A flow with variable fluid mass density represents an optical disturbance, which changes the phase of a transmitted light wave with respect to an undisturbed wave. Before the flow of gas in the puff begins, an interference pattern is obtained and the fringes’ positions are determined by the initial phase difference between both beams. As the gas flows in the puff, the mass density of the gas increases, causing an additional phase difference. The time dependent additional phase difference results in time dependent shifting of the fringes and thus changes the intensity picked up by the photodiode. When a steady state flow is reached the mass density is everywhere constant in time and the fringes cease to shift (Fig 1).
Fig. 1: Interferometry measurement (blue line) taken at the center of the triple gas-puff with all three valve
plena open to air at atmospheric pressure. The green line indicates the signal sent by a piezoelectric detector
placed on a Delrin extender to measure the mechanical noise created by the valve operation.
Modified on: 2010-02-23