Analysis of the Photoacoustic Detector Signal for Thermal Diffusivities of Gases

A resonant and a nonresonant photoacoustic detector were used to determine thermal diffusivities of gases. With a nonresonant detector thermal diffusivities can be determined in a wide range between 10-3 and I 0 - 7m2 . s - 1 , whereas experiments with the resonant detector deliver thermal diffusivities in a range that is about a factor of 100 smaller. As refrigerants—HFCs, HCFCs, and hydrocarbons-are absorbents in the infrared at a wavelength of 3.39 um, their thermal diffusivity can be determined without the addition of a trace gas, particularly at pressures below 0.01 MPa. At pressures above 0.1 MPa, the addition of ammonia as a trace gas is recommended. The absorption wavelength is then l.531 um. A simulation model for the nonresonant photoacoustic detector is preseInted for the design of a detector and for an extended error analysis.