Laboratory characterization of a commerc

Laboratory Characterization of a Commercial Capacitance Sensor for Estimating

Permittivity and Inferring Soil Water Content

Mike Schwank,* Timothy R. Green, Christian Ma¨ tzler, Hansruedi Benedickter, and Hannes Flu¨ hler

ABSTRACT

Ring-capacitor sensors are used widely for real-time estimation of

volumetric soil water content u from measured resonant frequency fr

,

which is directly affected by the bulk soil permittivity e. However, the

relationship fr(e) requires improved quantification. We conducted lab￾oratory experiments to characterize the response of the Sentek En￾viroSMART sensor system for a full range of e values from air to water

and a range of temperatures. Water–dioxane mixtures were placed into a

solvent-resistant container equipped with custom tools for heating and

mixing the fluid, removing air bubbles from sensitive surfaces, measuring

permittivity in situ, and creating an axisymmetric metal disturbance to

the electric field. Total capacitance C was measured using a vector net￾work analyzer (VNA) connected to one sensor, while four other sensors

provided replicated fr readings. The measured temperature response of

free water permittivity was linear with a negative slope, which is

qualitatively consistent with theory. A precise nonlinear relationship

between e and normalized fr was derived. The instrumental error in e

was RMSEe 5 0.226 (for 3 , e , 43), which corresponds to a mea￾surement precision in u(e) derived from Topp’s equation of RMSEu 5

0.0034 m3 m

23

. Axisymmetric numerical simulations of the electric field

supplemented the experimental results. The characteristic length scale

for the distance measured radially from the access tube is 12.5 mm,

meaning that 80 and 95% of the signal are sensed within approximately

20 and 37 mm of the access tube, respectively. The results are crucial

for scientific applications of the investigated sensor type to environ￾mental media.

C

APACITANCE SENSORS have been developed commer￾cially and are being used globally for estimating soil

water content. The relative permittivity e, or “dielectric

constant,” acts as a proxy for the volumetric soil water

content u in units of cubic meters per cubic meter. Ca￾pacitance, or frequency domain, sensors are designed to

measure resonant frequency rather than directly measur￾ing capacitance, and the permittivity of a medium is most

directly related to the effective capacitance. Such capaci￾tance sensors have been evaluated previously on the basis

of measured water contents (Baumhardt et al., 2000; Evett

and Steiner, 1995; Paltineanu and Starr, 1997). With the

exception of Kelleners et al. (2004a, 2004b), the relation￾ship between the sensor reading (or resonant frequency)

and added capacitance or permittivity of the measured

medium has not been well characterized. Kelleners et al.

(2004a, 2004b) focused on the effects of ionic conductivity

and dielectric losses rather than on quantifying a relation￾ship between soil permittivity and the sensor reading for a

full range of soil permittivity values (i.e., no data for the

approximate range of 3 , e , 20).

Available field data from near-surface sensors at

30- to 60-cm depths measured with the Sentek Enviro￾SMART (Sentek Sensor Technologies, Stepney, SA, Aus￾tralia) sensor system display variations at diurnal and other

time scales associated with measured temperature fluctua￾tions (Green et al., 2004).1 The exact causes and quanti￾fication (i.e., correction) of these temperature effects on

apparent water content measurements are currently un￾known, although the phenomenon has been observed in the

laboratory (Baumhardt et al., 2000) and theories have been

postulated (Or and Wraith, 1999; Robinson et al., 2003).

To gain further insight into dielectric processes in soils

from long-term time series of data collected with the

capacitive EnviroSMART system, the relation between

the sensor reading and the soil permittivity e is nec￾essary. For that purpose we performed the laboratory

characterization using this specific sensor type. How￾ever, the procedures presented here are adaptable for

characterizing similar new measuring systems before

their field application.

Knowing the exact relation between the soil permit￾tivity e and the sensor reading is also required for val￾idating and improving dielectric mixing models used for

soil moisture estimation from proxy data e. The scope of

the present study is limited to inferring u from e based on

previous work (e.g., Topp et al., 1980), noting that

quantification of e is not sufficient to determine the accu￾racy of u in real soils due to complex losses associated

with the imaginary part of e.

BACKGROUND AND THEORY

Design and Deployment of the

EnviroSMART Probe

The Sentek EnviroSMART probe and capacitance sen￾sors have been designed to determine soil water content in

the field with or without local surface access. Figure 1a

shows field installation of the plastic access tube using a

hand auger to ensure contact between the soil and outer

M. Schwank, Institute of Terrestrial Ecosystems (ITES), Swiss Federal

Institute of Technology (ETH), CHN E29, Universita¨ tstr. 16, CH-8092

Zu¨ rich, Switzerland; T.R. Green, USDA-ARS, Great Plains Systems

Research Unit, Fort Collins, CO, USA; C. Ma¨ tzler, Institute of Ap￾plied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Swit￾zerland; H. Benedickter, Laboratory for Electromagnetic Fields and

Microwave Electronics, ETHZ, ETZ K 88, Gloriastrasse 35, CH-8092

Zu¨ rich, Switzerland; H. Flu¨ hler, Institute of Terrestrial Ecosystems

(ITES), Swiss Federal Institute of Technology (ETH), CHN F 28.1,

Universita¨ tstr. 16, CH-8092 Zu¨ rich, Switzerland. Received 13 Jan. 2006.

*Corresponding author ([email protected]).

Published in Vadose Zone Journal 5:1048–1064 (2006).

Original Research

doi:10.2136/vzj2006.0009

ª Soil Science Society of America

677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: FEP, fluorinated ethylene-propylene; SMD, surface

mounted device.

1The EnviroSMART capacitance sensors evaluated here were de￾signed and manufactured by Sentek Pty. Ltd., Australia. Use of such

commercial products does not constitute endorsement by the ETHZ,

USDA-ARS, or University of Bern. Sentek did not provide any fi￾nancial assistance (cash or in-kind) for the project.

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

1048

Published online August 24, 2006