# Thread: CYA and SWG; check my math and ? of timing

1. ## CYA and SWG; check my math and ? of timing

Please see if I am doing this math correctly, I am way out of my comfort zone on this.

SWG is capable of making 283 gm chlorine in 24 hrs at 100% setting, per owners manual. What time and setting do we need to use for SWG? No one will be around for several days and so I am trying to get this right the first time. Owner returns Friday and can test on her own then and call me. Would be best if FC could stay where we left it. Hate for it to be too low and let black algae return before owner returns on Friday. Also not good for FC to be too high for kids that arrive on Friday.

I'm thinking if one gallon of water = 3780 gm per online calculator, then 17K gallon pool = 64,260,000 gm water. If SWG is on 100% for 8 hrs (or 80% for 10 hrs) it should make 94 gm chlorine. That is 94/64,260,000 = 0.000001 or 1 ppm. Is this right????
So it may be that this SWG can makeup 1.0 ppm/day. The SWG, Jandy AquaPure700 is undersized for the pool size and heat of this area. And it is 3 yrs old.

We are ready to increase the CYA level from 30 (during shocking) to 85 for my friend's SWG pool at the beach. FC is still high and fell from 22.5 ppm at dawn to 11 ppm at 5 PM yesterday. Or about 50% drop at CYA 30.

What FC % drop should we expect at CYA = 80?
Trying to decide whether to add CYA now (FC 10) or later today (FC might be 5). Once it dissolves, is it actually effective? Or is it effective a week later when it registers on test?

We can hope for FC to fall to 5-ish first, then add CYA and hope FC holds when we turn on the SWG --- or we can add the CYA now and hope the FC still falls once we turn the SWG back on.

2. ## Re: CYA and SWG; check my math and ? of timing

I get a similar number, but not exactly the same as what you get. 283 g of chlorine is about 10 oz. 10 oz of chlorine gas in 17,000 gallons will raise the FC level by 4.4 over 24 hours, or about 1.5 in 8 hours.

Daily FC loss with CYA around 80 should be right around 1.5 ppm, though it could be a little higher or lower. So I get that this SWG is marginal but should be usable.

3. ## Re: CYA and SWG; check my math and ? of timing

That is wonderful. We added the CYA when the FC was at 7.5 and it was exactly there when we retested at 3 PM. SWG is set at 80% for now.

I really appreciate the pool calculator that helped to predict the pH effect of the CYA, I had not thought about that before since I use trichlor pucks in my pool when CYA is low.

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