I'm generally successful at keeping my pool's pH in the 7.2-7.4 range
without too much hassle. However, every time it rains, it takes what
I consider to be a dramatic drop. We got about .1" of rain yesterday
and, as expected, the pH dropped to 6.8. The pool is a 24x48 round
above ground, 13,000 gallons or so. If my math is correct, .1" is
approximately .2% of the total volume. With that much of an effect
on the pH of the pool water, I would expect to see my concrete patio
etched clean! I'm able to bring it back up to 7.2-7.4 with about a
pound of soda ash, so it's not a major problem. I'm just confused at
how so little rain can have so big an effect.
What am I missing?
Rain - contains lots of things and in turn that changes
the pool's ph. You won't see etching etc. from those
strengths unless a very long period of time is studied,
but obviously it's what's in the rain that causes it.
Collect some rain and check its ph; you'll see what
I'm talking about. Don't overthink it; just use common
sense and logic.
As the other poster suggested, check the pH of your rain water. Even
pristine rain water will run in the mid sixes. That comes from
rainwater scrubbing CO2 from the air, making carbonic acid. If that
rain is getting heavier oxides in the air the pH will be lower. Bear
in mind pH is a log scale so a 5 is 10 times stronger than a 6.
Maybe your pH tests or procedures are no good. (What is your basis for
trusting them? False results are easier to get than true, and it takes
knowledge and diligence to keep them in line. If you've never calibrated
your tests, then they can't be trusted.) Adding about 1 part in 1000 of
rainwater does not seem like it could have such an effect.
The first step is to confirm the readings.
Whats your total alkalinity number? Alkalinity is your buffer (kinda like
buffered aspirin). It is there to prevent wild pH swings. If its within
nominal than I would just say you have some very serious acid rain and alot
things around you should be dissolving by now.
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