Calibration and Characterization of an Improved Low-Cost Water Content Sensor

1. Fig. 3. Rockwool calibration at various electrical conductivities (in parenthesis). Fig. 1. Calibration of five EC-5 probes in various mineral soils at different ECs (in parenthesis). Fig. 4. EC-5 dielectric calibration using a TDR-100 Fig. 2. EC-5 calibration in various mixtures of potting soil at different ECs (in parenthesis) Calibration and Characterization of an Improved Low-Cost Water Content Sensor Colin S. Campbell*, Gaylon S. Campbell,Douglas R. Cobos, Brody L. Teare, and Jeff Ritter Decagon Devices, Inc. Introduction Inexpensive, accurate, and reliable soil moisture measurements are necessary in countless applications from research to hydroponics. Many have dreamed of these types of sensors being so inexpensive that they could be spread over a wide area to monitor water status at numerous locations in a watershed, greenhouse, or golf course to name a few. However, measurements of this sort are only as valuable as their ability to truly portray conditions in which they measure. Cheap moisture measurement devices have been available for years, but could only give rough, relative moisture information. Likewise, accurate volumetric water content (VWC) sensors have been available for a long time, but were much too expensive for extensive placement. New, less expensive sensors show considerable promise but must be tested to show if they maintain the measurement quality of the higher ones Results Mineral Soil Calibration: Five EC-5 probes were calibrated in a variety of mineral soils at varying electrical conductivities (Fig. 1). Probe data show very little dependence on soil type or electrical conductivity (EC) and no probe-to-probe variation either. Results indicate there is no need for soil specific calibration. Rockwool Calibration: Rockwool (Master, Grodan BV), is a greenish fibrous mat visually similar to fiberglass insulation, used to grow greenhouse crops in hydroponics. Probe output in rockwool is nonlinear (Fig. 3, different from the other media tested), but has good sensitivity over the range of rockwool VWC (0 to ~ 97%, rockwool has a porosity of 97%) and appears to have a low sensitivity to changes in solution EC. Dielectric Permittivity Calibration: EC-5 output was calibrated against a TDR-100 system to give a dielectric calibration (Fig. 4). Although the probe is more sensitive at lower dielectric permittivity, there is good resolution across the entire range. Objective Test a new soil water content sensor to determine if it can accurately and reliably determine dielectric permittivity and VWC in a range of soils and soil-less media. Potting Soil Calibration: The same five EC-5 probes were calibrated in three types of potting soil (Fig. 2). According to the data, the same calibration equation can be used for any of the potting soils tested, regardless of potting soil mixture or electrical conductivity. Probe Characterization: One of the most important pieces of information from any soil moisture probe is the accuracy of their factory calibration. A single EC-5 was tested along with an ML2 sensor in three mineral soils and a potting soil to characterize their readings with respect to actual water content. Both probes performed very well across a wide range of VWC (Fig. 5), with standard deviations of 0.0088 and 0.013 m3/m3 for the EC-5 and ML2, respectively. Materials and Methods The probe was calibrated in the laboratory in a series of soils and soil-less media to correlate probe output to dielectric and VWC. Probes Type: EC-5 Water Content sensor (Decagon Devices, Inc) Soil and Soil-less media: Several mineral soils, along with potting soil and rockwool. Dielectric: A TDR-100 system (Campbell Scientific, Inc.) was used to determine actual dielectric of the material Actual Water Content: Determined by gravimetric analysis using a known measurement volume. Electrical Conductivity: Saturation extract Probe Characterization: Delta-T ML2 Soil Moisture Sensor Fig. 5. EC-5 and ML2 characterization using factory calibration versus known volumetric water contents. Conclusions Although inexpensive, the new EC-5 soil moisture sensor calibrated well over a variety of soil types and electrical conductivities, showing very little sensitivity to the EC and soil type variations that have caused problems for inexpensive probes in the past. Further characterization of the probe using the factory calibration showed that reliable, accurate measurements can be obtained in a variety of different soils. Decagon Devices, Inc. 950 NE Nelson Ct. Pullman, WA, USA 509-332-2756 *colin@decagon.com www.decagon.com