shocking oxidizes cya

[ant 83]

This is something that I've known for a long time. There is a post about it from some time ago:

Degradation of Cyanuric Acid (CYA))

Wojtowicz also shows a strong temperature dependence on the chlorine oxidation of cynauric acid where every 10F increase in temperature results in roughly doubling the rate of degradation. So his data was with pools at 85F so pools at 90F could have degradation rates about 1.4 times higher.

Higher temps in hot tubs speeds this up. Higher chlorine levels do as well.

Some folks report that their cya levels don't drop very much over time. I have no reason not to believe them, but it's not my experience with my hot tub. I still need to test for cya levels frequently because I cannot predict what the test will say before running it. I frequently shock my tub up to the shock level on the cya/fc chart. Cya needs to be added back at roughly 1ppm per day averaged out over time in my hot tub.

The other week I was out of town and I asked my wife to add bleach to the tub, but we had ran out of bleach. She bought a half gallon of clorox from the gas station up the road to use. When I got home, I just assumed that the gas station bleach was old and stale, and just dumped the rest of it in and then threw the bottle away. I even added some more regular bleach that I had just bought on top of the clorox at the same time. As it turned out the clorox wasn't so bad. When I ran the tests about an hour later, my FC was running 80ppm. My CYA was at about 80-100, when I had tested it a week earlier.

This shocking event drove my cya levels down to about 20-30 when I tested it again the next day.

It also broke lose a bunch of bio-film from the plumbing. I did the best I could wiping out that dark bubblegum from the waterline with paper towel, then added some ahh-some to see if I could bust lose some more. The bleach seem to have gotten the vast majority of it, so I just skimmed off the foam and didn't change out the water.

The water has been extra clear this week like what happens when I buy a new filter. I also ordered a new filter to put in, but haven't done so yet.

How often does TFP recommend new filter replacement?

This new one will be our 5th filter since we got the tub 2.5 years ago. So, I guess that averages out to a new filter every 6 months. Does that sound about right?

Edit to note: my tub has only one filter. I see in pictures that others have multiple filters on their tubs. So, I suppose that makes a difference.

 

[YippeeSkippy]

Let me understand this... you're soaking in Ahh-some and a super high FC?

I have purchased both disposable and cleanable filters for my spa. I toss 'em and buy new when they get really ratty looking even after cleaning.

Hot water probably has more affect on CYA than chlorine.

Maddie

 

[JoyfulNoise]

Couple of thoughts here -

I have seen higher oxidation levels of CYA in my pool than what TFP commonly quotes. I live in a very hot and sunny climate with plenty of UV and, during the hottest months of the summer (May/June/July), I have measured a CYA loss of 20ppm/month in some instances. My pool is not particularly hot, but it can easily get to about 92F during that time. So CYA oxidation by chlorine is not unheard of and, as you say, you can definitely measure it in a hot tub. The point of Richard's post you cite and the Wojtowicz study was to confirm that CYA does have several degradation pathways. The problem is none of them are at all useful for lowering CYA levels in an over-stabilized pool in any reasonable time frame.

Second, when measuring CYA with the melamine test, very high FC levels can interfere with the test causing a negative interference. At 80ppm FC, you are well beyond any level of chlorine that is considered safe and FC levels that high will almost always mess up the various indicator dyes used. Even the DPD indicator is not safe from interference. For the melamine test specifically, there are acid buffers to drop the pH of the test sample to a level where most of the CYA is as cyanuric acid as opposed to a chlorinated cyanurate form. The chlorinated form of the cyanurate will not react with the melamine as readily as the cyanuric acid form as the chlorine atoms take up spots on the cyanurate chain which would normally cause the hydrogen bonding needed to form the melamine cyanurate precipitate. So, very high chlorine levels can throw off the pH buffering of the solution and also interfere with the formation of the precipitate. This can lead to a measured CYA level LESS THAN the true value. And, of course, there's always increased oxidation of the CYA when the FC is high.

 

[ant 83]

Thanks for your comments joyful.

I did wait until the next day to test for cya. FC had fell down into the single digits by then. It was an unexpected test result, and the reason that I tested the cya after.

FC never remains high for very long if cya levels are relatively lower. It gases out I suspect.