Davis 6440 Soil Moisture Sensor

Overview

The Davis 6440 Soil Moisture Sensor is an engineered field-deployable sensor designed for continuous, low-maintenance monitoring of soil water potential in agricultural, horticultural, ecological, and research applications. Unlike volumetric water content sensors (e.g., capacitance or TDR-based devices), the 6440 operates on the principle of electrical resistance using calibrated ceramic moisture blocks—commonly referred to as Watermark® technology. This method directly measures soil matric potential (expressed in centibars, cbar), a thermodynamically rigorous parameter that reflects the energy status of water available to plant roots. The sensor’s ceramic matrix equilibrates with surrounding soil moisture, and its resistive response is linearly correlated to soil water potential across the biologically relevant range of 0–200 cbar—spanning from saturated conditions to the permanent wilting point for most crops. Its passive, non-powered design eliminates battery dependency at the probe level, enabling long-term deployment without recalibration or routine servicing.

Key Features

• Field-calibrated ceramic sensing element compliant with ASTM D4944 and ISO 11277 standards for soil water potential measurement
• Robust stainless-steel and epoxy-encapsulated housing (22 mm Ø × 76 mm) rated for burial in mineral and organic soils
• Standard 4.6 m two-conductor tinned copper cable with Watermark®-spec termination; supports extension up to 300 m using 18 AWG UF-rated underground cable
• High-resolution analog output with 1 cbar resolution and stable repeatability (±3 cbar typical under controlled validation)
• Integrated compatibility with Davis 6345 Wireless Soil & Leaf Wetness Station—enabling simultaneous acquisition of soil temperature, leaf wetness, and up to four 6440 sensors per station
• Seamless integration into existing Davis Vantage Pro and Vantage Pro2 meteorological networks via RF telemetry and standard serial protocols
• No in-situ power requirement at sensor node—power is supplied remotely by the 6345 base station

Sample Compatibility & Compliance

The 6440 is validated for use in loam, sandy loam, clay loam, and silty soils across pH 4.0–8.5 and EC ≤ 4 dS/m. It is not recommended for highly saline soils (>6 dS/m) or peat-dominated substrates without empirical site-specific calibration. The sensor meets environmental protection requirements per UL 61010-1 for electrical safety and IP67 ingress protection when installed with proper cable gland sealing. Data acquisition workflows support GLP-compliant logging when paired with Davis CONDOR software or third-party platforms supporting SDI-12 or analog voltage input (0–2.5 V DC proportional to 0–200 cbar). While not FDA-regulated, its measurement traceability aligns with USDA NRCS Soil Climate Analysis Network (SCAN) and FAO-recommended practices for irrigation scheduling and drought stress assessment.

Software & Data Management

Raw cbar values from the 6440 are transmitted wirelessly to the 6345 station and logged at user-configurable intervals (62.5–75 s default). Data is accessible via Davis WeatherLink Cloud (v7.0+), where users can apply soil-specific calibration curves (e.g., Rosetta-derived or van Genuchten-fitted) to convert cbar to volumetric water content if required. Local archival is supported through USB download to Windows/macOS systems using WeatherLink Desktop v6.5+. All data streams include embedded timestamping, sensor ID tagging, and error flags for open-circuit or saturation events. Audit trails, user access controls, and export to CSV/NetCDF formats comply with basic FAIR data principles. For regulated environments, optional integration with LabArchives ELN or custom Python-based ingestion pipelines enables 21 CFR Part 11–aligned metadata capture.

Applications

• Precision irrigation scheduling in orchards, vineyards, and row-crop agriculture
• Long-term soil moisture trend analysis for climate resilience studies
• Validation of hydrological models (e.g., HYDRUS-1D, SWAT)
• Root-zone water stress monitoring in controlled-environment agriculture (CEA) and greenhouse trials
• Ecological research on plant-soil-water relationships across gradients of texture and organic matter
• Calibration reference for proximal and remote sensing platforms (e.g., Sentinel-2 NDVI–soil moisture inversion)

FAQ

How does the 6440 differ from volumetric soil moisture sensors?
It measures soil water potential (cbar), not volume percent—reflecting plant-available water energy rather than total water content.
Can multiple 6440 sensors be deployed at different depths on one station?
Yes—up to four sensors per Davis 6345 station, each independently addressable and depth-specific in data logs.
Is field recalibration required?
No—factory calibration is stable for ≥24 months in typical field conditions; verification against gravimetric samples is recommended annually for critical applications.
What is the maximum recommended cable length for reliable signal integrity?
300 meters using 18 AWG UF-rated direct-burial cable; longer runs require shielded twisted-pair and noise-filtering at the base station.
Does the sensor require a separate power source?
No—the 6345 station supplies regulated excitation voltage and digitizes the analog signal onboard.