Abstract
Accurate moisture sensing is an important need for many research programs as well as in control of industrial processes. This paper describes the development of a high accuracy frequency domain sensing probe for use in obtaining dielectric measurements of materials suitable for work ranging from 300 MHz to 1 GHz. The probe was developed to accommodate a wide range of permittivity’s ranging from εr = 2.5 to elevated permittivity’s as high as εr = 40. The design provides a well-matched interface between the soil and the interconnecting cables. A key advantage of the frequency domain approach is that a change of salt concentration has a significantly reduced effect on ε′, versus the traditional time-domain reflectometry, TDR, measured apparent permittivity, Ka.
Highlights
In the pursuit of high accuracy moisture sensors and systems, of critical need is a high quality reference probe by which to establish a standard by which to judge the accuracy of new theoretical models and more practical systems that are suitable for mass-production
As accurate sensing of soil moisture requires a large sensing volume; of particular need is a method to transition from 50 Ohm coax cables to the sensing structure that preserves signal integrity and allows for high-quality measurements to be obtained at frequencies ranging from direct-current, DC, well into the microwave region
As such all measurements were performed directly in the frequency domain, with the sole exception of the time‐domain reflectometry (TDR) results, which were obtained in the normal manner
Summary
In the pursuit of high accuracy moisture sensors and systems, of critical need is a high quality reference probe by which to establish a standard by which to judge the accuracy of new theoretical models and more practical systems that are suitable for mass-production. Due to soil’s inherently local variability and porosity, the best soil moisture sensors typically utilize a high frequency design that strives to balance signal integrity versus obtaining a reasonable sensing volume of soil. This tradeoff led to wide-spread adoption of high-frequency, HF, time-domain reflectometry probes and techniques, TDR [1]. Recent research has noted errors [2,3] and accuracy limitations of TDR, in heavy soils with high salt contents or with high clay contents [1,4,5] To mitigate these errors, research efforts have striven to develop algorithmic corrections [6,7,8]. As time domain signals are captured using very high-speed analog to digital converters, the signals do not have a large dynamic range
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