David It's called a Papenberg depth gauge, and it is basically used for precision depth reading (plus a scale showing the remaining buoyancy) when operating a submarine at periscope depth. The measurements are based on the compression of an air cushion. Hence, the level reading in the vertical view glass (tube) is not linear (although it is quite simple to calculate a corrected scale for the instrument). It is a very sensitive depth measuring device that you can easily made yourself, and it would be practical down to about 65 - 80 ft. Minute tendencies of depth change can easily be observed on the curvature of the meniscus even before the water level in the tube moves. Best regards, Jens Laland
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