The Medway Handyman presented the following explanation :
So, what reading is 'normal' & what reading is 'damp'?

That will vary from meter to meter, depending upon the voltage it uses
at the probes. The higher the voltage, the more the damp plaster breaks
down to conduction and the lower the reading displayed. Perfectly dry
may produce an infinity reading.

The best bet is try it on some known to be dry plaster, then work from
there.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk