I had this problem last year and I have it again this year. I have to add 14 oz per day of 29% MA to maintain a PH of 7.5. I usually test at 7:30 AM. The PH reading is 8, I then add 14 oz of acid and 1 hour later I test again and the PH is 7.5. The next morning it's back to 8. The only answer I have for this is that it has been windy. Could the dust blown dirt cause the PH to rise that much?
I drained the pool over the winter and acid washed it so I'm starting the season with fresh water.
Test results are:
FC 3 (I add about 24oz per day after the sun goes down)
PH 7.5 - 8
CYA 40 (I used pucks at first to raise the cya, now I use 10% liquid)
Pool Volume 10500 gallons