I have been using a pH meter of the kind commonly available in garden centres - it was a replacement for one that I wasn't sure was reliable.

It turns out that it's completely useless. I discovered today when I plunged the probe into a lemon, only to find that it read pH 8! I then tested it in 5% (0.5M) acetic acid (white vinegar) and it read pH 10. This is complete nonsense, of course!

This may be a 'two off' experience, but the meter is obviously completely oddball. It does correctly read neutral values, but I wonder if both circuits are reading positive because of an electronic defect.

Clearly, any corrective interventions I may have made if I'd relied on the meter would have been disastrous for my plants!