ON THE IMPORTANCE OF INTERNATIONAL STANDARDIZATION OF METHODOLOGIES AND TECHNIQUES FOR LABORATORY AND FIELD CALIBRATION OF SOIL WATER MEASUREMENT SENSORS BASED ON CAPACITANCE, IMPEDANCE, AND TDT

One of the most important facets of technological and research development trajectory, in a globalized world, is standardization of methodologies and techniques for both the manufacturers “bench testing” techniques, and for the more elaborate, independent, laboratory and field calibration methodologies of soil water measurement sensors in more diverse, but very well described conditions. This paper will introduce a global review and critique of past and current methods and techniques for direct correlations of sensors outputs to known dielectric constants of specific liquids, as well as correlations between apparent dielectric constant of soil matrix vs. volumetric water content determined experimentally in the main zone of influence of electromagnetic fields generated by sensors operating at different frequencies. Published and unpublished experimental data will be used to show the similarities and differences between some well known soil water measurement sensors and probes based on capacitance, impedance and TDT. A minimum of experimental protocols describing sensors and test requirements, as well as statistical analysis and interpretation, will be proposed such as: frequencies of operation; response of sensors in air and distilled water at room temperature ( ~22 deg. C); soil intrinsic characteristics (texture, clay mineralogy, EC, organic material, gravel content, etc..); method and uniformity of soil packing (dry and wet bulk density); axial and radial sensitivity of sensors in distilled water, in air – water and in air – soil interfaces, as well as in soil at different depths; rotational orientation response of cylindrical sensors inside access tubes; sensors response in controlled levels of temperature in air, water, and soil; sensors response at different soil EC and temperature conditions; scaled output(frequency, voltage, etc..) vs. volumetric water content; description of specific liquids of known dielectric constant, etc..

Keywords: soil water sensors, capacitance, impedance, TDT, calibration methods, standardization