A B S T R A C TMaintaining depth is a diver's essential task, and that is why he/she uses a buoyancy vest for maintaining and changing depth [the so-called buoyancy control device (BCD)]. Changing depth is controlled by manually actuating pneumatic valves, which causes the BCD volume to expand or shrink and consequently change its buoyancy.Divers' desires for devices that automatically change or maintain depth have been present since the first arrival of modern diving using a buoyancy vest. This need has arisen, particularly when diving, in regard to the following: where poor visibility is present; when both hands are needed for the job in amateur diving; where decompression procedures are needed; during safety stop procedures; during automatic ascending when a diver's life functions are critical; and also when using the same technology for changing depth in small modern submarines and submarine-like vessels.The presented prototype device has been developed for automatic buoyancy control using flexible BCD. This device can limit ascending and descending velocities, allow a diving diver to request a depth at a requested velocity and hold that requested depth, ensure a diver's requested depth and velocity are controlled, minimize any depth and velocity errors in relation to disturbances from the environment, and record all captured data for dive analyses. Controlling velocity is important for proper decompression.This paper presents a mathematical model of a diver's buoyancy, the prototype development of an automatic BCD, and simulation and actual diving results. The prototype device has been fully tested by one of the leading manufacturers of diving equipment.

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