PH plateau?

[steveinaz]

Normally my pool runs on the high-side with ph; typically 7.8-8. I do my acid thing on Weds and Sats usually needing about 20 oz to keep the pool at 7.5.

I've noticed lately, when I'm lazy and forget to check/adjust ph, it lowers on its own. What's up with this? This weekend, (after being lazy for 2 weeks), I checked my ph expecting it to be sky-high...it was perfect, 7.5.

I also noticed this in the middle of the summer. We were gone for 9 days. I was fully expecting to come back to really high ph readings--but it was perfect at 7.5...after 9 DAYS. I'm confused. Up to that trip, I was adding acid every 3rd day to keep ph in check.

What causes the ph to lower on it's own in a typically high ph pool?

Conditions:
Pool is uncovered...water temp is approx 52 degrees
I don't close the pool
I have lowered chlorine output on SWCG to about 3ppm for the winter