Scala no-decompressione

The no-decompression scale is not an aesthetic detail: it is a diving safety instrument engraved or printed on the bezel, indicating the maximum time of stay at a given depth before direct ascent to the surface becomes dangerous. Once that limit is exceeded, the diver must perform decompression stops to allow nitrogen dissolved in the blood to be released gradually.

The historical reference is the US Navy no-decompression table, adopted as the standard for civilian and military dive training since the postwar period. Doxa incorporated its values on the Sub 300 bezel as early as 1967, patenting a dual reading: the outer scale shows depth in feet, the inner scale shows maximum time in minutes without mandatory stops. This double-ring system is the origin of what collectors commonly call the no-deco bezel or shark-toothed bezel, because of the characteristic serrations of the unidirectional click with reduced grip. Doxa extended the same scale to the Sub 600T and Sub 750T, with values sometimes expressed in meters in European versions.

It is important to distinguish two uses of the term that circulate confusingly in magazines. The first, technical and correct, indicates a bezel graduated according to no-decompression tables: an active instrument that the diver reads during immersion. The second, imprecise but widespread, uses no-deco as an abbreviation for no decoration, referring to bezels without superfluous graphic elements, smooth lacquer or monochrome enamel. The two meanings are not interchangeable, although they sometimes coincide on the same watch.

Brands like Jenny and Ollech & Wajs drew inspiration from the Doxa system for their dive watches in the 1960s and 1970s. An isolated but documented case is the Casio Aqualand CQ-1012-50 from 1985, where the no-deco tables were printed directly on the rubber strap, a practical solution but evidently vulnerable to wear. The Synchron Ti300M Poseidon offers another variant: unidirectional bezel with 120 clicks and integrated no-deco scale, an unusual number of clicks that slows down reading but increases positioning accuracy. Not all dive watches mount this scale: the Doxa Army, for example, uses the classic 60-minute scale, oriented toward generic timekeeping rather than decompression.

From my workbench, I add that many buyers confuse how the scale works with that of a simple countdown. It is not: the bezel must be rotated until the minute marker aligns with the immersion start time, then read the value corresponding to the estimated depth. It is a quick calculation, but it requires that the diver already knows the reference table. Those without dive training treat this bezel as decorative. In that case, technically, it is.