Thermal-wave resonant-cavity measurements of the thermal diffusivity of air: A comparison between cavity-length and modulation-frequency scans

The application of a thermal-wave resonant cavity to thermal-diffusivity measurements of gases has been investigated. The cavity was constructed using a thin aluminum foil wall as the intensity-modulated laser-beam oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer. Theoretically, cavity-length and modulation-frequency scans both produce resonance-like extrema in lock-in in-phase and quadrature curves. These extrema can be used to measure the thermal diffusivity of the gas within the cavity. It was found experimentally that one can obtain very accurate and reproducible measurements of the thermal diffusivity of the gas by using simple cavity-length scanning without any signal normalization procedure, rather than traditional modulation-frequency scanning normalized by the frequency-dependent transfer function of the instrumentation. By scanning the cavity length, the thermal diffusivity of room air at 299 K was measured with three-significant figure precision as 0.216 +/- 0.001 cm(2) . s(-1), with a standard deviation 0.5 %. Only two significant figure accuracy could be obtained by scanning the frequency: 0.22 +/- 0.03 cm(2) . s(-1), with a standard deviation of 14 %. Cavity-length scanning consistently exhibited a much higher signal-to-noise ratio.