Timekeeping: The Dive Watch, Deconstructed

In the early days of diving, the way to track dive time was to pull out the crown of your watch before descending, set the minute hand to twelve o’clock, push in the crown and then start the dive. Of course, this was clumsy and always meant having to reset the time once back on the boat. The introduction of the rotating elapsed time bezel was a big leap forward. Contrary to some misconceptions, the bezel does not track the amount of air in the SCUBA tank. The bezel allows a diver to twist the bezel so that the arrow, or descent marker, aligns with the minute hand. As time passes, the minute hand is read against the bezel markings to quickly tell elapsed dive time. Knowing his depth and this time, along with the no-deco limit for his depth, a diver can safely carry out his dive.

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Why Does the Bezel Only Turn One Way?

On early dive watches, the timing bezel turned both directions. But now, it is de rigueur for all dive watch bezels to only turn counter-clockwise. The reason for this is so that if the bezel is bumped accidentally during a dive, it will only move one direction, subtracting time from the dive and prompting the diver to end his dive early. If there was the possibility of extending a dive (as a bi-directional bezel could) a diver could be in danger of surpassing the no-deco limit.

Why Are the First 15 Minutes of a Bezel Marked Differently?

On many dive watch bezels, the first 15 or 20 minutes of markings are colored – yellow, orange, red – and have individual minute markers shown. This is again a remnant from past usage. Upon ascent, if a diver needs to time a shorter interval with more precision, most likely a decompression stop, it is helpful to be able to reset the zero marker to the minute hand and use the more finely marked area to track his time. Some dive watches, such as the military versions that Rolex and Omega built for the British Royal Navy, display minute markers on the full bezel.

How Deep is Deep Enough?

The first dive watches of the 1950s from Rolex, Blancpain, Omega and others had rather meager depth ratings by today’s standards – about 100 meters. In the decades since, a sort of “depth race” began among watch companies and 1,000 meter depth ratings were seen on dive watches as early as the late 1960s. Today, there are some dive watches with ratings as deep as 12,000 meters. These ultra-deep divers appeal to watch fans for the same reason supercars with high top speeds appeal to car enthusiasts, displaying engineering prowess and ultimate performance. But for the vast majority of recreational or even professional divers, a 200 meter rated dive watch is more than enough. The International Standards Organization, or ISO, has a detailed standard for dive watches. The document specifies exactly how a dive watch’s water resistance is to be tested and says that to be called a dive watch, a timepiece must have a minimum water resistance of 100 meters and have a method to track elapsed time (see above). And no, moving your arms underwater does not significantly increase water pressure on your watch.

How Does a Helium Release Valve Work?

One of the most misunderstood aspects of a dive watch is the helium release valve. Along with extraordinary depth ratings, many modern divers feature them yet most watch owners couldn’t explain how they work. The fact of the matter is, for 99.9% of dive watch owners, the helium release valve is utterly useless. It was developed to solve a peculiar problem faced by a very small and elite group of divers.

Professional saturation divers are those who work at very deep depths for days on end. The air mixture that they breathe replaces nitrogen with helium, an inert gas that is harmless to breathe and doesn’t cause the narcotic effect that nitrogen does at depth. In order to continue to work for long periods of time underwater, these divers live in a dry, pressurized chamber suspended from a ship or platform at the surface. They enter the water to work via an airlock and breathe the same pressure and mix of air as they do inside the chamber. When their work is done, the chamber is hoisted to the surface and pressure reduced gradually for days until the divers can emerge to surface air and pressure.

The problem with helium is, while it is safe to breathe, its molecules are very small. They can penetrate inside a dive watch while the diver is in the dry chamber between stints in the water. As the chamber is depressurized, the helium molecules expand inside the watch and look for any means to escape. The quickest route is usually by blowing the watch crystal off, an obvious hazard to both man and watch. So Rolex patented a one-way pressure relief valve that will pop open to allow helium to escape safely. The valve, some manually unscrewed, some automatic, can be found on many dive watches nowadays. If you think you will ever need this valve, chances are you’re not reading this article right now.

Why Do Some Rubber Straps Look Like An Accordion?

Maybe you’ve noticed on many dive watches that the rubber straps have a section, typically near its attachment points on the watch case, which is rippled or vented. Contrary to some misconceptions, this is not to allow your wrist to breathe. Rather it allows the wearer to pull the strap tight over a wetsuit sleeve at the surface but remain tight as the diver descends. You see, neoprene wetsuits compress as water pressure increases. Even a strap that was tight at the surface will loosen up around this shrinking circumference causing the watch to flop around on the wrist. By pulling the vented strap tight at sea level, the vents will contract to take up the slack as the diver descends.

The Dive Watch is Dead. Long Live the Dive Watch.

Despite its absence from dive boats, the dive watch is alive and well, as a nod to history, an exercise in extreme engineering and a symbol of adventure. It is a talisman from days of exploration, when men descended into the sea for the first time to explore wrecks, discover reefs and do clandestine battle. And just as watch collectors are keepers of a tradition of a mechanical art, so too are dive watch owners keepers of a legacy of rugged instruments that accompanied man on some of his greatest adventures. Wear your dive watch proudly. And know how it works.

Photos by Gishani for Gear Patrol