A dive-watch bezel is the rotating ring around the crystal. Done well it is a complete, mechanical dive computer. Done badly it is a useless decoration. The four kinds you'll see on modern dive watches all originate in patents filed between 1956 and 1972, and one of them came from Aquastar.
The unidirectional elapsed-time bezel
The standard 60-minute bezel marks elapsed time. The diver aligns the 12-o'clock triangle with the minute hand at start-of-dive. Reading the minute hand against the bezel scale gives elapsed bottom time. The bezel only rotates counter-clockwise, so an accidental knock can only shorten the read, never extend it. That safety mechanic was first patented in 1956, then standardised in ISO 6425 in 1996. Every Aquastar Benthos, every Model 60, uses one.
The no-decompression bezel (Aquastar, 1962)
The no-decompression bezel does more. It marks the actual depth-vs-time curves of a recreational dive table, the maximum bottom time at each depth before you have to make a decompression stop on ascent. The diver looks at the depth gauge, finds that depth on the bezel, reads off the residual safe bottom time, and watches it run down to zero on the chronograph hand. No table, no slate, no maths.
Aquastar founder Frédéric Robert filed the patent in Geneva in 1964. The bezel had already shipped in 1962 on the original Deepstar Chronograph, the world's first dive chronograph with the entire decompression table marked on the bezel. The Marine Nationale picked it up nine years later as their standard issue diving watch. The Aquastar Deepstar of 2026 still uses the same bezel pattern, lifted directly from the 1962 archive drawings. Read the patents.
The internal compressor bezel
A "compressor" case, patented by Ervin Piquerez in 1956, has the bezel inside the crystal. The diver rotates it via a second crown at 2 or 4 o'clock. Advantage: a fully sealed external case (nothing rotates at the gasket line). Disadvantage: harder to read, harder to grip. Most 1960s diving Skin-Diver watches used the compressor system; modern revivals usually go back to the external bezel for legibility.
The GMT bezel
A 24-hour bezel paired with a fourth hand reading 24-hour time. Useful for tracking surface time at a second location (commercial divers travelling between operations), but mostly used by pilots and travellers. Aquastar's Greenwich editions of the Model 60 and Deepstar II carry this complication where the dive function is secondary.
Reading a bezel underwater
The two visibility constraints are lume and contrast. Older lume on a bezel disc (paint mixed with phosphor) fades within five years of UV exposure. Modern X1-grade Super-LumiNova on a lumed pip, the dot at 12 o'clock, gives sufficient legibility for the only number that actually matters underwater (where the minute hand is relative to the start point). Every Aquastar bezel uses lumed pips; the rest of the disc is high-contrast print, not lume, so the disc stays legible after a decade.
Which bezel matters
For recreational diving with a wrist computer: the elapsed-time bezel is plenty. For technical diving where the watch is a backup: an Aquastar-pattern decompression bezel still calculates a working schedule even when the computer fails. That redundancy is the reason the Marine Nationale specified Aquastar over quartz alternatives for six years.
Frequently asked questions
How does a dive watch bezel work?
A dive watch bezel is a rotating ring around the crystal marked with an elapsed-time scale. Before entering the water, the diver aligns the zero marker with the minute hand. As the dive progresses, the minute hand sweeps around and points to elapsed bottom time on the bezel scale, so total dive time is readable at a glance without any calculation. The bezel is unidirectional, rotating counter-clockwise only, so an accidental knock during the dive can only make the watch read more elapsed time than has actually passed, never less. That fail-safe makes the diver surface early rather than late.
Why must a dive bezel rotate in one direction only?
A unidirectional bezel rotates only counter-clockwise so it cannot be knocked forward mid-dive. If a wetsuit cuff bumps it, the bezel can only rotate to show more elapsed time than has truly passed, which pushes the diver to surface earlier. A bidirectional bezel that could under-read is a genuine safety flaw underwater, which is why every standardised dive-watch specification since the 1960s requires unidirectional rotation.
What is a no-decompression bezel?
A no-decompression bezel is a diving bezel calibrated not just in elapsed minutes but with a logarithmic scale showing the maximum time a diver can stay at a given depth without requiring a decompression stop on ascent. Filed 15 May 1964 by Marc Jasinski for Aquastar S.A. of Geneva (CH 436 140), the Deepstar bezel is the world's first multiple-dive no-decompression chronograph mechanism. It made the watch a dive-planning tool in the tool-watch tradition. It carries two scales, an inner 60-minute elapsed-time scale and an outer no-decompression scale calibrated to the French Navy tables of the period, and lets a diver plan a first dive, time the surface interval, and execute a repetitive second dive directly off the wrist, without a slide rule and without a separate table.
What are the main types of dive-watch bezel?
Three main types. An external unidirectional elapsed-time bezel is the standard on almost every modern dive watch. An internal rotating bezel is operated by a second crown and sits under the crystal, protected from knocks. A no-decompression bezel is calibrated with a decompression scale rather than just elapsed minutes; it is the type Aquastar patented in 1964 and still produces today.
How accurate is a dive-watch bezel compared to a dive computer?
A mechanical bezel is analog and less granular than a modern dive computer, but it needs no battery and no boot time and never fails silently. Serious divers still carry a mechanical bezel as backup even when a computer is on the wrist, because a bezel and minute hand together provide bottom-time information the second the computer fails. That redundancy is the reason the Marine Nationale specified mechanical Aquastar bezel watches over quartz alternatives from 1979 to 1984.
Related reading: The Aquastar Deepstar, the original no-decompression chronograph · The Aquastar Benthos 500 · Aquastar's eleven patents, 1957 to 1977
