Abstract
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.
Highlights
Gases such as carbon dioxide (CO2), oxygen (O2), methane (CH4) and hydrogen (H2) are important for various environmental and technical processes
A phenomenological description was developed for multi-gas diffusion into a closed chamber coated with a planar gas-selective membrane
This description allows for exact calculations of both the concentration and pressure within the membrane and measurement chamber of a sensor cell that forms the basis of membrane-based gas sensor technology
Summary
Gases such as carbon dioxide (CO2), oxygen (O2), methane (CH4) and hydrogen (H2) are important for various environmental and technical processes. A suitable technology, which was first introduced in [1], was developed [2] based on gas diffusion through the wall of a gas-selective membrane into a closed measurement chamber (Figure 1). The sensor must be calibrated for the targeted gas component within a given gas matrix, e.g., air. This interesting feature was successfully used to monitor different mixtures of air and O2 or CO2 in situ within a lysimeter filled with soil [4]. A previous work demonstrated a means of overcoming this disadvantage by solving a system of equations using a set of measurement chambers coated with diverse gas-selective membranes [2]. The corresponding material parameters are available from current gas separation research and material data collections [6,7,8,9]
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