Pyroelectric Thermal-Wave Resonant Cavity: A Precision Thermal Diffusivity Sensor for Gases and Vapors

A novel thermal-wave resonant cavity (TWRC) was constructed and used for thermophysical measurements of gases and vapors, with an AC current-heated thin-film resistive element acting as a thermal-wave source. A thin-film pyroelectric element was used both as a cavity wall and as a signal transducer. A theoretical model of the cavity length-scanned thermal-wave field was developed to quantify the standing-wave resonance antinode patterns in the demodulated lock-in signal output in-phase and quadrature channels. These resonance extrema were used to measure precisely the thermal diffusivity of the intracavity gas or vapor. Seven high-purity gases (nitrogen, dry air, oxygen, methane, hydrogen in nitrogen, pure hydrogen, and helium) were measured using the cavity. Fourth-significant-figure precision was obtained for this parameter, with standard deviations less than 0.32% for the five measurements performed with each gas. Furthermore, three grades of gasoline vapors from Imperial Oil were studied with the cavity. The measured thermal diffusivities showed that the TWRC can monitor fundamental evaporation kinetics as an analytical qualitycontrol instrument. These results, together with the simplicity of TWRC sensor fabrication, are indicative of its potential to become a new standard measurement instrument for the determination of gas thermal diffusivity with improved precision, and a new in situ monitor of chemical evaporation kinetics over conventional methodologies, such as gas chromatography and mass spectrometry.