Published online 26 May 2006
Published in Vadose Zone J 5:805-808 (2006)
DOI: 10.2136/vzj2006.0139
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
NOTES
A Flexible Water Content Probe for Unsaturated Soil Column Experiments
E. D. Mattsona,*,
K. E. Bakera,
C. D. Palmera,
C. R. Breckenridgeb,
J. M. Svobodac and
R. W. Smithd
a Geosciences Dep.
b Biological Sciences Dep.
c Information and Communication Systems Dep., Idaho National Lab., Idaho Falls, ID 83415
d Biological and Agricultural Engineering Dep., Univ. of Idaho, Idaho Falls, ID 83415
* Corresponding author (earl.mattson{at}inl.gov)
1 The description of commercial products does not constitute an endorsement of those products by the INL nor the U.S. Department of Energy. 
Received 28 November 2005.
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ABSTRACT
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A commercially available soil moisture probe was modified by replacing rigid electrode waveguides with flexible electrode traces that can be attached to the interior of soil column walls. This new design minimizes soil packing difficulties and potential bias in water and solute flow pathways commonly associated with rigid probe installations in column experiments. Tests demonstrated that the modified flexible electrode design maintains a voltage response that is proportional to the electrode trace length and is reduced by approximately one-half when only one side of the electrode trace is in contact with the soil media. Laboratory experiments confirmed that the longer electrode traces attached to the interior of a column wall have a comparable sensitivity of that of a commercially available rigid probe inserted horizontally across the diameter of a soil column. The replacement of the rigid electrode with a flexible electrode offers an improved method of measuring the water content in solute transport experiments while minimizing sensor intrusion into laboratory soil columns.
Abbreviations: TDR, time domain reflectometry
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INTRODUCTION
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SMALL diameter (5–7 cm) column experiments are often used to evaluate water flow and solute transport through unsaturated soils in controlled laboratory-scale experiments. The distribution of water in the soil column is an important variable affecting variably unsaturated fluid and solute transport. However, there are some difficulties with current techniques for measuring water content in soils in unsaturated soil column studies.
Direct measurement (Topp and Ferré, 2002a) of soil water content by destructive sampling of the soil and measuring the amount of water contained in a known volume or mass of soil only provides a one-time "snapshot" of the water content for that particular volume of soil and does not allow repeated testing of the same soil volume. Indirect methods measure a physical or chemical property of the soil (e.g., permittivity, electrical conductivity, heat capacity, hydrogen content, and magnetic susceptibility) that has a predictable relationship to its water content often require sensors to be embedded in the soil. These embedded sensors can locally disturb the soil either through the insertion process into undisturbed soil or through packing of the soil around the sensor.
Soil disturbance due to sensor emplacement has been a concern for laboratory- and field-scale water and solute studies (e.g., Corwin, 2000; Costello and Horst, 1991). Sensors that vertically penetrate the soil can create macropores and modify water and solute flow pathways. Intrusive sensors can hamper uniform packing of soil columns and, depending on their size, can create flow obstructions in the column. Sensors placed in the soil after the column is assembled disturb the soil structure adjacent to the sensor affecting instrument calibration and increasing uncertainty in the measured water content. Traditional commercially available sensors have been designed primarily for field applications and investigate soil volumes that are not feasible in most laboratory-scale column experiments. Therefore, there is a need for less intrusive water content sensors for laboratory-scale soil column experiments.
Printed circuit boards have been used as waveguides to miniaturize time domain reflectrometry (TDR) probes (e.g., Nissen et al., 1999, 2003) and capacitance probes (Decagon, 2004). The advantage of these probes is that the sensing portion of the probe can be reduced to sizes more amenable for laboratory-scale column experiments. The disadvantage of the reduction is that the modified probes measure a smaller volume and are therefore more sensitive to the degree of contact with the porous medium being measured (Nissen et al., 2003). In addition, the measurement volume includes both the soil medium as well as the circuit board material, and the measurement magnitude in each material is dependent on the permittivity of the porous medium (Robinson et al., 2003). Commercially available printed circuit board probes thus far have been made of rigid materials that in general must be inserted into the porous medium to be measured. It is the insertion of these rigid waveguides into undisturbed soils and the potential of soil packing biases around these probes in repacked soil experiments that contribute to the concern about soil disturbance.
To overcome this concern, we replaced the rigid electrode waveguides of a commercially available capacitance probe with flexible electrode traces in an effort to minimize soil disturbance while maintaining the sensitivity of the probe for use in small-diameter column studies. A capacitance probe is an electromagnetic sensor designed to measure soil water content by utilizing the soil surrounding the electrodes as part of a capacitor. The measurement is a function of the bulk dielectric constant of the soil, the imposed frequency, and the electrode geometry (Starr and Paltineanu, 2002). Commercially available sensors incorporate a fixed electrode geometry (length, electrode spacing) and material properties to obtain consistent readings between probes. Although we used the capacitance technique in these tests, the result should be applicable to other permittivity measurement techniques. This note describes the design of the flexible electrodes and presents results of laboratory testing conducted to quantify the response of the modified probes in relation to commercially available rigid probes.
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MATERIALS AND METHODS
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Our flexible probe design consisted of modifying commercially available ECH2O probes (EC-10; Decagon Devices, Inc. Pullman WA) by replacing the three rigid, parallel copper waveguides of the FR 4 circuit board with a geometrically identical pattern on a flexible Kapton circuit board (Advanced Circuits, Aurora, CO).1 This modification allows experimentalists to attach the sensing electrodes around the inside circumference of soil columns to minimize the sensor intrusion into the soil column, while utilizing the commercially available electronic circuitry of the ECH2O probe. The electronics of the commercial ECH2O probe were attached to the Kapton circuit board by cutting the bulk of the fiberglass board off, leaving a 2-cm stub projecting from the electronics, milling the fiberglass board to expose the copper waveguides in the fiberglass boards, and then connecting the copper traces in the Kapton circuit board to the copper waveguides of the fiberglass board using three stainless-steel bolts (Fig. 1
).
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Fig. 1. (a) Plan and (b) cross-sectional view of the flexible capacitance probe attachment to the column. (c) Picture of a flexible electrode prototype in a soil column; (d) picture of the standard EC-10 probe inserted horizontally across the column.
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Three sets of laboratory experiments were conducted to examine the impact of probe electrode trace modifications on sensor voltage response and sensor calibration to water content. The first set of tests examined voltage response as a function of flexible electrode trace length to determine application limitations. The second set of experiments examined voltage response of electrode traces in a homogeneous measurement medium (quartz sand) and a heterogeneous medium consisting of quartz sand against one face and a sheet of plastic against the other face. The last set of experiments compared sensor responses between a nonintrusive flexible electrode trace mounted on a column wall and a commercially available rigid probe that is horizontally placed across the column.
Ottawa F-110 quartz sand and 330 mg kg–1 KBr solution were used during the calibration experiments. The Ottawa sand was rinsed with ultrapure water (
18 M
) to remove excess salts and then oven dried for 24 h (e.g., Topp and Ferré, 2002b). The sand was wetted to the desired water content with a 330 mg kg–1 KBr solution (specific conductance of 405 µS cm–1), mixed well, and packed into test chambers.
The test chamber for the first two test sets consisted of a plastic tray (2.7 cm high by 6.7 cm wide by 22 cm long). For the first test set, sand was packed into the lower half of the tray to approximately 1.3 cm thickness; the electrode was then placed on top of the sand and additional sand was packed on top of the electrode until it was level with the tray surface. A series of probe voltage responses were measured where the probe's electrodes were shortened from 9.5 to 5.6 cm with 10% gravimetric water content quartz sand in contact with both sides of the electrode surfaces.
For the second test set, the rectangular test tray was slightly modified to examine the effect of attaching one side of the probe against a plastic wall. For these tests, a 0.6-cm (0.25 inch) piece of clear plastic was cut to fit in the bottom of the tray. The Kapton circuit board was attached to the clear plastic, and the electronics were bolted to the base of the tray. Due to chemical incapability between the Kapton circuit board, the plastic column, and numerous glue adhesives, we used vinyl tape to attach the edges of the Kapton board to the column wall.
For the third test set, we compared the response of a flexible probe wrapped partially around the inner circumference of a soil column to that of a rigid probe cut to the length that would fit across the diameter of a column. Both columns were packed with Ottawa sand at water contents ranging from 0 to 20% by weight. The flexible electrode length was 13.4 cm and was attached to the 0.6-cm-thick column wall using the vinyl tape along the edges of the electrode. Although the flexible Kapton sensor circuit board could be trimmed to a length that would totally encompass the column circumference, we cut the Kapton sensor to cover approximately 75% of the column circumference, allowing space to concurrently monitor soil-matric potential using a tensiometer (Fig. 1c). For comparison, a standard ECH2O EC-10 probe was cut from 9.5 to 5.6 cm (i.e., the inner diameter of the column) and fitted into a slot such that the electrodes fit across the diameter of the column (Fig. 1d). Soil was subsequently packed around the probe at the desired water content.
Calibration procedures followed those described by Starr and Paltineanu (2002), with modifications to accommodate the flexible electrodes. An excitation voltage of 2.5 V was applied to the probes using a Hewlett-Packard (Palo Alto, CA) power supply, Model 6206B, and the differential output voltage was measured with a Fluke (Everett, WA) multimeter, Model 87 III. After the voltage measurement was recorded, the container and the probe were weighed for soil bulk density calculations and a subsample of the soil was taken for gravimetric water content (ratio of water weight to dry soil weight) determination by the oven drying method. These procedures were repeated for gravimetric water content from 0 to 20% in 5% increments. Soil bulk densities were calculated from the gravimetric subsamples, initial weight of the moist sand, and the volume of the test chambers. Volumetric water contents were calculated using the gravimetric water content and bulk density measurements.
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RESULTS AND DISCUSSION
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Results of the first test set showed that the voltage response of both the rigid and the flexible circuit board probes were linearly related to electrode length. Linear regression analysis of these data calculated similar equations: V = (0.0205 ± 0.0011)L + (0.206 ± 0.008) for the rigid probes (r2 = 0.994) and V = (0.0180 ± 0.0022)L + (0.193 ± 0.017) for the flexible probes (r2 = 0.956), where V equals voltage and L equals trace length in centimeters. Both the slope and the intercept for the rigid probes are significantly different from zero (t = 19.0, P = 0.0028 df = 2; t = 24.9, P = 0.0016, df = 2, respectively). Similarly, both the slope and intercept for the flexible probes are significantly different from zero (t = 8.10, P = 0.0039, df = 3; t = 11.4 P (>|t|) = 0.0015, df = 3, respectively). A comparison of the two curves (e.g., Glantz, 2002) shows that the two curves are not coincident (F = 35.4, df in numerator = 2, df in the denominator = 5, P = 0.0011). Student t tests indicate that the slopes are not significantly different (t = 0.97, df = 5, P = 0.181) but that the intercepts are significantly different (t = 4.85, df = 5, P = 0.002). When the results of this test were normalized (Fig. 2
) to the probe output at the maximum electrode length (9.5 cm), the intercepts (0.00077 ± 0.0419 and –0.00028 ± 0.0963 for the rigid and flexible traces, respectively) are not significantly different from zero (t = 0.0184, P = 0.987, df = 3 for the rigid trace; t = –0.0029, P = 0.998, df = 2 for the flexible trace), nor are they significantly different from one another (t = 0.008, P = 0.497, df = 5). The slopes of the voltage response curves for both the rigid and flexible probes (0.9857 ± 0.05182 and 0.0969 ± 0.012) are not significantly different from 1 (t = 0.276, P = 0.404, df = 2; t = 0.258, P = 0.407, df = 3 for the rigid and flexible traces, respectively), nor are they significantly different from on another (t = 0.122, P = 0.454, df = 5).
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Fig. 2. Probe response (normalized to an electrode length of 9.5 cm) versus normalized electrode length (cm/9.5 cm) for the flexible and rigid probes.
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The results of the second test set show that the sensitivity of the probe response as a function of the water content change in the quartz sand was reduced when the sand was only in contact with a single surface of the electrode traces. The probe responses in both cases were linearly related to water content with r2 > 0.98 (Fig. 3
). The slope of the probe response with only a single side of the electrode in contact with the soil probe (0.0037 ± 0.03) was 55% of the slope when soil was in contact with both sides of the electrode (0.0067 ± 0.02). The relatively small standard errors for the fitted parameters suggest that for our laboratory experimental conditions and capacitance electronics, any shift of the flexible electrode sampling volume was not significant enough to require higher-order regression analysis. Common plastics typically used in the construction of column experiments (e.g., PVC, Plexiglas) have a permittivity of approximately 3 (Lide, 1996, p. 13–12), a value similar to that of the common mineral components of soil (3–12; Telford et al., 1990, Table 5.5, p. 291). On the column side of the electrical sensors, we expect the low permittivity of the plastic and the air (1) to be invariant, whereas the permittivity on the inside of the column will depend on the soil used and will vary with the soil water content and the specific conductance of the soil solution. Numerical analysis of the sampling volume interrogated by parallel electrodes supported on a plastic backing (Robinson et al., 2003) shifts from a measurement volume within the plastic backing when the soil has a permittivity less than the plastic backing to a sampling volume within the soil as soil permittivity increases. The linear relationship between probe voltage output and soil water content for our test results suggest that any variability of the electrode sampling volume location due to changing the soil/water permittivity value did not bias our test results.
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Fig. 3. Plot of probe response versus water content for a flexible electrode with soil in contact on both sides and a flexible electrode attached to a plastic sheet with soil in contact on a single side.
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The results of the first two test sets clearly demonstrate that the electrode response is a function of both the length of the waveguides and the configuration of the electrode (i.e., soil on one side or both sides of the electrodes). A flexible electrode installed along the column wall should decrease its sensitivity by 55%. However, the flexible probe wrapped around the total internal circumference of a column could be 
times longer than a rigid probe placed across the column. Thus, despite the 55% sensitivity reduction when one side is placed along a plastic wall, the resultant probe sensitivity could be as great as 1.7 times (0.55) than that of a probe being placed across the column. The experiments in the third test set were designed to test this hypothesis.
Results for the third test set showed that both the flexible probe and the rigid probe responded linearly as a function of volumetric water content with r2 > 0.98 (Fig. 4
). The expected slope ratios can be calculated by multiplying the ratio of the flexible electrode traces to the rigid waveguides lengths by the reduced sensitivity of having the soil in contact with a single side of the flexible electrode traces. For our column experiment, the expected value is 132% (i.e., 11.4 cm/5.6 cm x 55%). From Fig. 4, the slope of the flexible electrode probe is 111% (i.e., 0.0062/0.0056 x 100) of the slope of the rigid probe and is in general agreement to the expected value. This comparison result suggests that the reduced flexible probe length covering only three-fourths of the circumference of the column wall measured water content is at least as precise as a rigid probe emplaced horizontally across the soil column.
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Fig. 4. Plot of a rigid probe inserted through a soil column and a flexible probe attached to a column wall as a function of water content of the quartz sand.
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CONCLUSIONS
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A commercially available probe was re-engineered as a nonintrusive water content sensor for soil column studies by replacing rigid electrodes with flexible electrode traces. The modified flexible probe is fairly inexpensive, easy to build, and minimizes the intrusion of the sensor into the soil profile. A linear response of the probe output with respect to water content was observed in quartz sands and with solutions of fairly low electrical conductivity. The improved sensitivity of the flexible probe due to its long length of flexible electrode is sufficient to make up for the reduced sensitivity due to soil only being on one side of the electrodes.
The study presented in this technical note evaluated only the geometric effects of replacing the rigid electrodes with a flexible electrode in repacked quartz sand. It should be noted that specific difficulties (e.g., effects of varying electrical conductivity) of using the capacitance technology are not overcome using flexible electrodes. The effects of soil properties (e.g., bulk density, mineralogy) and electrode contact with intact soil cores were also not investigated. Soil specific calibrations are recommended for individual probes.
In addition to monitoring water content in cylindrical laboratory soil columns, flexible electrodes could also be advantageous in measuring water content in deformable porous media and in media with nonplanar contact surfaces. Potential applications include adapting the flexible electrodes to monitor water content in other circular objects such as trees, deformable materials in the case of freeze–thaw conditions and swelling soils, and against irregular surfaces encountered in deep boreholes in the vadose zone as described by Wyatt et al. (1999) or Dahan et al. (2003).
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ACKNOWLEDGMENTS
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This research was supported by the Environmental Management Science Program of the Office of Science, U.S. Department of Energy (DOE). I would also like to acknowledge the assistance of Ms. Jena Davis (DOE Office of Science Pre-service Teacher Program) and Ms. Melanie McCandless (DOE office of Science Student Undergraduate Laboratory Internship Program) in the laboratory studies.
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REFERENCES
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ABSTRACT
INTRODUCTION
