Towards a passive contactless sensor for monitoring resistivity in porous materials

Abstract A passive contactless sensor is presented for monitoring resistivity in porous materials. Electrical resistivity (ER) is an important indicator of water and ion content in porous materials, especially man-made and natural composites such as concrete and soil. The ER sensor is small, battery-free and is energised and queried wirelessly for data by a radio-frequency identification (RFID) reader or a near-field communication (NFC) enabled mobile device (a tablet or Smartphone). By applying Topsoe coefficients and a calibration function to the sensor output, the ER of the material is reliably estimated. Experimental results in the laboratory with a model aggregate material (sand) wetted with sodium chloride solutions demonstrate excellent agreement between the new sensor and a standard laboratory conductivity meter. Embedded ER sensors can provide long-term and repeatable data from soil, concrete and similar porous materials where ER is an accepted proxy for water and ion content. We think that passive contactless resistivity sensors will become commonplace as the Internet of Things (IoT) develops in the construction and precision agriculture sectors, and as the demand for long-lived, battery-free sensors increases.