Soil Science Society of America Journal 65:1081-1083 (2001)
© 2001 Soil Science Society of America
DIVISION S-1—NOTES
An inexpensive, portable meter for measuring soil moisture
Joseph J. O'Brien* and
Steven F. Oberbauer
Dep. of Biological Sciences, Florida International Univ., University Park, Miami, FL 33199
* Corresponding author (obrienj{at}fiu.edu)
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ABSTRACT
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Until now, dielectric-based soil moisture measurements required expensive cable testers or commercial systems to read probes. Here we describe a method of constructing an inexpensive meter from a multimeter and a simple power supply. When coupled with a Campbell Scientific (Logan, UT) CS615 probe, the entire system costs 
$350 US. A single meter can be used to measure multiple probes and the entire system is quite small and portable. The new system reads soil moisture probes capable of measuring a soil's dieclectric constant. Measurements taken with the meter described here and a CR10 data logger recommended by the probe manufacturer did not differ significantly. Nor was there any measurement offset between the data logger and the meter.
Abbreviations: TDR, time domain reflectometry • VDC, volts direct current
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INTRODUCTION
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TIME DOMAIN REFLECTOMETRY (TDR) has proven to be a very effective means to infer in situ soil water content. Time domain reflectometry measures the velocity of electromagnetic pulses as they traverse a wave-guide of a known length inserted in the soil (Topp et al., 1980). The velocity of the signal changes with the dielectric constant of the soil, which is largely a function of the soil water content. Wave-guides are inexpensive and relatively easy to construct (Zegelin et al., 1989), however, measuring the EM pulse travel time requires an expensive cable tester.
The CS615 water content reflectometer (Campbell Scientific, Logan, UT) provides a less expensive means to measure soil moisture though changes in a soil's dieclectric constant. The CS615 probe consists of a small, portable, epoxy-encapsulated circuit board with two 30-cm steel wave-guides (Bilskie, 1997). The wave-guides are inserted into the soil, and the probe outputs a square wave with an amplitude of 2.5 volts direct current (VDC). The probe is powered by 9 to 18 VDC and is activated with a minimum 1.3-VDC signal. Like TDR, the CS615 is sensitive to changes in signal propagation time along the wave-guide, driven by changes in a soil's dieclectric constant. The CS615 differs from TDR in that it uses a point on the signal reflected off the end of a wave-guide to trigger an event and generate output that varies in frequency, instead of analyzing the entire waveform as in TDR (J. Bilskie, 2000, personal communication). The output frequency (Hz) can be measured by a data logger, which inverts the output to give a period (milliseconds) and applies a polynomial function, converting the period to volumetric soil moisture content (Anonymous, 1996). Since soil dielectric measurements are influenced by soil physical and chemical properties such as texture (% clay and % organic matter) and pore water conductivity, the manufacturer recommends site-specific (or soil-specific) calibration of the probe. Campbell Scientific reported the probe accuracy as ± 2% soil volumetric water content when using a specific soil calibration.
Data loggers and other commercial soil moisture meters, while considerably less expensive than a cable tester, are still costly. Furthermore, when the user does not need to continuously log the probes or they are widely spaced, measuring them with a data logger monopolizes a versatile instrument that might be better used for other measurements if alternatives were available.
Here we describe the construction of a small, lightweight, and inexpensive meter for the Campbell CS615 water content reflectometer. We also compare the results of readings taken with our meter and a data logger recommended by the probe manufacturer.
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Materials and Methods
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The meter was built from a digital multimeter (Goldstar DM-332, LG Precision, Los Angeles, CA) capable of reading frequency, a 12-VDC power source, a switch, and a 7805 5-VDC voltage regulator integrated circuit (Fig. 1). Any multimeter capable of measuring frequencies in the range of 0 to 2 kHz could be used to construct the meter. For humid or wet environments, water-resistant multimeters are available. All of these parts are readily available off the shelf at a local electronics supplier. Eight 1.5 V AA batteries connected in series supplied the 12 V required to power the probe and the voltage regulator. The battery output was reduced to 5 VDC by the voltage regulator that activates the CS615. A toggle switch was added to conserve battery power when the probe was not in use. The batteries and circuitry were enclosed in a small, hinged plastic box attached directly to the back of the multimeter, and the probe was connected using a five-conductor audio plug. The probes can be connected directly to the meter, or with connectors allowing repetitive reading of permanently installed probes.
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Fig. 1. Schematic diagram of the meter for the CS-615 probe. The pin numbers of the 7805 chip refer to (1) voltage in, (2) ground, and (3) voltage out.
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This study compares the frequencies measured by our meter to a commercially available solid-state data logger (CR10, Campbell Scientific, Logan, UT). A CS615 probe was inserted, following the published procedures (Anonymous, 1996), in 15 scattered sites in oxic Humitropept soil at La Selva Biological Station, Costa Rica. The probe output was read with the meter and the data logger in random order at each site. The probe was enabled for 1 min before each reading to allow the probe output to stabilize. In order to put the frequencies into a more meaningful context, we applied an empirically-derived calibration to the frequency output (Veldkamp and O'Brien, 2000) to convert the probe output frequency to volumetric water content. This calibration function was used instead of the Campbell Scientific function because the soils at La Selva have high clay and organic matter content and low bulk densities.
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Results
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The CS615 drew 70 mA when activated, so AA batteries will last 21 h if the probes are read continuously, and much longer if intermittent readings are taken. The estimated mean soil volumetric water content values as measured by the meter (37.0%) and CR10 (37.4%) did not differ (t = 9.278, df = 14, P = 0.000). The values shown nearly perfectly correlated (R2 = 0.9998, F1,13 = 62987.61, P = 0.000) across the range of values we sampled, with a y intercept of 0 and a slope of 1 (Fig. 2).
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Conclusion
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The total cost for the complete system described in this paper, including a CS615 probe, is $350 U.S. ($140 for the meter and $210 for the probe). In comparison, commercially available soil moisture meters or data loggers cost more than double this amount. Our meter has the added benefit of a smaller size and lighter weight than a data logger. The combined probe and meter weigh less than a kilogram, and can easily fit into a small backpack. Furthermore, we found no difference in the readings between a CR10 data logger and our probe.
We use the portable meter to measure soil moisture with CS615 probes installed in widely scattered plots in tropical rain forest. The meter and probes have performed well. Readings can be taken rapidly: with a five-conductor plug in place, the measurement takes 2 min, including 1 min to allow readings to stabilize.
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ACKNOWLEDGMENTS
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This paper is contribution 28 of the Tropical Biology Program of Florida International University.
Received for publication May 13, 2000.
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REFERENCES
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- Anonymous. 1996. CS615 Water Content Reflectometer Instruction Manual. Campbell Scientific, Logan, UT.
- Bilskie, J. 1997. Using dielectric properties to measure soil water content. Sensors Magazine 14:26–32.
- Topp, G.C., J.L. Davis, and A.P. Annan. 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resour. Res. 16:574–582.
- Veldkamp, E., and J.J. O'Brien. 2000. Calibration of a frequency domain reflectometry sensor for humid tropical soils of volcanic origin. Soil Sci. Soc. Am. J. 64:1549–1553.[Abstract/Free Full Text]
- Zegelin, S.J., I. White, and D.R. Jenkins. 1989. Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry. Water Resour. Res. 25:2367–2376.
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ABSTRACT
INTRODUCTION
