Standardizing Characterization of Electromagnetic Water Content Sensors: Part 1. Methodology

cases salinity. However, it is sometimes the case that a new sensor is promoted and distributed only to be Performancedifferences inthe growingnumber ofelectromagnetic disapproved of years later due to poor measurement (EM) sensors designed to estimate soil water content from a variety of indirect measurements (e.g., from measured travel time, capacitance, performance. The cost to users in unreliable experimenfrequency shift) suggests the need for a standardized sensor character- talresultsorreducedproductivityforgrowersultimately ization methodology. We suggest that characterization and evaluation falls back on the company in the form of a damaged of EM sensors, which currently lack citable standards, be performed reputation. These costs could be minimized or avoided in a homogeneous fluid of known permittivity rather than in a porous by appropriate assessment of sensors and dissemination medium of unknown permittivity. Our objectives were to (i) develop ofstandardized performancecriteria. Studiesevaluating a methodology for evaluating EM sensor measurement attributes EM sensor performance have typically evaluated perreferencing sensor-specific characteristics and targeted soil properties mittivity determinations in a number of soils over a and (ii) suggest standards for characterization and comparison of sensors. range of water contents (Evett et al., 2002; Leib et al., Criteria for qualitative assessment of sensors include determination 2003; Seyfried and Murdock, 2004). These studies are of effective measurement frequency; susceptibility to variations in salinity, dielectric relaxation, and temperature; and a look at spatial useful in demonstrating the general water content meavariationinsensorsamplingarea.Measurementfrequenciesforbroad- surement capability in specific soils. However, these band sensors can be inferred from correlated network analyzer and results are often misleading and conflicting due to consensor measurements or from manufacturer suggestions. Fluids were founding effects arising from bound water or salinity selected to provide surrogate soil-related effects such as relaxation that can be disguised by soil-specific calibrations showoccurring both within and outside of the effective measurement fre- ing improved apparent sensor performance. The quesquency range of common sensors. Test conditions included dielec- tion we ask is, “Does the sensor provide a unique and trically relaxing (R) and nonrelaxing (NR) as well as electrically con- therefore reliable permittivity response (e.g., voltage, ducting(C)andnonconducting(NC)liquidsandcombinationsthereof travel time) from water content changes when tempera(e.g., NR-C). No suitable combinationof relaxing and conducting (R-C) ture, salinity, and even soil type are also varied?” Estab