On 8 June, 22:12, Homer Sparrow Homer.Sparrow.
wrote:
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!

--
Homer Sparrow

If you were a regular reader here you would have seen me posting about
this problem a few times.
If you are handy with a saldering iron then all you need to do is to
open it up and swap the two wires around then it will read OK.
David hill