In the Laboratory
Substituting an Inexpensive Function Generator for the Pulsed Laser in the Experiment “Laser Measurement of the Speed of Sound in Gases” Mark G. Epstein, Matthew W. Laszlo, and Steven G. Mayer* Department of Chemistry, University of Portland, Portland, Oregon 97203, United States *
[email protected] The recent article by Baum, Compton, and Feigerie (1), “Laser Measurement of the Speed of Sound in Gases: A Novel Approach to Determine Heat Capacity Ratios and Gas Composition”, illustrated an experimental method for determining the speed of sound and heat capacity for gases. This experiment differs from those presented in the textbooks by Garland, Nibler, and Shoemaker (2) and Halpern and McBane (3) in that it uses a short sound pulse to measure transit time of the shockwave rather than a standing wave at a set frequency to measure the distance between nodes in a Kundt's tube. Because turbulence caused by the flow of gas affects the position of the nodes, the flow must be stopped just prior to making measurements, which introduces error due to diffusion of the gas. This effect is especially problematic for helium because it diffuses so rapidly that it is virtually impossible to make repeatable measurements. The pulsed experiment is not affected by diffusion of the gas because the gas can be admitted to the tube at a low, constant flow rate throughout the experiment. Although the method described by Baum and co-workers is clever and produces excellent results, even for helium, it requires the use of an expensive, high-powered, pulsed laser. The authors state that the transit times are short (∼10-3 s), so this technique is possible only by using short (
