CYA and Clarity

pb4uswim · Apr 30, 2024

Couple questions/observations:

I opened my pool and my CYA was between 30 and 40 so let’s just say 40. When I closed my pool it was closer to 70. I thought CYA didn’t go anywhere (I’ve read stories about draining a pool to get CYA down) so why did it go down over winter? I did dump a few inches before closing (almost to the bottom of the skimmer) and it did fill back up over winter, but does that really explain about a 30 point loss in CYA?

Is there any objective way to measure clarity? I can see the bottom of my pool at the 6ft end and I’m sure someone walking up that isn’t a TFP’er would say it’s clear, but it just looks a little dull to me. I’ll do another OCLT tonight but I did pass that after I SLAM’d it. I’m thinking I just got a false sense of completion.

Which brings me back to CYA. Is there any point in adding CYA at this point. Seems like it would just make SLAM’ing more costly (more chlorine). Is there a desired level of CYA for an opening SLAM? I retested today and I’m still between 30 & 40.

The good news is that my SWG is working. For some reason, the salt really dropped over winter. I put in two 50lb bags and got it up to around 3200ppm (per my test kit). Interesting thing is that the SWG did not recognize the increase in salt for about a day. It went from 2100 (too low, light on) to 2700 (the low point per the manual) pretty quickly but the light would still not go off. I was about to put another bag in but the next morning the SWG read over 3000ppm and the indicator light was off and the “generating” light was on. Patience Grasshopper.

I’ll start the heater in a couple weeks. It’s still too cold to swim here in S.E. MI.

HermanTX · Apr 30, 2024

pb4uswim said:
I opened my pool and my CYA was between 30 and 40 so let’s just say 40. When I closed my pool it was closer to 70. I thought CYA didn’t go anywhere (I’ve read stories about draining a pool to get CYA down) so why did it go down over winter? I did dump a few inches before closing (almost to the bottom of the skimmer) and it did fill back up over winter, but does that really explain about a 30 point loss in CYA?

CYA can have some natural degradation plus the dilution of any rain water or partial drain. It is not unusual for CYA to be lower from closing to opening.

pb4uswim said:
Is there any objective way to measure clarity? I can see the bottom of my pool at the 6ft end and I’m sure someone walking up that isn’t a TFP’er would say it’s clear, but it just looks a little dull to me. I’ll do another OCLT tonight but I did pass that after I SLAM’d it. I’m thinking I just got a false sense of completion.

This is very subjective as you have pointed out. One test is - can you see the head of the screws holding the main drain in place? (Not sure if you have a drain). Give it a day or so after your SLAM to let the filter clean it.

pb4uswim said:
Which brings me back to CYA. Is there any point in adding CYA at this point. Seems like it would just make SLAM’ing more costly (more chlorine). Is there a desired level of CYA for an opening SLAM? I retested today and I’m still between 30 & 40.

CYA has a purpose - to protect your FC from UV. With a SWCG, it is making a small amount of FC each hour which can be attacked by UV. That is why it is recommended to keep a slightly higher CYA with a SWCG vs using Liquid Chlorine. But do whatever you feel is necessary. Note - if the CYA/FC relationship (FC/CYA Levels is properly maintained then the risk of algae is very low, meaning no reason to SLAM. Some members have not had to SLAM in 5 yrs or more.

pb4uswim said:
The good news is that my SWG is working. For some reason, the salt really dropped over winter. I put in two 50lb bags and got it up to around 3200ppm (per my test kit). Interesting thing is that the SWG did not recognize the increase in salt for about a day. It went from 2100 (too low, light on) to 2700 (the low point per the manual) pretty quickly but the light would still not go off. I was about to put another bag in but the next morning the SWG read over 3000ppm and the indicator light was off and the “generating” light was on. Patience Grasshopper.

As noted with CYA, Salt can become lower due to dilution. It is always best to wait 48 hr between testing and any further additions to allow the pool to mix thoroughly. Everyone could be more patient in doing these additions.

Overall, thanks for be a TFP member.

Newdude · Apr 30, 2024

+1. CYA degrades slowly and is unnoticeable during the season but it very noticeable when we walk away from the pool for 6 to 8 months.

Your salt dropping similar means you took on that much rain water (1/3 your volume). But the CYA dropped a little more because it degraded.

If you weren't occasionally pumping it, the excess was leaking out under your patio. Pay closer attention to the water level next year. I pumped a foot 3 times this winter, it was a wet one. It's easiest to pop a skimmer lid and judge from there how high the water level is. If it's 2 inches above the top of the skimmer throat, it's about to overflow.

I tested the water at various depths last year and it was 0s at the top, increasing to how I left it on the bottom. I always just tossed the pump on the bottom and unknowingly pumping the good stuff out. So this year I hung the pump in a bucket to get only top water.

After pumping, the top layer still tested 0s, even with the pump providing some mixing up top. I opened this year with the most salt and CYA I ever retained with 2500 / 50. It was previously always 1200 ish / under 30.

borjis · Apr 30, 2024

Yep. Every year I open to 20 ppm cya.
I then add enough to get to 50 on March first. Needs another 20 ppm added in June to maintain the 50 ppm. It's been this way, every year for 9 years. I did try 30 ppm at first but my chlorine lasts best with 50.

Pro tip I learned a few years ago:
Test the cya with the water sample warm to the touch. Cold water samples will give a false higher cya reading. (Below 70 f)

SJPoe · Apr 30, 2024

Flint here, so similar weather over the winter.

At opening, I'd lost close to 15% of my salt, and probably close to the same CYA. We had a lot of rain over the winter, my pool was almost overflowing when we opened at the beginning of April, despite being drained well below the skimmer/returns at closing.

In my pool, every inch of water (at least for the first couple feet) equals 400 gallons, and we drain at least a foot out, which is 5000 gallons, which is close to 20%...and it was refilled with rain water...so the math kinda maths

Your pool figures around 300 gallons/inch (all things being equal) - so each inch is 2% - 70->40 CYA might be a bit much, but completely out of the ballpark either.

pb4uswim said:
I’ll start the heater in a couple weeks. It’s still too cold to swim here in S.E. MI.

Way ahead of you

It's Michigan...summer could be over already...that might have been it...

mgtfp · Apr 30, 2024

borjis said:
Cold water samples will give a false higher cya reading.

Is that what you are experiencing? I usually see the opposite which always made sense to me as colder temperatures will slow down the Melamine reacting with CYA.

Sunny Blues · Apr 30, 2024

…my CYA was between 30 and 40 so let’s just say 40. When I closed my pool it was closer to 70. I thought CYA didn’t go anywhere (I’ve read stories about draining a pool to get CYA down) so why did it go down over winter? I did dump a few inches before closing (almost to the bottom of the skimmer) and it did fill back up over winter, but does that really explain about a 30 point loss in CYA?

The way I’ve always understood it is (and pardon my unscientific novice-level explanation) that chlorine will continue to be used up during the winter to keep the water clean, and when it starts to run low the chlorine that is bound up and protected by the CYA is released to keep it maintained. CYA naturally declines in this way throughout the year but during swim season we are regularly testing and adding chlorine before it gets too low so CYA drops more slowly during season. But over winter, you add chlorine once before you close and that’s it for the next six months! So the CYA will be used up a tad more quickly over that time as it helps to maintain the chlorine. Also, backwashing out water to close for the season lowers some CYA, and adding new water to the pool at opening is going to lower the amount too. So between that and the amount it needed to expend over winter to manage chlorine, losing 30ppm is nothing and should be expected.

Hopefully our experts here can correct anything wonky I’ve said or gotten wrong. This is just how I’ve always understood it in layman’s terms.

mgtfp · Apr 30, 2024

Sunny Blues said:
The way I’ve always understood it is (and pardon my unscientific novice-level explanation) that chlorine will continue to be used up during the winter to keep the water clean, and when it starts to run low the chlorine that is bound up and protected by the CYA is released to keep it maintained. CYA naturally declines in this way throughout the year

CYA doesn't disappear when chlorine gets released from it, it just changes from being a chlorinated cyanurate to being cyanurate ion. When adding more chlorine, this will change back again.

There may be an effect that chlorinated cyanurates don't show up (as much) in the CYA test, so at very high FC levels the CYA may read a bit lower, but that's not usually something to be worried about.

CYA gets slowly oxidised by chlorine, an effect that is more pronounced at higher water temperatures, which is why pools in hot desert climates tend to lose more CYA than other pools.

The main effect over winter should be dilution from precipitation causing overflow.

Another effect is water stratification, where fresh rain / snow melt water sits on top of higher density "old" water. This makes people who just opened their pool often panic when testing before allowing the water to properly mix.

An extreme case is when a certain type of soil bacteria gets into a pool with no FC residual left that feeds on CYA turning it into ammonia. A pool owner then opens a pool with no FC, no or at least very low CYA, low pH and not being able to hold FC due to the quite violent reaction between ammonia and chlorine.

Sunny Blues · Apr 30, 2024

mgtfp said:
CYA doesn't disappear when chlorine gets released from it, it just changes from being a chlorinated cyanurate to being cyanurate ion. When adding more chlorine, this will change back again.

There may be an effect that chlorinated cyanurates don't show up (as much) in the CYA test, so at very high FC levels the CYA may read a bit lower, but that's not usually something to be worried about.

CYA gets slowly oxidised by chlorine, an effect that is more pronounced at higher water temperatures, which is why pools in hot desert climates tend to lose more CYA than other pools.

The main effect over winter should be dilution from precipitation causing overflow.

Another effect is water stratification, where fresh rain / snow melt water sits on top of higher density "old" water. This makes people who just opened their pool often panic when testing before allowing the water to properly mix.

An extreme case is when a certain type of soil bacteria gets into a pool with no FC residual left that feeds on CYA turning it into ammonia. A pool owner then opens a pool with no FC, no or at least very low CYA, low pH and not being able to hold FC due to the quite violent reaction between ammonia and chlorine.

Well my spewed nonsense was more fun to read

Thanks for the clearer explanation of what happens under the cover in winter

borjis · May 1, 2024

mgtfp said:
Is that what you are experiencing? I usually see the opposite which always made sense to me as colder temperatures will slow down the Melamine reacting with CYA.

Ya definitely. If I test 50f water it's definitely higher cya reading unless I warm it up to room temp first. Others have had that experience as well.

Newdude · May 1, 2024

Newdude said:
It's easiest to pop a skimmer lid and judge from there how high the water level is.

Adding pics for clarity in case anyone needs.

The skimmer lids should be easy to remove with most of them outside the safety cover. This inside edge is the same front and back, and if the water inside the skimmer is higher than the lip, it's time to drain at your earliest convenience.

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CYA and Clarity

pb4uswim

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