CYA and Testing Free Chlorine

[Metron9]

I’m understanding that high CYA means your free chlorine has to be higher according to the chart. Is it the CYA binds with the chlorine but the free chlorine test still shows it as free chlorine but the bound chlorine is not active ?
So that if I have 150 CYA and I need over 10ppm chlorine it’s still ok to swim in it since a portion is not active ?

it’s taking about 3 gal of 12% liq chlorine to push it to 10 in a 20x40 pool. With a 150 CYA but staying clear now. We are going to drain down a foot of water after this weekend and are now using softened water as our well water is very high in calcium.

 

[CrystalRiver]

Right. The CYA binds with a portion of the chlorine, which protects the chlorine from UV and also renders that portion of the chlorine inactive. That's why it's not so harsh. But the CYA will also release the chlorine quickly as the unbound chlorine level drops, so it all shows up as "free" chlorine during testing.

Keep in mind that with FC over 10, the pH test is unreliable.

 

[JohnT]

All of the FC levels in the chlorine/CYA chart are about the same effectively. The 10ppm at CYA 150ppm is less active than 3ppm at CYA 0ppm.

BTW, because the CYA test is non-linear, don't bank on high readings being very accurate. You could drain a foot and still measure 150ppm after refilling.

 

[Metron9]

All of the FC levels in the chlorine/CYA chart are about the same effectively. The 10ppm at CYA 150ppm is less active than 3ppm at CYA 0ppm.

BTW, because the CYA test is non-linear, don't bank on high readings being very accurate. You could drain a foot and still measure 150ppm after refilling.

Update August 22,2021
For three months I have been using 2 gal liquid chlorine 12.5% . Morning and evening doses. I checked daily chlorine levels using FAS-DPD . Every morning there was very little chlorine left. I stopped monitoring and just used tge 2 gal a day regimen. Recently I started testing again. A week ago I was at about 8ppm so I waited a few days and added another gallon. The last tree days without adding any chlorine I am now at about 3ppm. I decided to check the CYA and holy cow the sample was almost clear, less than 40 to cover the dot.
Previously as I wrote above it was over 150 off the scale.
I thought it was going to take 2 gal a day but now looks like a gallon a week. I will start tracking daily when I add chlorine. Water has stayed perfectly crystal clear ever since I started using liquid chlorine. After cleaning my cartridge filter each month I used diatom powder for tge first two to months, after that the water was perfect and I am no longer using the diatom powder.
It’s an indoor pool as well so I’m happy to see the CYA is still falling, it did fall to I think a bit under 100 after the large water change. We also add about 150 gal a day for evaporation but I bought a new water softener for the pool so we can keep the calcium hardness at bay.
I have also added about 5 gallons of muriatic acid over the 4 months to keep the ph down as liquid chlorine adds to the ph level.
I want to thank the folks here for opening my eyes to the CYA levels. Ph water temps stay at 89 degrees, I use a scroll compressor 2.5 ton heat exchanger about 2 hours a day since my indoor temp in the greenhouse (that’s where my pool is) usually is over 90 degrees for the afternoon hours when it’s full sun. It’s Florida in Minnesota for us up to November. I shut down for the winter though as sub zero temps keep me inside under heavy blankets as I await warmer temperatures in the spring.

 

[Metron9]

Update August 22,2021
For three months I have been using 2 gal liquid chlorine 12.5% . Morning and evening doses. I checked daily chlorine levels using FAS-DPD . Every morning there was very little chlorine left. I stopped monitoring and just used tge 2 gal a day regimen. Recently I started testing again. A week ago I was at about 8ppm so I waited a few days and added another gallon. The last tree days without adding any chlorine I am now at about 3ppm. I decided to check the CYA and holy cow the sample was almost clear, less than 40 to cover the dot.
Previously as I wrote above it was over 150 off the scale.
I thought it was going to take 2 gal a day but now looks like a gallon a week. I will start tracking daily when I add chlorine. Water has stayed perfectly crystal clear ever since I started using liquid chlorine. After cleaning my cartridge filter each month I used diatom powder for tge first two to months, after that the water was perfect and I am no longer using the diatom powder.
It’s an indoor pool as well so I’m happy to see the CYA is still falling, it did fall to I think a bit under 100 after the large water change. We also add about 150 gal a day for evaporation but I bought a new water softener for the pool so we can keep the calcium hardness at bay.
I have also added about 5 gallons of muriatic acid over the 4 months to keep the ph down as liquid chlorine adds to the ph level.
I want to thank the folks here for opening my eyes to the CYA levels. Ph water temps stay at 89 degrees, I use a scroll compressor 2.5 ton heat exchanger about 2 hours a day since my indoor temp in the greenhouse (that’s where my pool is) usually is over 90 degrees for the afternoon hours when it’s full sun. It’s Florida in Minnesota for us up to November. I shut down for the winter though as sub zero temps keep me inside under heavy blankets as I await warmer temperatures in the spring.

 

[JohnT]

Nice!