Nowadays, these dive watch markings seen on vintage and “vintage-inspired” divers can seem arcane, almost hieroglyphic, even to those of who dive and know the theory behind decompression. So here is an attempt to decipher some of these markings with various well known (and some not so well known) examples from the beloved dive watch category.

Perhaps the best starting point for this primer is to explain decompression, without turning this into a PADI course. Though some technical divers breathe different gas mixtures that use various percentages of helium, nitrogen and oxygen, for simplicity’s sake, let’s look at a diver who’s got a tank of good old compressed air on his back. The air we breathe is composed of roughly 79% nitrogen and 21% oxygen. This is compressed to 3,000 pounds per square inch when it’s crammed into an aluminum or steel scuba cylinder. The regulator the diver attaches to the tank valve, and breathes through, adjusts the pressure of that compressed air to match the surrounding water pressure, depending on depth. This is because at too high a pressure, lung damage is a possibility, and at too low a pressure, the lungs aren’t able inflate against the water pressure pressing on the body.

The effects of breathing compressed air at depth are fairly benign, if you follow certain basic rules. Don’t hold your breath as you ascend (that air will expand and burst a lung), don’t go too deep (nitrogen induces a narcotic effect the deeper you go and the oxygen in the air becomes toxic below a certain depth), and pay attention to the time you spend at depth. And this latter rule is where a dive watch comes in.

Roughly speaking, each depth has a maximum allowable time to which a diver can spend before he must pause on the way to the surface to decompress. This is because the longer he stays down, the more of that inert compressed nitrogen seeps into tissues. Once this “no-decompression limit” is exceeded, a diver must take breaks at specific depths on the way back to the surface, and “hang” there while his respiration rids his tissues of this excess nitrogen. Fail to adhere to the no-deco limits, or skip the decompression routine, and that nitrogen will expand, fizzing in larger and larger bubbles within the joints, the lungs, and the spinal column. This can cause great pain, paralysis, and even death upon returning to surface sea level pressure. The pain can cause a person to double over, giving decompression illness its nickname, “the bends.”

The simplest form of dive bezel is used in conjunction with a set of tables that indicates the no-decompression limit for each depth. You set the zero mark (usually an arrow) opposite the minute hand, and as time passes, the dive time is shown on the bezel. Knowing the maximum time allowable against the maximum depth indicated on a depth gauge makes for a safe dive. There is an old and questionably reliable rule, known as the “120 Rule” that says if you subtract your max depth from 120, you’ll get your no-deco time. So an 80-foot dive gives you 40 minutes before it’s time to head back to the surface. In a pinch, sure, but multi-level diving and time spent at each depth also plays a factor.

A step beyond the simple elapsed time bezel is the so-called “no-deco” bezel, patented by Doxa in 1967. This double scale bezel takes the place of those clunky and not exactly waterproof tables, by engraving the no-deco limits right on the outer ring. Set the zero mark to the minute hand when you descend, and the scale indicates when to surface for depths from 60 feet (60 minutes) down to 190 feet (4 minutes). This bezel type was also adopted by other brands like Eterna and Heuer, and is mainly aimed at the sport diver, who is sticking to recreational depths and doing strictly no-decompression diving. Similarly, Citizen printed the no-deco limit scale on the rubber strap provided with its Aqualand dive watches of the 1980s.

But this bezel (and Citizen’s strap) has its limits. Since residual nitrogen remains in the tissue even after a no-deco dive, there is a new table that has to be consulted for successive dives, and these new, adjusted no-deco limits are not accounted for on these no-deco bezels.

For a diver who wants to remain underwater beyond the no-deco limits, or go deeper, decompression becomes necessary. This is called advanced, or “technical” diving, since effectively there is no direct path to the surface. Time becomes a ceiling, as hard as a cave roof or frozen lake surface, under which a diver must remain – at least if he doesn’t want to get the bends. For decompression, there is no simple rule or easily remembered formula, since it depends upon how long a diver spends at various depths and, more recently, what mix of gases he might be using.

For example, when I dove the wreck of the British aircraft carrier, HMS Hermes, last summer, with a maximum depth of 175 feet, my “bottom time” spent on the wreck was 21 minutes, but I had to decompress at various depths on the way up, for a total of 39 additional minutes. I switched from air to a 50% oxygen mix, to finally a 100% oxygen mixture at a shallow depth, carrying four tanks with me. And though my dive computer did most of the calculations for me, I still found my simple elapsed time bezel useful. I had also written my entire decompression plan (as calculated using a desktop application) on a wrist slate and at each deco stop on my ascent would use the bezel to serve as a backup timer. Here is where the finer demarcated hashes that you see on bezels (often in the first 15-20 minute arc) come in handy.

In the early 1960s, Dr. Hannes Keller (a Swiss renaissance man, who was a physicist, artist, and diver) set out to break the world depth record, breathing new helium gas mixes. He worked with Vulcain to create a watch that would not only alert his depth-addled brain when it was time to ascend via an audible alarm, but also assist him in calculating his decompression stops on the way back to the surface. The resulting Cricket Nautical, released in 1961, was revolutionary, at a time when most dive watches were simple three-handers with a rotating bezel. The Vulcain not only had the mechanical alarm function and an inner rotating timing ring but also a multi-layer dial with decompression protocols. The entire dial was rotated using one of the crowns, so that a long aperture revealed decompression times printed on a lower layer for the depths printed in concentric rings on the dial. Using the elapsed time from the outer ring, with this circular table and the adaptive inner scale, a diver could safely make his stops on his ascent.

In the 1970s, Fortis released a similar, though less complex version of this decompression table for the dial of its Marinemaster, which did not have an alarm function. In place of the aperture and rotating dial, the Fortis made use of a color-coded dial with decompression tables. A diver would find his depth on the Meter/Feet scale and follow the appropriate ring around until he found his bottom time, and then decompress for that amount of time at a depth of 15 feet (5 meters), a common practice then that would be considered very unsafe nowadays, when deeper stops are standard practice.

The Swiss brand, Jenny, known for being the first brand to achieve a 1,000-meter depth rating with a watch, also was groundbreaking for the patent it achieved in 1971 for a decompression scale bezel. Similar to the Fortis Marinemaster’s dial, the Jenny Caribbean’s bezel shows depths which corresponded to times of immersion, above which are printed the number of minutes required to decompress at 15 feet (again, a now outdated practice). This bezel is far simpler to read (assuming you aren’t far-sighted) and understand than tracing the circles around a dial, which could be covered by the hands. Simply use the rotating bezel to track elapsed time using the inner scale, then check your depth gauge and find the depth on the bezel, and proceed outward to the closest time you spent at that depth and finally use the outermost ring to get your required deco time.

Finally, the most arcane dive watch markings, and the ones that have baffled me the most, belong to one of the most cultish of all dive watches. The Aquastar Deepstar chronograph was developed in the mid-60s at the behest of Jacques-Yves Cousteau himself, whose cadre of frogmen was pushing boundaries of depth and undersea living. Aquastar, a brand dedicated to aquatic timepieces, took on the challenge of creating a watch that could not only time a dive but also account for the residual nitrogen in the body during successive dives. Remember earlier I wrote about how the dive tables change after dive number 1? Well, the Deepstar is equipped with a dual scale rotating bezel that tracks elapsed time as usual, but also then has a scale that is set against the HOUR hand to calculate how soon excess nitrogen is flushed from the body tissues. To find the correct co-efficient, Aquastar provided a separate table to cross-reference. When the hour hand moved into the “Normal” region of the bezel, a diver could assume he can go back to using standard dive tables for further diving. Diving is surely a science as much as it is a sport, and the Aquastar Deepstar is evidence of that.

By the late 1980s, the advent of the digital dive computer sounded the death knell for the traditional dive watch. Nowadays, nary a diver on a boat can be seen wearing an analog watch, despite its continued benefit as a backup device or for a navigational aid (timing swim distances, etc.). A good dive computer will calculate no-deco limits in real time, adjusting for time spent at various depths during a dive, the use of enriched air mixes that lengthen no-deco times, and will also alert you when it’s time to ascend or when you’re going up too fast. More advanced ones will calculate decompression times, even allowing for gas switches mid-dive, and read the amount of gas left in your cylinder.

But through this brief look at the history of dive watch markings, we can see how problems were solved along the way, by analog means only, and how, despite their seeming simplicity, dive watches have been useful to take divers on some of the world’s greatest undersea adventures.

Special thanks to Chris Scott and Chris Sohl for their help with the Aquastar bezel, and to Analog/Shift for some of the photos.