One Sensor.
Seven Soil Properties.

ThermoTDR goes far beyond soil moisture — measuring bulk density, porosity, thermal conductivity and more, simultaneously, in situ, at a fraction of the cost.

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The Problem

Why Soil Moisture Alone Isn't Enough

Conventional soil moisture sensors tell you one thing: how much water is in the soil. But understanding soil behaviour — infiltration, compaction, aeration, root growth — requires a much richer picture. Properties like bulk density, porosity, air-filled porosity and thermal conductivity are critical for soil process modelling, yet they typically require destructive sampling, expensive lab equipment, or separate instruments.

Soil Modellers

Coupled heat-water transfer models need thermal conductivity and heat capacity as inputs — not just water content. ThermoTDR provides these directly.

Soil Scientists

Tracking bulk density changes over time (compaction, tillage, settling) currently requires repeated coring. ThermoTDR does it continuously, non-destructively.

Precision Agriculture

Air-filled porosity controls root aeration and gas exchange. Knowing it in real time alongside moisture opens new management possibilities.

The Solution

What ThermoTDR Measures — From One Installation

By combining a heat pulse method with Time Domain Reflectometry in a single probe, ThermoTDR simultaneously determines:

Volumetric Water Content (θ) Thermal Conductivity (λ) Heat Capacity (C) Thermal Diffusivity (κ) Bulk Density (ρb) Porosity (n) Air-Filled Porosity (nₐ)

These aren't independent instruments — they are derived from two complementary physics on the same soil volume: electromagnetic wave travel time for water content, and heat diffusion for thermal-structural properties.

Traditional Approach

  • Soil moisture sensor (one property)
  • Core sampling for bulk density
  • Lab analysis for porosity
  • Separate thermal needle probe
  • Multiple site visits, destructive
  • $15,000–$25,000 equivalent setup

ThermoTDR Approach

  • All 7 properties simultaneously
  • Single buried installation
  • Non-destructive, continuous
  • Automated, IoT-ready (ESP32)
  • Real-time field monitoring
  • ~$500 proof-of-concept system
Heavy Lifting — Done

What We've Already Built, Tested & Validated

This wasn't a paper exercise. We studied the underlying thermal physics, understood why the CPC model is mandatory for our probe geometry, designed and built custom hardware, wrote firmware and analysis software from scratch, ran dozens of test cycles, and produced a comprehensive peer-reviewed technical report. The conceptual and prototyping groundwork is complete.

🔬
Concept

Deep-dived into thermo-TDR literature. Understood why CPC model (not simpler ILS) is essential for large-diameter heaters. Mapped every parameter derivation chain.

🔧
Hardware

Designed and built a custom heater assembly piggybacked onto a CS655 TDR sensor. ESP32-S3 microcontroller with WiFi-based real-time data acquisition. Multiple design iterations.

💻
Software

Full ESP32 firmware (FreeRTOS). Complete Python GUI analyser with CPC model fitting, parameter extraction, and quality diagnostics. All built from scratch.

📊
Testing

Soil tests with R² > 0.99 model fit. Agar calibration runs to characterise probe geometry. Iterative power and timing optimisation across multiple test cycles.

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Documentation

Comprehensive technical report with annexures, peer-reviewed by domain experts. Every design decision, limitation, and result transparently documented.

Project Gallery
Hardware Assembly Hardware Assembly CS655 + Heater Probe CS655 + Heater Probe ESP32 Web Interface ESP32 Web Interface Python Analyser GUI Python Analyser GUI CPC Model Fit CPC Model Fit (R²=0.99) Agar Calibration Setup Agar Calibration Setup

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~$500
vs. $15,000–$25,000 for equivalent commercial capability
Proof-of-concept cost using off-the-shelf components
The Path Forward

What's Needed Next

The science is validated. The next step is engineering a purpose-built sensor with optimised geometry — moving from proof-of-concept to a field-deployable instrument.

Custom Integrated Probe Design

Design and fabricate a probe with standardised geometry — correct heater diameter, probe spacing, and length ratios aligned with published literature (e.g., 2.4 mm heater, 10 mm spacing, 70 mm length). This is the single most impactful improvement.

Integrated or Add-On TDR

Either build TDR rods into the custom probe, or develop a robust attachment system for commercial sensors (CS655, SM300 etc.) — maintaining flexibility across platforms.

Calibration & Validation

Rigorous calibration in reference media (agar, glass beads, known soils), followed by field validation across soil types. The optimised geometry will dramatically improve accuracy.

Field Deployment & Testing

Long-term monitoring in real soil conditions — tillage effects, seasonal compaction, irrigation impacts — demonstrating the practical value for soil science and land management.

Collaboration

Support This Work

We are seeking support through Pan BSI (Better Start Initiative) or similar funding pathways to advance from proof-of-concept to a field-ready instrument. The core science is proven — what's needed now is targeted hardware engineering and testing.

Custom Probe Fabrication Geometry Optimisation Laboratory Calibration Field Validation Firmware Refinement

This technology has the potential to transform how we monitor soil physical properties — making multi-parameter soil sensing accessible, affordable, and continuous.

Kishor Kumar · Landcare Research (Manaaki Whenua) · New Zealand