Unstable stabiliser?

[roger888]

Another chemistry question for the experts, please: what factors govern the persistence or degradation of cyanuric acid? I manage our pool using (more or less) just liquid chlor and crystalline CYA. From all the warnings about the danger of CYA build-up from prolonged galette usage, I assumed that CYA was itself pretty stable. But mine seems to drift down by perhaps 10ppm per week. Is this normal?

[Théière]

Hi Roger, how do you measure your CYA level?

Usually it is a very stable compound which easily forms a chemical bond with chlorine. The process is also a reversable one which means the bond will break and chlorine can be utilised albeit slowly.

There is a bacteria that seems to attack the occasional pool over winter when chlorine levels are at their lowest.

Are your losses accumalative? Is the temperature of the sample the same?

[roger888]

I use the Scuba+ for all my measurements. It's a bit difficult to do controlled tests, because I top up the CYA from time to time to keep it broadly in the 50-70ppm range.

Since posting yesterday, I've found this (rather technical) thread by Chem Geek on the TFP site, which suggests that there are a number of degradation pathways.

http://www.troublefreepool.com/degradation-of-cyanuric-acid-cya-t8880.html

[chem geek]

The degradation of CYA by bacteria requires living bacteria to occur and that should not be present in a pool with virtually any measurable chlorine in it.  The other pathway is from a slow oxidation of CYA by chlorine itself.  We see this in hot spas at around 5 ppm CYA loss per month at roughly a 2-4 ppm FC with 20-30 ppm CYA level.  In pools, it is usually lower -- in my own pool it seems to be around 2-3 ppm CYA per month (I have minimal water dilution in my pool with an oversized cartridge filter cleaned only once a year and with no summer rains).  There are some people who report somewhat higher losses, but not usually more than 10 ppm CYA per month unless the pools are small and there is significant water dilution from backwashing and rain overflow.

Is only your CYA level dropping or are you finding other levels dropping as well such as Calcium Hardness (CH) and salt (TDS)?  Maybe you have a leak in the pool where auto-fill is preventing it from being obvious.