Performance and calibration of moisture sensors in silane-coated granular materials

Abstract

Granular materials, such as soils, with synthetic water-repellent coatings have drawn increasing attention due to their potential to manipulate the response of the shallow ground to water infiltration. Within this context, the performance of soil moisture sensors in synthetic water-repellent soils needs to be evaluated. Water repellency can be achieved through silanisation, whereby particles are coated with thin silane coatings. Here, two soil moisture sensors (ECH2O EC-5 and ThetaProbe ML2x) were tested in three dimethyldichlorosilane (DMDCS)-treated granular materials (Fujian sand, crushed silica and completely decomposed granite (CDG)), by obtaining the soil-specific calibration equations using a two-step calibration approach. Electrical conductivity (EC) was incorporated into the equations to take DMDCS treatment into consideration. The results revealed that after silanisation, 3–4 d are necessary to establish water repellency, with the EC and contact angle used as indicators of its completion. The results also revealed that the output of the volumetric moisture sensors was increased in the DMDCS-treated soils. This influence was more acute at higher DMDCS concentrations and soil water contents, and regardless of the soil and sensor type. Differences in the calibration equations were documented and linked to the changes in the pore-water chemistry.