My question concerns how reliable pH testing is, in the setting of an
active fermentation.

Last week, I started with several crates of fresh red grapes. The
initial chemistries were poor...the pH was in the 3.9 to 4.0 range,
and the TA was as low as 0.40. Everytime I "under-adjust", I end up
with flabby though inoffensive wine. This time, I figured that I'd
give it a significant dose of tartaric (about 2.5 g/liter of must). I
plan to cold stabilize as well as do an MLF.

But I'm concerned that I may have overdone it. When I test the pH
now, at the top of the carboy with active fermentation going on, the
pH is in the 3.2 range. I'd like to be in the 3.4-3.5 range.

My question is whether these measurements are accurate. Does the
significant amount of CO2 that's obviously at the top of the carboy
affect pH measurements?

Has anyone had any experience with carbonate additions?

Thanks in advance,

Lee