OCLT Fail-Questions About SLAM

[RexO]

Merged threads - TFP Mod
Went on vacation for 1 week. Upon my return, all was expected except the FC was 2.5. When I left, the CYA was 70 so I was a bit concerned about the low FC. And, I was running an iChlor30 for 11 hours a day at 80%, so I thought I would hold my target FC of 5. I turned off the SWG and added liquid chlorine to bump the FC to 7. There was plenty of leaves at the bottom of the pool so I vacuumed and brushed. The water was still clear, but I lost 2 FC on the OCLT that evening. I kept up the FC through the day and started a SLAM last night. Prior to SLAM, my CYA was 60, so I targeted 24 FC for the SLAM, overshot just a bit to 25...tested at 10 PM. The water looks crystal clear, but this morning before sunrise the pool tested at 20 FC. So, I guess I am going to maintain this SLAM for another day...just added more liquid chlorine.
So, finally my question. I know TFP does not recommend Enzymes. I am convinced to discontinue the use. I was planning just to finish up the bottle of Orenda CV-600 that I had purchased, then be done with it. So, there was a maintenance dose of CV-600 in my pool. Could the enzyme be throwing off my OCLT? Is the SLAM burning up the enzyme and that could be the cause for my FC loss?

 

[RexO]

After returning from 1 week of vacation, I found my FC fell well below target. My iChlor30 ran at 80%, 11 hours daily. I bumped up the FC to 7, vacuumed and brushed, then did the OCLT. Dropped from 2 FC overnight so I started the SLAM last night. 60 CYA...so bump FC to 25 but lost 5 overnight....Fail. New I could not add Chlorine frequently while working so bumped it to 28 FC in the morning. Cheched 6 hours later at 1 PM and back down to 20 FC. The Chlorine seems to be burning off super fast for a pool with crystal clear water and no visual signs of algea or debris in water. Am I right to stay with the SLAM? I do have some residual Enzymes (Orenda CV-600) in the pool. I planned to discontinue based upon advice at TFP. Could the residual enzymes be the cause of my FC loss?

 

[Newdude]

An overnight loss of 5 is definitely something. You could very well be on the brink of an outbreak but not enough to be visible. This is where we have the most problems with other new people believing us when the pool appears to be fine. Your tests always tell the real story. Listen to them. Also remove, empty or clean any of the usual trouble spots. Ladder, lights, handrails, weir doors etc and run your water features good if you have them to ensure all the old/bad water is not hiding anywhere.

I don’t know much about the enzymes but imagine they could be a source of issue too. @JoyfulNoise is the guru on all things science.

 

[JoyfulNoise]

Try to stay as close to SLAM level as possible. Adding too much extra FC is only a waste as the rate of UV loss increases with increasing FC. The 40% FC/CYA target is intended to fix issues (organics, algae, etc) without wasting too much chlorine.

Daily losses during the sunlight hours really don't tell the whole story as it is pretty easy to lose 2-4ppm during the day in a clean pool, just keep an eye on overnight losses and, when you think the FC is holding pretty well overnight, then do a really careful OCLT. It's best if you can measure and dose after sunset and before sunrise and then not worry too much about the daylight hours unless someone can help you out.

 

[RexO]

Try to stay as close to SLAM level as possible. Adding too much extra FC is only a waste as the rate of UV loss increases with increasing FC. The 40% FC/CYA target is intended to fix issues (organics, algae, etc) without wasting too much chlorine.

Daily losses during the sunlight hours really don't tell the whole story as it is pretty easy to lose 2-4ppm during the day in a clean pool, just keep an eye on overnight losses and, when you think the FC is holding pretty well overnight, then do a really careful OCLT. It's best if you can measure and dose after sunset and before sunrise and then not worry too much about the daylight hours unless someone can help you out.

Thanks. Ok, pool just burned off another 5 ppm in 3 hours during the heat of the day...25 back down to 20 ppm. Basically burned off 10 ppm in less than 11 hours. But thats mostly because of the high level of FC, right? Not as crazy as it seems? It is hot and the pool is getting direct sunlight all day. Just wonding if my real SLAM target is 20 cause is losing it so quickly above 20 ppm. But, that CYA test looked like a 60 to me.

 

[JoyfulNoise]

Yeah, that’s not outrageous to me. TFP normally recommends a 7.5% FC/CYA ratio. SLAM is a 40% ratio. Loss of chlorine to sunlight increases with increasing FC concentration and the loss is exponential. So you can expect a much faster loss of chlorine to sunlight at high FC than at normal FC. The OCLT will be your best guide for when the pool is in good shape.

If your CYA test looks like it’s 60ppm, then just call it 60ppm. It’s always better to err on the high side with CYA.