Disapearing Cyanuric Acid

[Desiato]

12 months of TFP (and spreading the word to others over here in Spain) and the pools looking great but I've suddenly developed a CYA problem. Like most I started off with high CYA (160) and after a partial drain it was:

80 - on 23rd May last year.
70 - a week later
60 - the next day (31st May)
55 - 4th June
60- 3rd August
70 - 19th September.

I don't know what was causing the CYA to fall and rise as I only added bleach but it stopped at 70 before it started another slow fall over the winter. I'm not a lover of the disappearing black dot as I can get varying results with the same water sample so a better tester is on my shopping list but for now...I keep the pool open over winter and recorded...

70 - 25th October
60 - 7th November through to May this year where it was...
55 - 14th May

I rarely backwash now (once every 2/3 months) so despite being pleased with the slight drop in CYA I did wonder how it was happening. In May I switched my attention to my PH and TA (which I'll come back to) and didn't check the CYA until...

40 - 19th June.
40 - 1st July
30 - 6th July.

Having read that many on here like to keep their CYA at 50 during the summer, I added 5 left over pucks to a floater but surprisingly it didn't make any difference.

30 - 8th July

And that brings be to today and:

<20 - 11th July

My tube with the black dot only goes to 20 and it was still clearly visible so I'd guess my CYA has dropped from 30 to 10 in 3 days? I never thought I'd need to add CYA and the pucks don't seem to be doing it either so I may have to run out and buy some....grrrr.

To complete the picture, I took my eye of the CYA when it was at 55 (14th May) as my PH and TA had risen from 7.4 and 70 to 7.9 and 90. (between 12th Feb and 14th may). I then spent the next 3 weeks adding acid and aerating the pool and by 7th June I was at 7.2 and 75. I looked back at last years records and saw that I had maintained a TA of 65/70 so pushed a little further but had somehow managed to overshoot it and on 19th June had readings of 7.2 and 50. It was at this point I did the CYA test of 40. I have no idea if the sudden drop in TA is related to the drop in CYA but they happened at the same time so I've included them here to cover all bases.

I should also mention that despite leaving instructions when to add bleach to the pool, when returning from a week away, I had an algae problem and performed a slam (1st July). It cleared up very quickly but I had a strange FC reading when doing an overnight FC loss test.

3rd July 08:30 - 11ppm
3rd July 23:00 - 9.5ppm (a surprisingly small drop when I usually get 2/3 at this time of year)
4th July 06:30 - 11ppm eh?

No idea what happened there but the pool is clear again so...

I now know from reading this site that the pucks lower PH and that low TA can do the same which was why my top up hose (for 2 hours pointing at the surface) and upturned aerating pipes on the returns could only halt the PH fall and not increase it. I've added 1kg of Sodium bicarbonate which has raised the TA from 55 to 65 (it had crept up from 50 on 19th June to 55 on 5th July) and 2 large cups of PH+ (also left over from last year that I didn't know I had) which has raised the PH from 7.1 to 7.2. I'll need to add some more as 7.4 seems to work best for me so whilst I am on track to get my PH and TA back to normal, what on Earth is going on with my CYA?

 

[Mr Bruce]

You don't by chance have an auto-fill do you?

 

[Desiato]

No, I don't have anything

Just a basic pool with a push along vacuum a brush and lots of bottles of bleach.

 

[needsajet]

Maybe the CYA testing tips are helpful:

Ensure the sample is warm enough - 75F (24C) and above - (CYA is the only temperature sensitive routine test)

Don't wrap your fingers around the test tube (light needs to get in to light up the particles)

Hold the test tube at waist height with your back to the sun - you need bright light, but not direct sunlight, to get a good reading

Glance at the dot, look away, glance again. If you see it right away, keep going, if you don't see it with a glance, you're done

Pour the test solution back into the mixing vial, give it a shake, then pour into the test tube again. Repeat as needed to get a consistent reading

CYA is very slowly broken down, but not what you're seeing, other than the long period through winter, so I'd say it's mostly just mastering the test - no easy chore! So many of us in the same boat, but these tips do make a difference.

 

[Desiato]

Panic over!

I know what I did, I didn't leave the sample long enough in the tube for it to fully dissolve...doh!

I just did two more tests, first using a pill from a different strip and then one from the original strip. On the first test I waited until the pill had dissolved and could tell right away the tube was much clearer than I've been used to. I assumed the position (back to the sun, tube at waist height, fingers above and below the tube) and the result was 20. Went on the hunt for another tube to return the sample to, found one, gave it a swirl, back to the measuring tube and it was noticeably cloudier. This time I got a reading of 30.

2nd test with a pill from the original strip, I watched it dissolve and when it had, could see the tube was still very clear. Gave it a swirl, sat it down, gave it another swirl and despite the pill being dissolved, I could see tiny particles swirling around so kept on shaking it about, leaving it to stand until after 5 minutes, did the test. 35

So that's what I did wrong this morning. I always leave the CYA to stand whilst I am testing the FC and PH but as I didn't need to do those this morning, I clearly rushed the CYA test.

Didn't know that CYA was temperature sensitive up to 75F, good to know. Guess I'll be bringing the sample indoors and letting it warm up before testing next winter. So I'm pleased I haven't lost 20ppm in 3 days but as for the gradual loss of CYA over time, guess I'll just have to accept it without fully knowing why.

Cheers for your help.

 

[needsajet]

You can read about CYA degradation, but it's not fully understood or easy to predict. If you want to learn more, you can poke around this thread: Degradation of Cyanuric Acid (CYA))

I've looked through that thread a couple of time and it makes my head spin. The key takeaway is to get good at the test, test monthly, trust the test, and adjust CYA accordingly.

Have fun with your pool!