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Research Note #003BME688 · Gas Resistance · VOC ProfilingRESEARCH HYPOTHESIS

Root Zone Gas Resistance Profiling Under Controlled Moisture Deficit Conditions

Published: August 2026·Status: METHODOLOGY DRAFT — Experiment Pending Phase 05 Hardware

METHODOLOGY DRAFT — NOT YET EXPERIMENTALLY EXECUTED

This note documents the planned experimental methodology. Telemetry below is illustrative modelled output — not empirical measurements from a physical benchtop trial. Physical execution is planned for Phase 05 (Prototype Validation).

01 — RESEARCH QUESTION

Can the Bosch BME688 MOX gas resistance sensor detect systematic shifts in ambient root zone VOC profiles associated with progressive soil moisture deficit — and can these shifts be distinguished from temperature-driven baseline drift?

02 — HYPOTHESIS

As substrate moisture decreases, root zone microbial activity and root respiration rates change, producing detectable shifts in total volatile organic compound (VOC) concentration measurable by the BME688 MOX heater element as a systematic gas resistance shift — independent of ambient temperature fluctuation.

NULL HYPOTHESIS: Gas resistance shifts observed during substrate drying are entirely explainable by ambient temperature and humidity changes alone — not by root zone biological activity.

03 — SCIENTIFIC BACKGROUND

Metal oxide semiconductor (MOX) gas sensors detect changes in ambient gas composition via resistance changes across a heated ceramic substrate. The BME688 operates a resistive heater at 200–400 degrees C, causing oxidation and reduction reactions with target gas molecules on the SnO2 surface.

Soil microbial communities produce VOCs including alcohols, aldehydes, and terpenoids. Root respiration produces CO2 and water vapour. As soil moisture decreases, anaerobic microbial populations shift, potentially altering VOC emission profiles detectable as systematic gas resistance changes.

Key limitation: BME688 is a broad-spectrum MOX sensor — it cannot identify individual VOC species. Any claimed biological signal requires careful elimination of temperature, humidity, and CO2 confounds.

04 — PLANNED METHODOLOGY

Experimental Groups

  • Test plant — progressive drying from field capacity to 30% soil moisture
  • Control — same species maintained at 80–90% field capacity
  • Blank — empty pot with substrate only, same drying protocol

Measurement Protocol

  • 10-minute BME688 gas resistance readings every 30 minutes
  • SHT41 ambient T/RH logged every 5 minutes for compensation
  • Gravimetric soil moisture every 6 hours
  • Identical environmental enclosure for all groups
05 — TARGET HARDWARE

Bosch BME688

Primary gas resistance sensor

CANDIDATE

Sensirion SHT41

Ambient T/RH compensation

CANDIDATE

ESP32-S3 DevKit

Data logging and telemetry export

BENCHTOP TESTED
06 — ILLUSTRATIVE PROJECTED TELEMETRY
SIMULATED MODEL — NOT EXPERIMENTAL DATA

Interactive Telemetry Time-Series

Leaf Temperature Delta (ΔT) vs Vapor Pressure Deficit (VPD) Shift

Leaf ΔT (°C)
VPD (kPa)
00:0001:0002:0003:0004:00
Elapsed: 04:00Pre-Wilting Stress Signal

At VPD 2.8 kPa, leaf thermal delta reached +1.2°C.

Click points on graph to inspect readings

Projected illustrative gas resistance trajectory under progressive substrate drying. Actual telemetry replaces this upon physical trial execution.

07 — BME688 BASELINE DRIFT CHARACTERIZATION

BME688 MOX sensors exhibit significant long-term baseline drift due to MOX heater aging, ambient humidity history, and sensor poisoning from high VOC exposure. This experiment must account for drift before attributing gas resistance changes to root zone biology.

DRIFT SOURCES CATALOGUED

  • Ambient RH changes (major driver)
  • MOX heater temperature stability
  • Sensor warm-up period (24h minimum)
  • Long-term heater element aging

DRIFT CORRECTION APPROACH

  • SHT41 T/RH compensation model
  • Blank control subtraction
  • 24h stabilisation window before trials
  • Normalised resistance delta (not absolute)
08 — MEASUREMENT PIPELINE

MEASUREMENT

BME688 total gas resistance (Ohm)

FEATURE EXTRACTION

Delta-R normalised vs SHT41 humidity

DRIFT CORRECTION

Blank control subtraction

PATTERN DETECTION

Systematic shift > 2 sigma baseline

INFERENCE (IF FOUND)

Possible root zone activity change

CONFIDENCE

TBD — pending physical experiment

09 — KNOWN LIMITATIONS
  • BME688 cannot identify specific VOC species — only total reducing/oxidizing gas mixture shift.
  • Root zone gas signals must travel through substrate, drainage layer, and pot wall — significant signal attenuation expected.
  • MOX sensor cross-sensitivity: human exhalation CO2, cooking odors, and cleaning chemicals will produce larger signals than root zone biology.
  • Experiment requires strict environmental enclosure control — any ambient VOC contamination invalidates results.
  • Stomatal closure simultaneously affects leaf gas exchange AND ambient VOC levels — potential correlation without root zone causation.
10 — CURRENT STATUS AND NEXT STEPS

WHAT IS COMPLETE

  • Experimental methodology designed
  • BME688 baseline drift literature review
  • Compensation model approach defined
  • Data schema and collection protocol ready

PENDING PHYSICAL EXECUTION

  • BME688 + SHT41 benchtop PCB assembly
  • Environmental enclosure construction
  • 30-day trial execution (Phase 05)
  • Statistical analysis and null hypothesis testing