Why does pH seem more stable at a higher CYA level?

[ITR]

...or is it?

So I've been used to about a 0.6 pH rise over each week. After Hurricane Matthew I lowered the water seven inches and only got two inches of rain, which means I had to add about five inches of water to the pool. After adding the water back in last Sunday, I ran a full test with the following readings (and associated notes):

pH 7.6
FC 8.0
CC 1.0 (probably from stuff blowing in during the hurricane)
TA 40 (I keep TA low because raising it to 70-80 seems to make my pH rise even faster)
CH 300 (down 50ppm after the added "fresh" water)
CYA 30
Salt 2000
Temp: 80
CSI: -0.60

Notes: Added 40 lbs of salt, cleaned SWG and filter. Added 32oz by vol stabilizer to skimmer (in nylon in two 16oz doses), set SWG to 25% (was 15%) and started 4hr boost cycle. Added 5oz of Beautec.

I purposely did not add HCL/MA since I knew CYA would help keep the pH lower. Today I tested (sorry, I didn't do a full test, just pH - I'll do a full test when Jan gets back and I'm not managing the household by myself) and pH tested 7.8, so last week it raised about 0.2. I've had CYA at 40-50 because I started the pool two years ago with bleach. A couple of months ago I switched to SWG and never realized there should be a higher CYA for it...Not wanting to overdo it, I added 32oz thinking that should raise it to 50-ish and I can add more to bring it up to 70-80. According to PoolMath it should have raised CYA by 23 and lowered pH by .43 (which calculates the .2 rise since it's typically .6). I know it is typical for pH to lower when adding CYA, but will it affect my pH rise over time (long term), or just this week since it was dissolving?

ITR

 

[pooldv]

Your pH should not rise above 7.8 with TA between 40 and 50.

 

[JoyfulNoise]

If you'd like a technical explanation, see this thread - pH Buffer Capacity

Cyanurates act as a buffer in pool water. So, when you add more CYA to your pool, you increase the buffering capacity of the water against changes in pH. In the normal pH range of pool water, carbonates are the largest fraction of the TA, cyanurates are second and borates are third.

 

[JamesW]

Cyanuric acid contributes to the total alkalinity. Since it's not carbonate, it won't contribute to carbon dioxide offgassing. For the same TA, a higher CYA will result in a more stable ph.

 

[needsajet]

...and higher CYA is likely to allow shorter SWG run time, which can also reduce the upward pressure on pH.

 

[ITR]

Your pH should not rise above 7.8 with TA between 40 and 50.

Hmmmm...It does...I wish it wouldn't...but it does. The pool is about two years old so I would have thought the plaster was done "curing" which definitely was causing me to chase pH during the first year. What else would cause pH to rise (considering my test results)?

Good to know CYA contributes to TA. I didn't know that. I'll have to do a complete weekly test when Jan gets back to see where TA is. And I'll check out the Buffer link that Matt posted. Thanks for that!

 

[ITR]

Matt...my head aches just from reading that post. LOL I'm going to need a few days to digest that one fully and maybe breakout some of my old Chemistry books. Yowser!