Calibration of Capacitance Probe Sensors using Electric Circuit Theory

Abstract

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 gp is higher than the geometric factor for the medium gm, 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.

Calibration of a water content reflectometer and soil water dynamics for an agroforestry practice

“…This 92% predictability could be attributed to several factors. The relationship between k and h v is directly proportional to the free water content such as with sand (Zazueta and Xin 1994;Kelleners et al 2004a). The soils used in this study contained up to 54% clay.…”

“…Gravimetric, nuclear, electromagnetic, and tensiometer methods can be used to estimate soil water content (Zazueta and Xin 1994). Capacitance sensors (Dean et al 1987;Kelleners et al 2004a), impedance sensors (Hilhorst et al 1993;Seyfried and Murdock 2004), and transmission line oscillators (Campbell and Anderson 1998) are electromagnetic approaches to measure soil water content which are often preferred over neutron probe methodology. Relatively inexpensive CS616 water content reflectometer (WCR) sensors (Campbell Sci.…”

A time domain transmission sensor with TDR performance characteristics

“…Most of these instruments do not permit the measurement of bulk soil electrical conductivity, which can be useful for management purposes. Many of the capacitance sensors are sensitive to interference from bulk soil electrical conductivity (Robinson et al, 1998;Kelleners et al, 2004) and while many will continue to operate, the prediction of water content can be very poor (Baumhardt et al, 2000). In addition to cost and user-ability limitations, the length of cable that the probe can be attached to without signal attenuation compromising accurate permittivity and subsequently, water content estimations, limits TDR sensor systems.…”

Experimental study of the complex resistivity and dielectric constant of chrome-contaminated soil

“…Dielectric dispersion characteristics are often present that relate the sensor frequency to the permittivity of a medium (Kelleners et al, 2004). Some successful examples of the use of the surface ERT to monitor a tracer test in a complex environment have been tested (Monego et al, 2010;Wilczek et al, 2012).…”