Good ideas so far but how old is electrode? Is calibration taking longer?
+1. 31 years of field work and managing field work as an environmental consultant has left me with a deep abiding dislike of field pH meters.
I just had a guy go sample a spring for me, he calibrated the meter according to the manufacturer's instructions, it calibrated fine, and when he went to collect his field measurement of the spring water, it read....3.9. He couldn't believe it. So he recalibrated the meter again, it calibrated fine, and it read....3.9 again.
This is a pristine spring used by hikers along the Appalachian trail, near the top of the mount, far away from any mining activity that might change the pH. Fortunately, he had pH strips in his field kit. They read 6.8 to 7.0. Finally, a real number.
I have many similar stories from my younger days in the late 1980s and 1990s when I used field pH meters. My sampler was apologetic, I told him, "I got it."
I think you are better off using Martin's adjustments for your water and the type of malt, rather than worrying about 0.1 pH units off one way or the other. I call them, "bogus meters".