Methods and methodology for measuring moisture in materials and structures are thoroughly described by Nilsson et al (2006), see Info report (in Swedish).
Professor Lars-Olof Nilsson has been chairing a RILEM Technical Comittee TC-MMB on Methods for Moisture Measurements in Building Materials and Structures. The TC has produced a state-of-the-art-report that was completed in 2018. See Springer.com.