Improved Membrane-Based Sensor Network for Reliable Gas Monitoring in the Subsurface

Abstract

A conceptually improved sensor network to monitor the partial pressure of CO2 in different soil horizons was designed. Consisting of five membrane-based linear sensors (line-sensors) each with 10 m length, the set-up enables us to integrate over the locally fluctuating CO2 concentrations (typically lower 5%vol) up to the meter-scale gaining valuable concentration means with a repetition time of about 1 min. Preparatory tests in the laboratory resulted in a unexpected highly increased accuracy of better than 0.03%vol with respect to the previously published 0.08%vol. Thereby, the statistical uncertainties (standard deviations) of the line-sensors and the reference sensor (nondispersive infrared CO2-sensor) were close to each other. Whereas the uncertainty of the reference increases with the measurement value, the line-sensors show an inverse uncertainty trend resulting in a comparatively enhanced accuracy for concentrations >1%vol. Furthermore, a method for in situ maintenance was developed, enabling a proof of sensor quality and its effective calibration without demounting the line-sensors from the soil which would disturb the established structures and ongoing processes.

New Sensor Technology for Field‐Scale Quantification of Carbon Dioxide in Soil

“…The line sensor presented here covers the entire range of typical CO 2 concentrations in soil with high precision. This was demonstrated for <1.6% CO 2 by Lazik and Ebert (2012) and for concentrations <8% by Lazik and Sood (2016) for dry gases. In this study, we demonstrated the applicability of the line sensor for natural, humid soil conditions.…”

Approach for Self-Calibrating CO2 Measurements with Linear Membrane-Based Gas Sensors

“…Within these linearity ranges, such a membrane-based sensor can be calibrated by a gas-specific linear characteristic. To that end, the responses $a_1^I$ have to be compared with reference gases of known composition [27,28,31,32]. …”

“…Insofar as different sensor arrangements cause different slopes, also the replacement of a selective membrane by a membrane of the same chemical formulation and geometry could cause a change in slope, e.g., due to changes in manufacturing. For instance, we report an experimentally determined value of about 21% vol s/mbar for CO 2 measurement with a tubular PDMS membrane of 10 m length, an inner radius of 0.7 mm and an outer radius of 1.8 mm [31]. In contrast, for a 40 m-long PDMS membrane with an inner radius 2 mm and outer radius of 3.5 mm, we determined a slope of about 190% vol s/mbar for the same range of 0–5% vol CO 2 that was added to air [32].…”

“…Alternative to this direct calibration, the calibration gases can be flushed also through the line-sensor instead of the purge gas (inverse calibration), which enables the calibration of installed line-sensors without their dismounting [31]. …”

“…Already in 1939, this strong temperature dependency was considered for rubber-like polymeric membranes in [35]. The influence of this dependency on the measurement signal of a membrane-based gas sensor was investigated for an experimental temperature range of 297–302 K in [31]. However, negligible dependency was found within that 5 K range.…”