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

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.

Unlocking the Carbyne-Enriched Nanocoating Sensitivity to Volatile Organic Vapors with Plasma-Driven Deposition onto Bulk Micromachined Silicon Membranes

“…As a result, flexible membrane of silicon with different sizes (65 µm and 1000 µm of the square side) and geometries (patterned or non-patterned for the smaller membranes) were obtained (Figure 1a-d). The aim of studying different types of the membranes was to find the more appropriate sensitivity and response time, as it is known that these sensors' characteristics strongly depend on their geometry [12]. Figure 1e 2, 3, …), formed in the plasma sheaf are directed by electrode system to impinge upon the surface of the substrate where the poly-condensation of the carbon chains takes place.…”

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

“…The arrangement and the theoretical framework for quantifying the concentration of a single gas component was developed by Lazik et al (2009) and Lazik (2014). Since the pressure signal is evaluated near the steady‐state gas fluxes, the measurement technique allows comparatively fast measurement with respect to a classical diffusion probe that uses the same membrane for gas‐phase separation.…”

“…where superscripts ex and in define parameters outside and within the measurement chamber, respectively; the geometry factor g = 2pL/[Vln(R o /R i )] accounts for the cylindrical shape of the gas-selective cell (gas volume of the measurement chamber V [m 3 ], length of the membrane tube L [m], and its inner and outer radii R i and R o [m]); P s (m 2 /s) is the permeability coefficient of the membrane for gas component s, and f js = P j /P s is the selectivity coefficient of gas component j with respect to gas component s; c j in and c j ex are the mole fractions of gas component j within the measurement chamber and outside, respectively; and b = p in /p ex is the ratio of the gas pressures. The arrangement and the theoretical framework for quantifying the concentration of a single gas component was developed by Lazik et al (2009) andLazik (2014). Since the pressure signal is evaluated near the steady-state gas fluxes, the measurement technique allows comparatively fast measurement with respect to a classical diffusion probe that uses the same membrane for gasphase separation.…”

A New Principle for Measuring the Average Relative Humidity in Large Volumes of Non-Homogenous Gas

“…The combination of a self-calibrating sensor design [29] with a specially constructed water vapor compensating membrane [30] allowed the determination of small CO 2 concentrations due to soil respiration in face of a temperature-dependent varying water vapor pressure in humid soils. Such membrane-based sensors can be used to analyze various gases as shown by computer simulations [31] and experiments, e.g., [32].…”