Hacking a soil water content reflectometer to measure liquid level

A method for converting a soil water reflectometer to a water level sensor is presented. The sensor essentially records the time of travel of an electromagnetic pulse along two parallel rods, which is then used to infer the relative permittivity (dielectric constant) of the medium around these two rods. While the sensor is designed to capture the relative permittivity variation of soil with its moisture content, the immersion depth of the rods in water (or potentially other dielectric liquids) also influences the average dielectric constant of the air-water medium between the rods (consequently affecting its permittivity). Here we show how this change in permittivity can be calibrated to measure small liquid depths. An empirical calibration equation is developed; it accounts for the impact of water salinity, whereas the temperature effect on the measurements is shown to be negligible.