Fluoropolymer-based capacitive carbon dioxide sensor

We describe a thin film sensor of carbon dioxide which relies on the change in capacitance of a fluoropolymer thin film caused by the difference in dielectric constants between air and carbon dioxide and by the preferred adsorption by the polymer of carbon dioxide compared to that of air. The fluoropolymer, Teflon AF 2400™, selectively adsorbs large quantities of CO2 with sufficient diffusivity to allow response times of a few seconds to be achieved. The sensor demonstrates completely linear response over its operating range from ppmv to 100% CO2. Sensor performance is currently limited by thermal drift and the need to remove all water vapour from the sample flow. Precision levels, currently at the level of 5–10 ppm, can be reduced to well below that of 1 ppm with proper redesign of the sensor. As part of these studies, the absolute solubilities of several compounds in Teflon AF 2400 were measured and are as follows (in units of µmoles cm−3 atm−1): air (16.6), CO2 (177), C2F6 (≈ 600), CF3CFH2 (≈ 800) and CF2ClCF2Cl (≈ 4500).