Evaluation of Electromagnetic Sensors Using Acetic Acid/Water Mixtures - Dielectric Constant and Electrical Conductivity Responses.

The use of liquids with different dielectric constant and electrical conductivity (EC) has been proposed to test electromagnetic sensors (EM) designed to measure the soil water content and bulk EC. The main advantages of such an approach are the homogeneous background and uniform contact between the probe and medium provided by liquids. Another advantage is the fact such experiments can be reproduced under the same conditions in different laboratories. In this work we introduce the idea of using acetic acid/water mixtures to evaluate the response of EM sensors. Pure acetic acid exhibits a low dielectric constant (? = 6.2) and electrical conductivity close to zero. As acetic acid is diluted with water (? = 80) dielectric constant (?m) of the mixtures increases, producing a porous medium dielectric range from dry to saturation and beyond. The electrical conductivity of the dielectric mixtures varies with a maximum EC of around 2 mS cm-1. Measurements were conducted using 7 common EM sensors (i.e., CS616, Acclima TDT, Hydra Probe, Theta Probe, SM200, 5TE and TDR100) measured in the dielectric mixtures ranging from pure acidic acid up to pure water. Results obtained are discussed in terms of sensor linearity comparing the sensor output with the dielectric constant measured with the TDR100 as reference. Increasing levels of sodium chloride were added to pure water and to a specific mixture to evaluate the influence of EC on the sensor response. The ability of the Acclima TDT, Hydra Probe and 5TE sensors to measure EC was also evaluated. Advantages of the acetic acid-water mixtures as a dielectric standard for EM sensor characterization include the broad dielectric range as an alternative to more expensive liquids (e.g., isopropoxyethanol, dioxane).