Measurements of very low gas flow velocities by photothermal deflection spectroscopy

A new method based on photothermal deflection spectroscopy (PTDS) for measuring very low gas flow velocities when the transit time method becomes difficult to use is demonstrated. In this method, the probe and pump beams are aligned so that the centers of the two beams coincide, giving a null PTDS signal in a stationary medium. In the presence of a flow, a nonzero signal is obtained whose magnitude is proportional to the flow velocity. The theory of the method and the apparatus used are described, and experiments are reported in which velocities as low as a few mm/s could be measured.