

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. This normal relative humidity gradient is why problems occur when relying on the use of surface tests.
Moisture meters, no matter what type, fail to provide 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. 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.
The calcium chloride test method (moisture vapor emission), and the UK-based ‘hood’ method (RH) both have the same problem. They are surface tests and in reality, the surface of the concrete will more closely reflect the RH in the room or building than the conditions down inside the slab.

As surface moisture tests, hand held meters (qualitative), or calcium chloride or hood method tests (quantitative) are fine. But if you want the true, critical moisture picture of what’s happening deeper in the concrete, you have to get below the surface with an in situ relative humidity probe. An accurate relative humidity reading can inform your business decisions and prevent costly problems in the long run.