Another case of vanishing CYA

[svenpup]

First off, I want to say hello to all the old friends here. Miss you guys!!

I haven't been around for a while, but I thought this might add to the vanishing CYA body of knowledge.

The facts:

• Taylor K-2006 Test Kit (with refills from tftestkits)[/*:m:3lir1zkd]
• I last measured CYA when I opened the pool on March 9th: 60ppm[/*:m:3lir1zkd]
• Since then I have kept FC at a constant 5-6ppm with my SWCG and manage pH bi-weekly to 7.8 to 8.0.[/*:m:3lir1zkd]
• I had a cheapo blue 8mil solar cover that was always on when the pool wasn't in use[/*:m:3lir1zkd]
• Pool temps up to 92 degrees[/*:m:3lir1zkd]
• No splash out[/*:m:3lir1zkd]
• No leak (Before I got the cover I thought I had a leak because I was losing so much water. It turned out this this was primarily due to evaporation. Once I got the cover, water loss was reduced to ~0.5" per week. I continue to maintain meticulous water loss records out of habit.)[/*:m:3lir1zkd]
• After two years the cover was starting to fall apart, so I replaced it with this 16mil Clear cover.[/*:m:3lir1zkd]
• Two days later when next I measured FC it was at zero. I bumped up the SWCG to compensate, but I couldn't get it to hold. I could only attribute this sudden loss of FC to the clear cover, which I surmised must be letting UV though.[/*:m:3lir1zkd]
• I was debating abandoning the clear cover getting a new blue cover, but in the meantime I decided to bump up the CYA to compensate.[/*:m:3lir1zkd]
• I measured CYA to determine how much to add, and it was zero.[/*:m:3lir1zkd]
• Double checked the reagent with a 50ppm CYA standard and the reagent is fine.[/*:m:3lir1zkd]

So this is definitely not the over the winter version, the ammonia version, or the slow but steady (5ppm/month) version of CYA loss.
All I can think of is that CYA it loss is accelerated above 90 degrees.

 

[zea3]

It would be interesting if someone could experiment and see if CYA "cooks out". Someone who could do this outdoors! Don't know if fumes would be a problem or not.

 

[chem geek]

It's not the temperature -- we've measured CYA in spas and the chlorine oxidation of it at hot spa temps is still only around 5 ppm per month.

You noted that the FC got to zero so it's probably just that bacteria grew and had a wonderful and tasty CYA meal but converted it not just to ammonia but all the way to nitrogen gas. I describe this in Degredation of Cyanuric Acid (CYA) where bacteria don't have to stop at ammonia. Nitrosomonas bacteria can convert ammonia to nitrite and nitrate and denitrifying bacteria such as Pseudomonas aeruginosa can convert the nitrate to nitrogen gas. Also, Annamox bacteria can convert ammonia and nitrite into nitrogen gas. So there are several different pathways to get from CYA all the way to nitrogen gas and have no leftover ammonia nor partially degraded CYA.

Also, with your SWG still running, it was still trying to add chlorine so may have oxidized ammonia if some were leftover.

It's still pretty amazing though how getting behind rapidly turned into zero FC. That was most likely from algae, not bacteria, since very low levels of chlorine will kill bacteria. The algae was probably the first to start growing and their growth added a lot of organic material to the pool for chlorine to react with and that had the FC close to zero so that bacteria could then grow. However, I wouldn't expect all of this to occur in just 2 days. In a week, for sure, maybe even 4 days -- 2-3 days for the algae and 1-2 for the bacteria.