Sensitivity of a Capacitance Sensor to Artificial Macropores

Abstract

The effects of large macropores, cracks, and other large heterogeneities within the capacitance sensor sensitivity volume are of concern in application of these sensors. The objective of this work was to evaluate the sensitivity of the Enviroscan sensors to the presence of macropores. The artificial macropores were manufactured from polystyrene tubes and placed in contact with the access tube and at distances of 1.5 cm and 2.5 cm in air, water, and wetted sand. Measurements were obtained during sequential filling with water eight macropores at the same distance from the access tube. The dependencies of the raw frequencies on the number of filled macropores were linear with r 2 > 0.98. The macropore sensitivity, defined as the decrease in frequency per one filled macropore, deceased exponentially with the distance from the access tube. For a given distance from the access tube, the macropore sensitivity depended linearly on the scaled frequency. The effect of the water-filled macropores on the bulk soil water content estimate decreased with the increase in soil water content. The estimated errors of measurement were approximately 0.002 cm 3 cm -3 (0.001 cm 3 cm -3 ) per 1% of soil volume near the access tube occupied by the macropore for the water content of bulk soil of 0.10 cm 3 cm -3 (0.30 cm 3 cm -3 ) and were about three times smaller at the 1.5 cm distance from the access tube. Overall, macropores near the access tube had only moderate effect on the soil water content measurements with capacitance sensors when they occupied less than 13% of soil volume near the access tube.