Moisture meters should never be used to make the final determination as to whether or not a concrete slab is dry enough for a flooring installation. Here’s why:
No ‘moisture meter’ of any type can give consistently accurate ‘moisture’ readings across the different mixes and densities of concrete. Additionally, other components (metal reinforcing bar, aggregate size and amount, etc.) can cause false indications of ‘moisture’ especially with non-pin meters. But pin-type ‘moisture’ meters are also not practical for moisture measurement because variable chemical and physical characteristics in concrete can cause false readings due to changes in electrical resistance that have nothing to do with moisture.
Additionally, even IF the so-called ‘concrete moisture meters’ were sufficiently accurate (again, they are not), they only measure ‘moisture’ (not relative humidity) in a very small area near the surface of the concrete slab. This type of surface ‘moisture’ testing tells us nothing about the conditions down in the slab. Once a floor covering is placed on a concrete slab, the relative humidity (RH) within the slab will equilibrate throughout the thickness of the slab. This means that a slab that may have been “dry” (low RH) at the surface (without a floor covering) will see a higher RH (migrated from down in the concrete) at the surface, when the floor covering has been installed. This is where the problems occur. The calcium chloride test method (moisture vapor emission), and the UK-based ‘hood’ method (RH) both have the same problem, as they are also surface tests.
Keep in mind that an uncovered concrete slab will indeed have a relative humidity gradient (typically drier at the surface; much wetter at depth) throughout its thickness until a floor covering is put on top. Under normal conditions, the RH at roughly 50% slab depth will be significantly higher than the surface unless the slab has been down for a long time, and a vapor retarder is directly underneath the slab. In reality, the surface of the concrete will more closely reflect the RH in the room or building which gives no indication of the potential for a flooring failure due to the high RH deep in the slab that will migrate to the surface when the concrete can no longer emit water vapor due to the non-permeable floor covering.
Again, most major floor covering and adhesive manufacturers in North America and Europe already have concrete relative humidity specifications in their installation guidelines.
At best, moisture meters (pin-type or surface-type) may have some practicality as relative (qualitative, not quantitative) measurement devices for possibly indicating best placement for accurate, quantitative relative humidity sensors within the concrete. Moisture meters, similar to other non-quantitative test methods, are not final determination tools.
Learn more about Relative Humidity Probes.