Chipless Liquid Sensing Using a Slotted Cylindrical Resonator

A method for the wireless sensing of the permittivity and level of liquids is presented. The use of a simple, thin-film slotted cylindrical cavity wrapped around a standard polytetrafluoroethylene pipe is proposed. Wireless interrogation of the slot excites a resonant mode whose frequency is dependent on the liquid currently present within the pipe. The proposed method allows for measurements to be taken in situ with no requirement for taking samples of potentially hazardous liquids. The device is capable of sensing materials of high relative permittivity, including water, as well as very lossy liquids. A comprehensive set of results is presented, including measurements of butanol, ethanol, methanol, and water, for several device configurations. The proposed sensor is also shown to be sensitive to small changes in liquid level, allowing for accurate water-level measurements down to 0.1 ml. This sensor is a good candidate for very low-cost, low-complexity real-time monitoring of liquids.