what causes CYA to fall ? (no splash out!)

[jwfrank]

I am a very happy BBB user thanks to TFP. Very stable pH and TA (Liquidator as well, with solid results)

Opened the pool after last year's use of Dichlor (before BBB) with CYA of 110. Drained the pool water to start season at 85. A few months it was own to 75 and now 55, which I know is a good level. But why the decrease?

I can state categorically that there is extremely little splashout. Of course there is evaporation, rain water fill, and tap water fill

Is there another chemical reaction that is eating up CYA? Is there need to worry or just add Dichlor when CYA gets below 50?

 

[duraleigh]

Evaporation won't really reduce CYA....it stays in the pool more concentrated until the pool is refilled and then returns to it's original ppm.

Difficult to say what's causing your reduction (except for splashout).

If your other parameters are in line, I would suggest pucks for chlorination until your CYA comes back to it's appropriate level. Dichlor is such an instant producer of FC and CYA....I think pucks are a somewhat better method. That said, if you are happy with your results from dichlor, it will certainly do what you want it to.

 

[JCJR]

I read some where that some species of algae eats CYA. Also, I get a lot of rain sometimes and eventhough the pool does not overflow I think the water escapes from the skimmer and seaps into the pavers. So eventhough it looks like water is not over flowing it does. Can any of these things be happening to you?

 

[Beez]

It's funny, I was going to ask this exact question as well. Started the season at CYA 60, tested CYA yesterday for the first time since then, and I'm at 40! I was really surprised, because there has been NO water replacement at all over the summer. No backwashing, and no significant splash out(the pool was rarely used). Sure we have had some major downpours this season, but the pool never overflowed, and I did not drain any water out after the rains. I just let evaporation bring the water back down to normal levels.

I assume as JCJR mentioned, that although the pool never overflowed, I must have lost some water somehow after those rains. Maybe through the tile line, I'm pretty sure my skimmer doesn't leak...

 

[JCJR]

My skimmers do not leak either but when your pool is full of water open the skimmer cover and you will see the water level very high. I have not tested but it is possible that my skimmers are below the very top of the pool wall. Especially is some one has a pool with a raised coping the skimmer will be below that as well.

 

[PaulR]

I read some where that some species of algae eats CYA.

I've heard that about certain bacteria, but not algae.

There is a lot of fuzz in the CYA test, and going from 75 to 55, or 60 to 40, is within the test error range. This is not to say it isn't a real drop, just that it isn't necessarily a real drop.
--paulr

 

[chem geek]

Unless the chlorine got essentially to zero, the drop in CYA isn't due to bacteria. When bacteria get going consuming CYA, it tends to happen fairly quickly. In addition to water dilution, CYA can also drop by being slowly oxidized by chlorine, but that usually takes months to notice. It's possible that some chemicals in the water may make that reaction more rapid, but we don't know for sure.

 

[Beez]

There is a lot of fuzz in the CYA test, and going from 75 to 55, or 60 to 40, is within the test error range. This is not to say it isn't a real drop, just that it isn't necessarily a real drop.
--paulr

Of course I can't speak for anyone else, but the drop in CYA in my pool is not test error. I understand the potential for error in the CYA test, but I am very comfortable with it by now. I should also state that my CH level went from 390 to 360 in the same time period.

I know it must be dilution from rain, I just don't really understand the mechanism. There must be some sort of leak or water loss from the pool when the water gets really high. In any case the change is OK with me because I had wanted to reduce my CYA further anyway. It just struck me as odd when I noticed the change, then I came on here and someone else was experiencing the very same phenomenon!

 

[PaulR]

There is a lot of fuzz in the CYA test, and going from 75 to 55, or 60 to 40, is within the test error range.

Of course I can't speak for anyone else, but the drop in CYA in my pool is not test error. I understand the potential for error in the CYA test, but I am very comfortable with it by now.

Not meaning to imply a test mistake or even sloppiness; I'm the one being sloppy, I usually try to describe it as uncertainty meaning the test doesn't promise that degree of accuracy. But Taylor does try for repeatable results so if you've been seeing consistent results and then it changed, likely it is a real change.
--paulr

 

[jwfrank]

Richard raises an interesting point: chlorine could oxidize the CYA over months. (Rain water addition offsets evaporation so as long as there is no splashout, the CYA level should remain teh same)

Re this oxidation, if a pool isnt except by my wife and me during the summer, could it be that the chlorine (staying at a level of 4 or 5) hasn't much to do and hence goes about this oxidizing process on the CYA. (Sorry for the layperson's terms)

 

[chem geek]

Re this oxidation, if a pool isnt except by my wife and me during the summer, could it be that the chlorine (staying at a level of 4 or 5) hasn't much to do and hence goes about this oxidizing process on the CYA. (Sorry for the layperson's terms)

The oxidation of CYA by chlorine should have little to do with whether the pool is heavily used. It should only be related to the Free Chlorine (FC) and CYA levels and to other factors such as pH (probably faster at higher pH) and possibly some other chemical factors that might accelerate the reaction.

Unfortunately, there is a lot of variation and inconsistency in what we've seen in pools so it's just another factor to consider and the bottom line is just to periodically check the CYA level every few months or a couple of times a season (unless you know there will be large changes due to stabilized chlorine use or lots of dilution) and get a feel for how frequently it changes in your own pool.