CYA Usage Patterns

[MattM]

All,

Just finishing my first season with a new pool, and had another 'cya disappeared' episode. Spent most of the week rebalancing. This is now the third time I've had to reset cya levels.

Dec 2011 - Pool Filled
Jan 2012 - Set pool to 30-40 CYA
April 2012 - All CYA seemed to have gone, added again till it was brought up to ~50 CYA
Oct 2012 - Noticed that FC readings had been trending down for a 4-5 days, adjusted SWG to generate more, and then over 48hrs - the ORP sensor attached to my intellichem indicated that the pool CYA had disappeared, and sure enough a CYA test showed had fallen to under <20.

Note that the SWG is still reporting normal salt readings, and the borate test strips I have here are still showing borate readings within range. Also, given that I can tell how off the cya is by the ORP...the change would have had to have happened within a few days, and I know my autofill is controlled by a timer which limits max usage to no more than a few hours/day and there is no way that a water leak sufficient to drop CYA that much could have been masked by that limited of an autofill runtime.

What I suspect is that there was another algae outbreak into the pool and it went through the chlorine first and then attacked the CYA...we have kids in the neighborhood over every monday for swimming lessons. I'm assuming one of them brought something into the pool somehow...

Anything more I can do to prevent this type of event other than to be more responsive to sudden increases in chlorine usage? I'm not sure if I should fault myself for not shocking right away....swimming lessons were on Monday, I noticed CC was 1.5 on Tuesday...was concerned but held off on shocking right away (just increased SWG runtime)..but by Thursday morning, it was clear that the CYA had been removed/eaten..

 

[PAGirl]

I believe the target CYA for a SWG is 70. You don't mention what level of FC you were maintaining.

 

[UnderWaterVanya]

I would suggest shocking anytime you get > 1.0 ppm CC's but I might double check the results myself before starting the shocking process.

It it's odd that the CYA dropped so fast. Have you tested for ammonia?


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[MattM]

Minimum Target FC has been 7 with CYA 45-55, SWG usually kept in a 7-10 range.
CYA deliberately kept around 50 as that is the highest level compatible with an ORP controller.
No, I haven't tested ammonia as the test isn't included in the default tfp test kit, it doesn't appear to be a recommended test on this site, and the software I use (Simple Pool) doesn't have a field to record it.

 

[chem geek]

If the CYA drop is associated with the chlorine getting to near-zero and bacteria growing (and possibly the water getting dull/cloudy or having visible algae), then you just need to raise the chlorine level or the chlorine output to handle the higher bather loads. The bacterial conversion of CYA into ammonia, nitrite, nitrate or nitrogen gas is fast, taking only a few days (see this post for a log where the FC in my pool probably went to zero mid-week from higher demand from material in the newly turned on solar system and from the warming water and by the weekend the CYA had dropped significantly and ammonia was present along with a huge chlorine demand).

Or perhaps you've got something lowering the CYA level in spite of FC being present. That would be a different and currently unexplained mechanism than bacterial degradation. It would be great if you could bottle it and sell it as a CYA reducer, especially if it didn't create a huge chlorine demand.

 

[UnderWaterVanya]

As Richard (chem geek) alluded to - the point of asking about the ammonia is that there is a known way that bacteria can convert CYA into ammonia. It doesn't always happen that way - but if you have a huge chlorine demand it could be due to ammonia. The treatment doesn't change - dump more bleach in until it holds FC - but knowing more about how much (if any) ammonia you have would be useful and a way to track progress.

 

[barefooting]

The CYA in my pool decreases about 10 ppm per month, on average. So, my monthly ritual is to top it off, or "put a sock in it". I guess most of the drop is due to splash-out, but doesn't really account for all of the loss. My calcium level doesn't drop as fast, although the fill water has ~50 ppm (I only had to add water twice this year due to plenty of rain). I recall reading a few other posts on CYA dropping during the season. I can only assume their pool use is a lot like mine, cannonballs galore.