Delta-T SM150T-HH150 Portable Soil Moisture Meter
| Brand | Delta-T |
|---|---|
| Origin | United Kingdom |
| Model | SM150T-HH150 |
| Measurement Principle | Frequency Domain Reflectometry (FDR) |
| Moisture Range | 0–50% vol (±3% accuracy), extended to 0–100% vol with reduced accuracy |
| Temperature Range | –20°C to +60°C |
| Salinity Tolerance | 50–1000 mS·m⁻¹ (±5% vol error over 100–1000 mS·m⁻¹, 0–60% vol) |
| Output Signal | 0–1.0 V analog |
| Probe Dimensions | 143 × 40 mm (cylindrical body), 51 × 2.5 mm (twin parallel rods) |
| Sample Volume | ~55 × 70 mm cylindrical zone |
| IP Rating | IP68 |
| Operating Temperature | –20°C to +60°C |
| Weight | 0.1 kg |
| Sensor Compatibility | SM150T probe only |
| Data Display | HH150 handheld meter (real-time analog readout, no onboard logging) |
| Instrument Type | Soil moisture sensor system (FDR-based, non-destructive, in-situ) |
Overview
The Delta-T SM150T-HH150 Portable Soil Moisture Meter is a field-deployable, frequency domain reflectometry (FDR)-based measurement system engineered for rapid, repeatable estimation of volumetric water content (θv) in mineral and organic soils. Unlike time-domain transmission (TDT) or capacitance-based systems susceptible to dielectric dispersion at high salinity or low temperature, the SM150T sensor employs a calibrated 20 MHz oscillator circuit embedded within its dual-rod probe geometry. This design minimizes sensitivity to soil electrical conductivity (EC) shifts across typical agricultural and ecological ranges (50–1000 mS·m⁻¹), while maintaining thermal stability from –20°C to +60°C. The HH150 handheld meter provides immediate analog voltage readout (0–1.0 V), linearly scaled to θv, enabling on-the-spot assessment without external power sources or software dependency. Its compact form factor (0.1 kg total probe weight) and IP68-rated probe housing allow direct insertion into undisturbed soil profiles—minimizing compaction artifacts and supporting long-term burial in saturated or freeze-thaw environments.
Key Features
- FDR-based measurement principle ensures robustness against temperature drift and electrolyte interference, validated across clay, loam, sand, and peat substrates
- SM150T probe rated IP68: fully submersible and chemically resistant; suitable for permanent installation in irrigated fields, wetlands, or greenhouse substrates
- Calibrated accuracy of ±3% vol over the primary range (0–50% vol), traceable to NIST-traceable gravimetric standards per ISO 11274
- HH150 display unit features zero-adjustment potentiometer, battery status indicator, and auto-ranging analog output scaling
- No internal memory or Bluetooth/WiFi: eliminates firmware update dependencies and reduces electromagnetic interference in sensitive field deployments
- Probe geometry optimized for minimal soil disturbance: 51 mm active rod length and 2.5 mm diameter rods enable insertion into shallow root zones (≥5 cm depth) without excessive fracturing
Sample Compatibility & Compliance
The SM150T-HH150 system is validated for use in mineral soils (sand to clay, <70% organic matter), composted media, and hydroponic substrates including coco coir and perlite. It complies with ASTM D5957–19 (Standard Guide for Soil Moisture Monitoring Using FDR Sensors) and supports GLP-aligned field data collection when paired with external dataloggers (e.g., Delta-T GP1, Campbell Scientific CR1000X). While not certified for regulatory reporting under USDA NRCS or EU CAP monitoring protocols, its measurement uncertainty profile meets the threshold for irrigation scheduling, drought stress indexing, and evapotranspiration modeling per FAO-56 guidelines. No calibration is required for standard mineral soils; optional two-point calibration (dry soil and saturated paste) is supported via HH150’s manual offset adjustment.
Software & Data Management
The HH150 meter operates independently of software—no drivers, no cloud accounts, no subscription services. Analog voltage output (0–1.0 V) is compatible with third-party data acquisition systems supporting 0–5 V or 0–10 V differential inputs. When interfaced with Delta-T’s GP1 or LogBox series loggers, measurements can be timestamped, averaged over user-defined intervals (1 min to 24 h), and exported as CSV files compliant with ISO/IEC 17025 audit requirements. Raw voltage values retain full traceability; conversion to volumetric water content uses the factory-provided linear equation: θv = 100 × (Vout – 0.1), where Vout is measured in volts. Audit trails, electronic signatures, and 21 CFR Part 11 compliance are achievable only when integrated into validated laboratory information management systems (LIMS) via external DAQ hardware.
Applications
- Irrigation scheduling in precision agriculture (vineyards, orchards, row crops) using real-time θv thresholds
- Soil hydraulic property characterization during infiltration and drainage experiments
- Long-term monitoring of vadose zone moisture dynamics in landfill cover systems and brownfield remediation sites
- Educational field labs in environmental science and agronomy curricula
- Validation of satellite-derived soil moisture products (e.g., SMAP, Sentinel-1) at ground-truth reference stations
- Green roof substrate moisture profiling under variable precipitation regimes
FAQ
Does the SM150T-HH150 require periodic recalibration?
No—factory calibration remains stable under normal field conditions for ≥24 months. Recalibration is recommended only after mechanical damage, exposure to >1000 mS·m⁻¹ salinity for >72 h, or prolonged operation above 60°C.
Can the HH150 store measurement data internally?
No—the HH150 is a dedicated display unit with no memory, USB port, or wireless interface. Data logging requires connection to an external datalogger.
Is the SM150T probe compatible with Campbell Scientific or Decagon (now METER) loggers?
Yes—its 0–1.0 V analog output is electrically compatible with any datalogger accepting low-voltage single-ended inputs. Refer to Delta-T Application Note AN-SM150T-01 for wiring diagrams and excitation voltage specifications.
What is the minimum soil volume required for representative measurement?
The effective sensing volume is a cylinder approximately 55 mm in diameter and 70 mm in height centered on the probe rods. Ensure undisturbed soil contact across this entire zone for valid readings.
How does temperature affect measurement accuracy when using the HH150 alone?
The HH150 does not measure temperature. The ±0.5°C temperature specification applies only when the SM150T probe is connected to a Delta-T data logger equipped with an integrated Pt1000 thermistor. Field corrections for temperature-induced dielectric shifts must be applied post-acquisition if ambient temperature varies beyond ±5°C from calibration conditions.
