In this paper we propose a new gas monitaring system, which is based on selective diffusion through polymers. Combining the gas selectivity of nonporous passive polymers with the travel velocity of pressure changes (sound velocity) yields the physical bases of the proposed method: the partial pressure or the gas concentration measurement is transformed into a simple measurement of the total pressure inside a polymer tube, where the sampling points and the measurement equipment can be at different locations. Important to note that the same principle can be applied to different gases of interest; for instance to CO₂ and O₂ in order to control enhanced aerobic biodegradation through oxygen injection. In order to elucidate the measuring principle, we present an ideal isobaric experiment. Basedon a diffusion model with cylinder symmetry the linear time dependence of the experimental volume change can be understood and furthermore, an effective diffusion constant can be derived.