, some days more sunny, more or less bather load, adding water, water loss, etc...

I am just too lazy to go through the effort to try and micro-manage my SWCG.. I just set the % so to my FC stays at my Target or higher... This means I never have to worry about going too low and I don't care if I go a little too high.

I probably change my output % about once a month... if that..

Thanks,

Jim R.
 

riny

Gold Supporter
Aug 20, 2020
161
Poughkeepsie, NY
Pool Size
10800
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Jandy Truclear / Ei
Agreed... the exact FC change will vary a lot from day to day, even with the same SWG output. Personally I like to keep my FC in the 8-10 range and I have my SWG set to maintain that conservatively, which means it often creeps a little higher. I'll test about once a week and if the FC is 12 or more then I'll just turn off the SWG for a couple of days and let it drift back down. Remember that it's safe to swim up to SLAM level, which is around 28 at your CYA, so it's never a problem to let it go a little high.

Going low is a much bigger problem. Algae can take hold very quickly if your FC level is under, and I had it happen to me earlier this summer when I was trying to be as precise as you are. One bad storm turned my pool into a green monster overnight. So now I tend to err in the opposite direction and it's worked out well since then.

In fact, now that the weather is getting colder, I didn't even bother to adjust the SWG output. I'm just letting it push towards SLAM level which will be perfect for closing.
 
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