I have a Protimeter with prongs, it's far more accurate than the non invasive type. The latter are more prone to detect atmospheric moisture and also signal damp when reading aluminium especially in older vans.A significant amount do. A water ingress check will give you some baseline figures to check readings taken by yourself.
The more expensive , non invasive one's are the better ones to have, for obvious reasons.
A ingress check on our van through up elevated readings on the garage door. Knowing this I was able to put some measures in place to stop the ingress, and then watch the readings gradually drop. This years ingress check had the door back to 10%.
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