WET-2 Soil Tri-Parameter Sensor (FDR-Based)

Overview

The WET-2 Soil Tri-Parameter Sensor is a field-deployable, high-stability FDR (Frequency Domain Reflectometry) instrument engineered for simultaneous, in-situ measurement of three fundamental soil physical properties: volumetric water content (θ_v), bulk electrical conductivity (EC_b), and soil temperature. Unlike time-domain reflectometry (TDR) systems requiring complex signal interpretation, the WET-2 operates at a fixed 20 MHz excitation frequency to induce stable dielectric polarization within the soil matrix. Dielectric permittivity (ε_r) — measured in the range of 1–80 — serves as the primary raw parameter; θ_v is derived via empirical calibration against soil-specific texture and density relationships. EC_b is extracted from low-frequency impedance components of the same FDR signal, while temperature is measured using an integrated Pt1000-grade thermistor embedded within the sensor body. This co-located, single-probe architecture eliminates inter-sensor spatial mismatch and ensures temporal synchrony across all three parameters—critical for irrigation scheduling, salinity stress assessment, and biogeochemical flux modeling.

Key Features

• Integrated tri-parameter sensing in a single, compact probe (55 × 45 × 10 mm housing; 3 mm Ø × 66 mm stainless-steel electrodes)
• FDR-based measurement principle with 20 MHz carrier frequency, delivering robust performance across variable soil textures and moisture regimes
• High-resolution digital output: dielectric permittivity resolution 0.1, EC_b resolution 1 mS/m, temperature resolution 0.1 °C, θ_v resolution 0.1 %vol
• IP65-rated enclosure ensures reliable operation under outdoor environmental exposure, including rain, dust, and transient immersion
• Fast response time of ≤5 seconds per full measurement cycle, enabling high-frequency monitoring in dynamic field conditions
• Low mass (75 g) and minimal insertion footprint reduce soil disturbance during installation and long-term burial
• Standard 2 m shielded, UV-resistant cable with industrial-grade connector (M12 or equivalent) for compatibility with most data loggers and telemetry systems

Sample Compatibility & Compliance

The WET-2 is validated for direct insertion into mineral soils ranging from sandy loam to clay loam, with demonstrated stability in organic-rich topsoil (up to 20% OM) and moderately saline substrates (EC_b ≤ 200 mS/m). It is not recommended for saturated peat or highly conductive brine-saturated sediments (>300 mS/m), where electrode polarization effects may compromise EC_b accuracy. The sensor conforms to IEC 60529 (IP65 ingress protection), meets CE marking requirements for electromagnetic compatibility (EMC Directive 2014/30/EU), and complies with RoHS 2011/65/EU restrictions on hazardous substances. While not certified to ISO 11277 or ASTM D5116 for laboratory-standardized soil characterization, its field measurement protocol aligns with FAO-recommended practices for agronomic soil water monitoring and supports data traceability in GLP-aligned environmental monitoring programs.

Software & Data Management

The WET-2 outputs calibrated analog (0–5 V or 4–20 mA) or digital (RS-485 Modbus RTU) signals compatible with Campbell Scientific CR-series, Decagon EM50G, Onset HOBO, and other industry-standard data acquisition platforms. Raw dielectric, EC_b, and temperature values are transmitted independently, permitting post-acquisition recalibration using site-specific pedotransfer functions. When integrated into networked environmental monitoring systems, the sensor supports audit-trail-capable data logging—including timestamped metadata, measurement status flags (e.g., “probe contact error”, “temperature out-of-range”), and firmware version reporting—to satisfy basic requirements for data integrity under ISO/IEC 17025 and FDA 21 CFR Part 11 Annex 11 contexts where electronic records are subject to review.

Applications

• Agricultural water management: Real-time feedback for deficit irrigation, drip system optimization, and root-zone water balance modeling
• Soil salinity mapping and temporal tracking in irrigated croplands, coastal zones, and reclaimed mining sites
• Ecological research: Quantifying soil thermal buffering capacity, freeze-thaw dynamics, and moisture-driven microbial activity thresholds
• Environmental impact assessment: Monitoring leachate plume migration, landfill cap performance, and remediation efficacy
• Climate observatories: Long-term validation of satellite-derived soil moisture products (e.g., SMAP, Sentinel-1) at in situ reference stations

FAQ

How does the WET-2 differentiate between bulk EC (EC_b) and pore-water EC (EC_p)?

It measures EC_b directly via low-frequency impedance. EC_p is calculated post-hoc using the Rhoades or Hilhorst mixing model, which requires concurrent θ_v and assumes uniform solute distribution. Accuracy of EC_p is strongly dependent on θ_v precision and soil-specific calibration.
Can the WET-2 be buried permanently?

Yes—its IP65 rating, corrosion-resistant electrodes, and stable FDR electronics support multi-season deployment. However, mechanical protection (e.g., PVC conduit or gravel backfill) is recommended in high-traffic or tillage-prone areas.
What is the operational temperature limit during measurement?

The sensor performs measurements within –5 °C to +50 °C. For extended storage outside this range, consult the manufacturer’s thermal derating guidelines to preserve long-term calibration stability.
Is factory recalibration required annually?

No scheduled recalibration is mandated. Field verification against gravimetric θ_v and standard KCl solutions for EC_b is recommended every 12–24 months depending on deployment severity and data-criticality requirements.