I also take a sample of the must but I'm usually doing both a pH
reading and an acid titration. I usually dilute a 1 mL sample in about
1 oz. of hot (microwaved) distilled water to drive off CO2 gas. When
cooled I put the pH meter in to get a reading then proceed with the
titration to an end point of 8.2. Distilled water has virtually no
buffereing capacity - the dilution does not change the pH reading.

RD