Damp meters are a dangerous weapon in the hands of someone who doesn't know what they're doing. Not just because the two sharp points can inflict a nasty injury but because the readings that they display can't be trusted. It's not the fault of the meter, it's the plaster. You have to bear in mind that the meters were originally intended for timber. Plaster differs in that the particles that they are made from have very different properties to wood and it's not just a simple case of having a different scale. Different plasters have different properties so rather than reading the meter and declaring the moisture content shown, you need to take several readings in several areas daily or twice/thrice daily over an extended period (while recording the weather and humidity) and see how the results change. That tells you when to take a sample for determination of the moisture content.Yes. Many surveyors stick a probe in plaster and say "suspected" rising damp. Usually bollocks, it's internal humidity. If plaster had a high iron content, they'd condemn building as uninhabitable due to damp
Remember, they're protecting themselves above being open & honest to you
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A feature of many plasters is that repeated wetting and drying (as you would get as a result of rain/no rain) causes the release of waterproof salts which the meter indicates as dryness. Or the plaster may be electrically conductive, in which case the meter shows wetness.
It's like the old fable of the mechanic who got a siezed engine running by hitting it with a hammer and was asked to submit a detailed invoice - Tapping the engine with a hammer: £10. Knowing where to hit the engine with a hammer: £490.