The near-zero gravity environment of Spacelab presents the opportunity to perform many new fluids experiments which cannot be done in earth-bound laboratories. A large number of these investigations involve relatively small and confined volumes of fluid. Hence, fluid measurement techniques are required which do not disturb the flow. Several nonintrusive measurement techniques are being investigated. A photochromic dye method is being developed for flow measurement. Photochromic dyes are substances which darken upon exposure to ultraviolet (UV) radiation and then clear up again spontaneously. A small amount of dye (~.025% by weight) has been successfully dissolved in a low viscosity silicone oil and dye streaks created upon exposure to an UV source. The movement of the streaks reveals the flow. The streaks then clear up so that the procedure can be continued without the liquid becoming opaque. A laser Doppler dual-scatter system is also being developed for low flow speed measurement. An accuracy of a few percent has been demonstrated for flows of a few millimeters per sec. A double grid schlieren system is being developed for temperature measurement. The optical arrangement is such that an image of a Ronchi ruling is superposed on the original ruling giving a uniform grey field of view. Departures from the background are caused by light being refracted as it passes through the experimental volume.
William W. Fowlis,
"Remote Optical Techniques For Liquid Flow And Temperature Measurement For Spacelab Experiments," Optical Engineering 18(3), 183281 (1 June 1979). https://doi.org/10.1117/12.7972367