Calibration of capacitance probe sensors using electric circuit theory
Calibration of capacitance probe sensors using electric circuit theory
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DOI：
10.2136/sssaj2004.0430
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发表时间：
2004-03-01
影响因子：
2.9
通讯作者：
Skaggs, TH
中科院分区：
文献类型：
--
作者：
Kelleners, TJ;Soppe, RWO;Skaggs, TH
电容式探头传感器是一种很有吸引力的估算土壤水分的电磁技术。然而，人们担心土壤盐分和土壤温度对传感器的影响。我们提出了一个电路模型，它将传感器频率与介质的介电常数联系起来，并能够校正由于离子电导和弛豫引起的介电损耗。在改进的设置中，使用传感器读数优化了电路电感L，其中陶瓷电容器取代了传感器的电容板。模型中的其他三个参数使用传感器读数在一系列具有不同介电常数的非导电介质中进行了优化。塑料导通管的几何因子g(P)大于介质g(P)的几何因子，表明大部分电磁场不会超出导通管。离子导电性对传感器读数的影响是通过在三种介质中混合盐来评估的。其影响是深远的。传感器频率随电导率的增加而减小。这种影响在介电常数最低的介质中最为明显。该电路模型能够对传感器的电导效应进行校正。然而，随着介质损耗的增加，频率对介电常数变得相对不敏感，测量频率或传感器常数的微小误差会导致计算的介电常数出现较大误差。电容传感器的校准可以简化，只需确定其中两个常量，并使用传感器在空气和水中的读数计算其他两个常量。
Capacitance probe sensors are an attractive electromagnetic technique for estimating soil water content. There is concern, however, about the influence of soil salinity and soil temperature on the sensors. We present an electric circuit model that relates the sensor frequency to the permittivity of the medium and that is able to correct for dielectric losses due to ionic conductivity and relaxation. The circuit inductance L is optimized using sensor readings in a modified setup where ceramic capacitors replace the sensor's capacitance plates. The three other parameters in the model are optimized using sensor readings in a range of nonconductive media with different permittivities. The geometric factor for the plastic access tube g(p) is higher than the geometric factor for the medium g(p) indicating that most of the electromagnetic field does not go beyond the access tube. The effect of ionic conductivity on the sensor readings is assessed by mixing salts in three of the media. The influence is profound. The sensor frequency decreases with increasing conductivity. The effect is most pronounced for the medium with the lowest permittivity. The circuit model is able to correct for the conductivity effect on the sensors. However, as the dielectric losses increase, the frequency becomes relatively insensitive to permittivity and small inaccuracies in the measured frequency or in the sensor constants result in large errors in the calculated permittivity. Calibration of the capacitance sensors can be simplified by fixing two of the constants and calculating the other two using sensor readings in air and water.