An ‘isolated diffusion’ gravimetric calibration procedure for radar and microwave moisture measurement in porous building stone

Abstract Information about the presence and movement of water is crucial to understanding stone deterioration and rock weathering but hard to obtain. Non-destructive, non-invasive measurements of electromagnetic phenomena can provide proxy data on water contents within porous stone and rock. Commercial geophysical devices, such as radar and microwave moisture sensors, produce raw data or readings in arbitrary units, but can be related to absolute water contents through gravimetric calibration. Calibration procedures typically either equilibrate samples to a set of relative humidities (RH%) using salt solutions or environmental chambers (requiring specialised equipment), or monitor ambient drying which yields less homogenous moisture distributions and takes time. This study proposes and tests a cost- and time-effective ‘isolated diffusion’ gravimetric calibration procedure in which a set of samples are sealed at specific water contents and equilibrated. The procedure is compared to ambient drying over 120 h for three United Kingdom building stones and evaluated with modelled reflection coefficients and relative permittivities. The calibrations determined from isolated diffusion more closely follow modelled behaviour than those from ambient drying, as the calibrations developed from the latter were affected by uneven distributions of moisture. Calibrations for radar measurements developed from two types of back interfaces (air and metal) were very similar to one another, suggesting that measurements are consistent regardless of the type of back interface used. The isolated diffusion calibration procedure provides a cost-effective and simple method to facilitate comparison between different non-destructive testing methods and enables accurate measurement of water contents in porous geomaterials.