Photoacoustics of Small Gas Clouds

The photoacoustic effect - production of sound from light - may be exploited for detection and localization of gas leaks on the surface of otherwise sealed components. The technique involves filling the test component with a photoactive tracer gas, and irradiating the component to produce photoacoustic sound from any leak site where a tracer gas cloud forms. This presentation describes experiments utilizing 10.6-micron radiation from a carbon-dioxide laser with sulfur hexafluoride as a tracer gas. Here, photoacoustic sounds from a laminar plume of sulfur hexafluoride and several NIST-traceable calibrated leak sources with leak rates between 1 cubic centimeter in 4.6 hrs and 1cubic centimeter in 6.3 years were recorded with four or twelve microphones in a bandwidth from 3 kHz to ~100 kHz. The measured photoacoustic waveforms compare well with those from an approximate theoretical development based on the forced wave equation when the unmeasured size of the photoactive gas cloud is adjusted within the likely gas-cloud diffusion zone. However, for small gas clouds, the photoacoustic sound amplitudes predicted by the approximate theory fall far below the experimental observations and several potential reasons for this mismatch will be offered. Interestingly, the higher measured signal amplitudes imply that the sensitivity of photoacoustic leak testing may reach or even exceed the capabilities of the most sensitive commercial leak test systems based on helium mass-spectrometers.