Standardizing characterization of electromagnetic water content sensors: Part 2. Evaluation of seven sensing systems

Transmission line- type electromagnetic ( EM) methods for estimating soil volumetric water content (theta(v)) have advanced significantly in recent years, with many sensing systems now available. To estimate theta(v), EM systems make use of the dependence of soil dielectric permittivity on theta(v). However, a standard method for characterizing and comparing EM system measurement capability has not been established. Our objective was to evaluate the permittivity measurement ability of seven EM sensing systems using readily available media. Sensing system outputs were converted to real permittivity (epsilon') values and compared with reference epsilon' values in lossless and lossy dielectric liquids under four different test conditions: nonrelaxing and nonconducting ( NR- NC), relaxing and nonconducting ( R- NC), nonrelaxing and electrically, conducting ( NR- C), and temperature variation in NR- NC. The higher frequency broadband sensing systems, consisting of two time domain reflectometry ( TDR) systems and one time domain transmissometry ( TDT) system, deviated from a network analyzer by less than +/- 2.94 epsilon' units across a epsilon' range of 12.7 to 78.5 in NR- NC media. Two lower frequency impedance sensing systems deviated from the network analyzer by less than +/- 3.94 epsilon' units across a epsilon' range of 12.7 to 36.5 in the same media. Measurement of epsilon' using higher frequency broadband sensing systems was impacted more by bulk electrical conductivity (sigma b) and temperature ( T) than by dielectric relaxation. Imaginary permittivity values ( due only to relaxation, epsilon(rel)) of up to 14.5 in R- NC media resulted in epsilon' errors of +/- 0.511, whereas sigma(b) values ranging from 0 to 2 dS m(-1) in NR- C media resulted in epsilon' errors of +/- 2.69 and T values ranging from 5 to 40 degrees C resulted in epsilon' errors of +/- 4.89. Determination of epsilon' using lower frequency sensing systems - including one transmission line oscillator, two impedance probes, and one capacitance probe - was impacted more by sigma(b) than by T and epsilon(rel). For the lower frequency sensors ( and the same ranges of sigma(b), T, and epsilon(rel)), sigma(b) resulted in epsilon' errors of +/- 111, T resulted in epsilon' errors of +/- 6.59, and e(rel) resulted in epsilon' errors of +/- 3.28. The effects of epsilon(rel), sigma(b), and T on permittivity measurement accuracy is to a large extent dependent on measurement frequency, with higher frequency broadband sensing systems generally yielding better measurements.