Membrane Based Measurement Technology for in situ Monitoring of Gases in Soil

Lazik, Detlef; Ebert, Sebastian; Leuthold, Martin; Hagenau, Jens; Geistlinger, Helmut

doi:10.3390/s90200756

Cited by 26 publications

(25 citation statements)

References 7 publications

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“…The linearity between

Δ χ_{x}^{a}

and

a_{1}^{I}

in Equation (3) has already been experimentally demonstrated for mixing oxygen and nitrogen within a large mole fraction range of

χ_{O 2}^{a} = 0.01 - 1

[28] and for CO 2 mixed with air for a range

χ_{C O 2}^{a} \leq 1

[30] for laboratory conditions. Within these linearity ranges, such a membrane-based sensor can be calibrated by a gas-specific linear characteristic.…”

Section: Resultsmentioning

confidence: 92%

“…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]. …”

Section: Resultsmentioning

confidence: 99%

“…Equation (1) forms the basis for a multi gas component analysis [27] as well as for analyzing a single gas component. Considering a mixing process of a gas component with a gaseous matrix

{χ_{1}, χ_{2} \dots}

(referred to as

{χ_{j}}_{j = 1, 2 \dots}

), such single gas component analyses are demonstrated for O 2 and CO 2 using a single gas-selective membrane [28]. …”

Section: Theoretical Descriptionmentioning

confidence: 99%

“…During the consecutive measurement step, a characteristic change of mole numbers is caused within the measurement chamber near the adjusted steady-state gas flows. This change can be monitored either in terms of a volumetric change or a pressure change, and can be used to determine individual gas concentration, e.g., combining different selective measurement cells (combinations of membrane-coated chamber and a gas-tight reference chamber) [27,28]. Such membrane-based gas sensors could be advantageous in measurement tasks where only a single gas component changes.…”

Section: Introductionmentioning

confidence: 99%

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

Lazik

Sood

2016

Sensors

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Linear membrane-based gas sensors that can be advantageously applied for the measurement of a single gas component in large heterogeneous systems, e.g., for representative determination of CO2 in the subsurface, can be designed depending on the properties of the observation object. A resulting disadvantage is that the permeation-based sensor response depends on operating conditions, the individual site-adapted sensor geometry, the membrane material, and the target gas component. Therefore, calibration is needed, especially of the slope, which could change over several orders of magnitude. A calibration-free approach based on an internal gas standard is developed to overcome the multi-criterial slope dependency. This results in a normalization of sensor response and enables the sensor to assess the significance of measurement. The approach was proofed on the example of CO2 analysis in dry air with tubular PDMS membranes for various CO2 concentrations of an internal standard. Negligible temperature dependency was found within an 18 K range. The transformation behavior of the measurement signal and the influence of concentration variations of the internal standard on the measurement signal were shown. Offsets that were adjusted based on the stated theory for the given measurement conditions and material data from the literature were in agreement with the experimentally determined offsets. A measurement comparison with an NDIR reference sensor shows an unexpectedly low bias (<1%) of the non-calibrated sensor response, and comparable statistical uncertainty.

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“…The linearity between

Δ χ_{x}^{a}

and

a_{1}^{I}

in Equation (3) has already been experimentally demonstrated for mixing oxygen and nitrogen within a large mole fraction range of

χ_{O 2}^{a} = 0.01 - 1

[28] and for CO 2 mixed with air for a range

χ_{C O 2}^{a} \leq 1

[30] for laboratory conditions. Within these linearity ranges, such a membrane-based sensor can be calibrated by a gas-specific linear characteristic.…”

Section: Resultsmentioning

confidence: 92%

“…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]. …”

Section: Resultsmentioning

confidence: 99%

“…Equation (1) forms the basis for a multi gas component analysis [27] as well as for analyzing a single gas component. Considering a mixing process of a gas component with a gaseous matrix

{χ_{1}, χ_{2} \dots}

(referred to as

{χ_{j}}_{j = 1, 2 \dots}

), such single gas component analyses are demonstrated for O 2 and CO 2 using a single gas-selective membrane [28]. …”

Section: Theoretical Descriptionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Lazik

Sood

2016

Sensors

Self Cite

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“…For short times, Equation (7) yields the following linearized term:

a_{1}^{(I)} = {\frac{d p}{d t} |}_{t \to + t_{0}} = g P_{s} \sum_{j} f_{j, s} false(p_{0}^{j} - p_{L}^{j} false)

where the pressure change, dp / dt , within the measurement chamber relates for the case (I) to the partial pressure differences between the faces of the membrane. This relation was successfully used to both simultaneously estimate O 2 and N 2 concentrations using two measurement chambers covered with membranes of differing selectivities [2] and analyze a single gas, such as O 2 or CO 2 , mixed in air [4] using only a single membrane-based gas sensor (one cell). The notation “ a 1 ” originates from the measurement procedure based on approximating the recorded pressure evolution using a polynomial, p = a 0 + a 1 ( t – t 0 ) + a 2 ( t – t 0 ) 2 + … Disregarding the higher-order terms for short times, t – t 0 , and differentiating yields dp / dt = a 1 .…”

Section: Transient Sensor Theorymentioning

confidence: 99%

Membrane-Based Characterization of a Gas Component — A Transient Sensor Theory

Lazik

2014

Sensors

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Based on a multi-gas solution-diffusion problem for a dense symmetrical membrane this paper presents a transient theory of a planar, membrane-based sensor cell for measuring gas from both initial conditions: dynamic and thermodynamic equilibrium. Using this theory, the ranges for which previously developed, simpler approaches are valid will be discussed; these approaches are of vital interest for membrane-based gas sensor applications. Finally, a new theoretical approach is introduced to identify varying gas components by arranging sensor cell pairs resulting in a concentration independent gas-specific critical time. Literature data for the N2, O2, Ar, CH4, CO2, H2 and C4H10 diffusion coefficients and solubilities for a polydimethylsiloxane membrane were used to simulate gas specific sensor responses. The results demonstrate the influence of (i) the operational mode; (ii) sensor geometry and (iii) gas matrices (air, Ar) on that critical time. Based on the developed theory the case-specific suitable membrane materials can be determined and both operation and design options for these sensors can be optimized for individual applications. The results of mixing experiments for different gases (O2, CO2) in a gas matrix of air confirmed the theoretical predictions.

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