Abstract
Transparent windows used to perform optical measurements in `difficult' samples such as natural waters in the environment and sewage streams in treatment plant inevitably become coated with chemically and/or biologically deposited films. These usually dominate the instrument's performance and reliability. We describe here a windowless measurement configuration, which avoids this fouling by supporting the water sample from below on an air bubble. By controlling the air pressure in the bubble with a fast pressure-modulator and feedback controller, the air-water meniscus' curvature is stabilized, and serves as a reliable optical interface to the liquid medium. Round menisci up to 8 mm in diameter have been stably controlled, with electronically regulated curvature. By periodically purging the bubble with air, long-term measurements of scattering and long-path absorption are possible, substantially free from fouling problems.
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