Cotton Production and Processing Unit, USDA-ARS, Lubbock, TX 79403, USA.
Sensors (Basel). 2011;11(1):757-70. doi: 10.3390/s110100757. Epub 2011 Jan 12.
Accurate measurement of moisture content is a prime requirement in hydrological, geophysical and biogeochemical research as well as for material characterization and process control. Within these areas, accurate measurements of the surface area and bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, to accurate characterization of soil water content in soil water conservation research to bio-plant water utilization to chemical reactions and diffusions of ionic species across membranes in cells as well as in the dense suspensions that occur in surface films. One promising technique to address the increasing demands for higher accuracy water content measurements is utilization of electrical permittivity characterization of materials. This technique has enjoyed a strong following in the soil-science and geological community through measurements of apparent permittivity via time-domain-reflectometry (TDR) as well in many process control applications. Recent research however, is indicating a need to increase the accuracy beyond that available from traditional TDR. The most logical pathway then becomes a transition from TDR based measurements to network analyzer measurements of absolute permittivity that will remove the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity in traditional TDR applications.This research examines an observed experimental error for the coaxial probe, from which the modern TDR probe originated, which is hypothesized to be due to fringe capacitance. The research provides an experimental and theoretical basis for the cause of the error and provides a technique by which to correct the system to remove this source of error. To test this theory, a Poisson model of a coaxial cell was formulated to calculate the effective theoretical extra length caused by the fringe capacitance which is then used to correct the experimental results such that experimental measurements utilizing differing coaxial cell diameters and probe lengths, upon correction with the Poisson model derived correction factor, all produce the same results thereby lending support and for an augmented measurement technique for measurement of absolute permittivity.
准确测量水分含量是水文、地球物理和生物地球化学研究以及材料特性描述和过程控制的首要要求。在这些领域中，为了回答从棉花纤维成熟度的描述到土壤保水研究中土壤水分含量的精确描述，再到生物植物水分利用、离子物种在细胞膜内以及致密悬浮液中的化学反应和扩散等诸多基本问题，表面面积和结合水含量的准确测量变得越来越重要。在致密悬浮液中，离子物种在细胞膜内以及致密悬浮液中的化学反应和扩散等诸多基本问题，为了回答从棉花纤维成熟度的描述到土壤保水研究中土壤水分含量的精确描述，再到生物植物水分利用、离子物种在细胞膜内以及致密悬浮液中的化学反应和扩散等诸多基本问题，为了回答从棉花纤维成熟度的描述到土壤保水研究中土壤水分含量的精确描述，再到生物植物水分利用、离子物种在细胞膜内以及致密悬浮液中的化学反应和扩散等诸多基本问题，表面面积和结合水含量的准确测量变得越来越重要。满足更高精度水分测量要求的一种很有前景的技术是利用材料的介电常数特性进行描述。该技术在土壤科学和地质界通过时域反射（TDR）测量表观介电常数得到了广泛的应用，并且在许多过程控制应用中也得到了应用。然而，最近的研究表明，需要提高传统 TDR 所提供的精度。那么，最合乎逻辑的途径就是从基于 TDR 的测量过渡到网络分析仪对绝对介电常数的测量，这将消除高表面积土壤和电导率对传统 TDR 应用中表观介电常数测量的不利影响。本研究考察了同轴探头的一个观测到的实验误差，该探头是现代 TDR 探头的起源，假设该误差是由边缘电容引起的。该研究为误差的原因提供了实验和理论依据，并提供了一种校正系统的技术，以消除该误差源。为了验证这一理论，我们建立了一个同轴电池的泊松模型，计算出由边缘电容引起的有效理论附加长度，然后利用该泊松模型导出的修正因子对实验结果进行修正，使利用不同同轴电池直径和探头长度进行的实验测量在修正后都能得到相同的结果，从而为绝对介电常数测量技术的增强提供了支持。
