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 meter of any type can give consistently accurate readings across the different mixes and densities of concrete. Other components (metal reinforcing bar, aggregate size and amount, etc.) can also cause false indications of moisture levels, especially with non-pin meters. Pin-type 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.
Even if the so-called concrete moisture meters were sufficiently accurate, they only measure moisture (not relative humidity) in a very small area near the surface of the concrete slab. Surface moisture testing says almost nothing about the moisture conditions deep inside 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. A slab that may have been “dry” (low RH) at the surface (without a floor covering) may see a higher RH (due to moisture migration from within the concrete to the surface) after the floor covering or coating has been installed.
Compared to a concrete moisture meter, the Rapid RH® system gives an RH reading from deep within the slab. This provides a more accurate picture of the slab’s true moisture condition and what can be expected after a floor covering or coating is installed.