Explanation for previous high CYA loss found?

[rhawke]

Hi all. The first 4 seasons of having a pool I experienced high CYA loss of about 10ppm per Month. I read a bunch of posts on it but never a solution.

Well this year I decided to increase my chlorine target. I had always targeted 5 ppm for my SWG because I wanted as little chlorine as necessary for the toddlers, which then resulted in having to slam maybe once or twice mid season because chlorine dropped too much at one point before I caught it.

This season I wanted to be on the safe side and set my FC target at 8 ppm.

To my surprise this summer I am positively surprised :

- my SWG is now running at 50 % vs 65 % last summer

- my CYA is now in August still at 70-80 (I bumped it up to 80 in May)

- I didn't shock or Slam once

So could increasing the FC target really solve the issue of CYA loss? Or is this just a happy coincidence?

(secretly hoping chemgeek will chime in )

 

[mgtfp]

which then resulted in having to slam maybe once or twice mid season because chlorine dropped too much at one point before I caught it.

That could be part of the explanation.

Apart from draining water, there are two ways how CYA can be lost:

By chlorine oxidation which is more pronounced at higher temperatures (which is why it's more common in hot climates) and at higher chlorine levels. Having to SLAM regularly could result in higher CYA degradation, especially in hot climates.

The other way is bacteria turning CYA into ammonia. But that really requires FC to drop to zero, and afterwards you would observe a very high chlorine demand due to the ammonia (you wouldn't be able to hold FC for longer than a few minutes after adding chlorine), together with pH dropping. I don't think that happened in your case. That's more common when opening a pool after winter.

10ppm loss per month is probably at the higher end of what's still considered not unusual in hot climates. Not having to SLAM anymore may well have made a few ppm difference.

 

[AUSpool]

From what we currently know about CyA and the FC/CyA ratio, increasing the ratio will not decrease the potential loss of CyA. During the swim season it is not uncommon to loose 5-10pm CyA per month.

 

[Mdragger88]

The hotter the water the faster cya degrades. Especially more noticeable when it starts getting 90+.
Have you taken any steps to keep your pool cooler this season?

 

[AUSpool]

It’s really nice to see the historical records and how raising your CyA allows you to run your SWG a little lower. And by running your FC/CyA ratio closer to 10% you actually provide a safer pool to swim in. Every time a pool gets close to requiring a slam, or needs a slam, it is increasing the risk of microbial contamination or cross infection between bathers.

 

[rhawke]

Thanks for the interesting answers.

The CYA loss happened already before I slammed. I actually think the slam was necessary because cya degraded, I was too lazy didn't test CYA for 2 months, then realized it was down to like 40-50 ppm which explained why the FC couldn't keep up anymore and after a day with kids in the pool I would get to 1-2ppm FC which the pool wouldn't recover from without additional chlorine.

While I have pool shading that is keeping the pool below 90, I am in my 3rd year of those. And this year I was out of town all of June with no pool shade and the pool was 94 for a solid 3 weeks.

I will retest tomorrow just to make sure I didn't just make a mistake testing.

 

[AUSpool]

…which explained why the FC couldn't keep up anymore and after a day with kids in the pool I would get to 1-2ppm FC which the pool wouldn't recover from without additional chlorine.

You can kind of use that as an indicator that you CyA level has reduced. In the middle of summer when everything is balanced and ticking along nicely a sudden drop in FC is a good indicator that your CyA has reduced. Less CyA, more FC exposed to UV degradation and your FC will go down a bit.

It’s impossible to say for sure because there is so little research into CyA but I believe it is possible for FC to reduce the uv degradation of CyA and perhaps even the oxidation from FC. If it is possible maximum saturation of hypochlorous acid on the three possible cites would be when the FC/CyA ratio is somewhere between 7.5 - 10%. But that’s just pure speculation on my part.