Caravan damp detection - comparing pin & pinless moisture meters (nb: read description)

Trying out 3 moisture meters on "caravan like" materials to see how they behave. The first 2 meters have settings for different materials but the “bobble head” meter has a chart in the booklet that converts the “DISPLAYED” reading to a WOOD moisture % - you have to halve the displayed value so 20% displayed = 10% for wood and 50% displayed = 25% for wood. That is the maximum wood range. I think that works.
Btw - the thin plastic sheet over the wood (mimicking vinyl) made no difference to the pinless readings.