#1459: FocalDepth

Divers’ watches can be beautiful examples of precision engineering but they are also subject to damage, malfunction and misreading under pressure.

Given that their main function is to alert a diver to the need to surface, today’s invention achieves this in a simpler, cheaper way (and might be used as a low-tech backup).

Before descending, a diver would strap to his wrist a watch case containing an old-style 35 mm film cartridge. The film would lie over a high-contrast image of a downwards pointing arrow.

The diver would select a number of light filters to insert over the film, corresponding to the planned depth and duration of the dive. When the light available at depth had exposed the film enough to obscure the arrow, it would be time to surface again.

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