Just about the most popular type of tool watch there is, no true enthusiast’s collection is complete without at least one dive watch.
Dive or divers’ watches are some of the most common in the industry. Often seen as the archetypal sports watch, dive watches are useful tools that have a rich aesthetic history. The combination of those elements, which appeal to both the scientific and artistic mindsets, has resulted in a diverse corner of watchmaking, replete with industry classics. You need only imagine horological stalwarts such as the Rolex Submariner, the Omega Seamaster, or the IWC Aquatimer to get a strong idea of this genre’s ubiquity throughout the world of watchmaking.
But while we can all pretty much say we know one when we see one, what must a genuine dive watch have to set it apart from all other genres?
Although they have been around since the fifties, becoming evermore prevalent as recreational Scuba diving caught the public imagination (helped along by a certain fictional secret agent), official standards for what actually constituted a dive watch weren’t laid down until 1996.
It was then that the International Organization for Standardization drew up a set of base guidelines for what these timepieces were and were not required to do, which were given the catchy title of ISO 6425.
For a watch to “officially” be considered a dive watch, it must pass a series of tests based on these criteria. Only those models that successfully fulfilled these requirements were then permitted to include the word ‘Diver’ on their dials. ISO does not perform this test itself (any brand that says its watch has been tested by ISO is trying to pull one over on you).
Instead, this responsibility falls at the feet of the brand. A brand can either perform this test in-house or pay an external independent to test it for them. While the latter route may have more credibility for its guaranteed lack of bias, many major Maisons choose to perform the test themselves, and the kind of reputations we-re talking about here – brands like Rolex and Omega – are beyond repute in this regard.
Even so, a brand may not choose to put “dive watch” or “divers’ watch” on the dial. There is no requirement for them to do so. Some may see it as braggadocios, others may see it as stating the obvious, while it’s possible some (those companies that have been producing well-regarded diving tools for decades prior to the establishment of the ISO 6425) really don’t feel they have to dance to the tune of a johnny-come-lately.
However, as a number of watches had been used for diving for around 40 years beforehand without any problems, there are plenty of pieces out there that satisfy all the criteria that do not have the term visible anywhere—the Rolex Submariner being the most obvious and famous example.
Similarly, there are also models that look, sound and smell like a dive watch that, officially at any rate, are not.
So, what are the main criteria of the ISO 6425 and why should you care? Is it all unnecessary marketing garbage to the man on the street, or is there some tangible benefit to owning a watch that was designed to survive at least 100 meters beneath the sea when you (presumably) live on dry land?
The ISO’s tests cover a whole raft of aspects that are vital to the safe timing of a dive; from being robust enough to withstand the increase in pressure to actually being readable in the reduced light underwater.
Again, it’s important to remember that ISO only lays down the rules, they do not physically test the watches themselves—that’s down to the individual manufacturers.
So, what does the process entail?
Firstly, the watch must be water resistant to a depth of at least 100m, or 330ft. While that is far beyond the bounds of what anyone but the most specialized technical diver will achieve, the extra requirement is to account for the added pressure caused by the diver’s movement. As all watches are tested in an artificial, stationary environment, as well as with brand new gaskets and completely pristine cases, the additional rating is used to compensate for the discrepancies in conditions.
The case must be made of a material able to withstand the galvanic corrosive effects of seawater, and if metal, have a bracelet constructed of an alloy with comparable properties. For the most part, that means either 316L or, in Rolex’s case, 904L stainless steel. To test the resistance, dive watches are kept in a sodium chloride solution of 30 grams per liter (30 g/l NaCl ) for 24 hours, at between 18 and 25°C, and then inspected for pitting and oxidation.
Chemical resistance sounds cool, but for a watch used and kept in regular atmospheric conditions, it should be largely unnecessary. This test, which simulates the saltwater you’d find in the sea, is designed to measure the watch’s resistance to rust. Simply put, a modern watch on dry land should not rust. A bronze watch may patina (which isn’t a world away from fancy rust), but you knew that already. A contemporary 316L or 904L stainless steel timepiece simply does not need to be concerned about sudden and unwanted oxidization.
Next, all dive watches, whether digital or mechanical, must have some form of device to allow the wearer to measure elapsed time, up to one hour. Typically, for mechanical watches, this is taken care of by a rotating bezel.
Perhaps the most immediately identifiable physical trait of a dive watch is its rotating timing bezel. This is a really essential part of the dive watch DNA because it allows a diver to see for how long they have been submerged. Without it, tragedy could occur. Should a diver misjudge how long they’ve been underwater, they could underestimate their immersion time, leading to decompression sickness – better known as “the bends” among those in the diving community.
The timing bezel can be used in normal life, but it isn’t as accurate as a chronograph (nor as intuitive) and the lack of an alarm feature requires you to keep your eyes on it to make the best use of its capabilities.
That surround must show markings at least every five minutes, and be unidirectional—as in, it only turns one way. A fundamental safety feature, it means if it gets knocked for any reason, it will only read that the immersion time has been longer than in reality rather than shorter.
Diving watches are required to be highly legible. This is a good thing for humans wearing these watches above water as being able to read a watch is of prime concern. The minute markings on the dial, too, must be easily readable and they, as well as the hands, must have a visibility from 25cm in total darkness. An indication that the watch is running, such as luminous material on the seconds hand, is a further prerequisite.
Dive watches generally have distinguishable indexes at the four cardinal points, and particularly the 12 o’clock position, to help prevent disorientation in poor light.
To recreate the effects of typical recreational Scuba life, dive watches undergo a range of carefully regulated tests to simulate the conditions in which they are expected to operate.
As well as the case’s resistance to corrosion, the model’s reliability under water is also examined. This involves a whole series of checks.
First, the watch is immersed to a depth of 30cm for 50 hours, after which its functions are checked for accuracy.
It is then tested for shock-resistance, receiving a blow with a force of around 5,000gs, once to the face of the watch and again to the left side. To pass, it must retain timekeeping to within +/- 60 seconds a day.
To test the crown specifically, the weakest point of any mechanical watch design, the piece is pressurized to 125% of its depth rating (so, a 200m dive watch will be subjected to a force equivalent to 250m) and a weight of five Newtons will be applied to the top of the crown for 10 minutes.
The thermal shock test again involves submerging the watch in 30cm of water, before heating it up to 40°C, then down to 5°C, then back up to 40°C for five minutes each, with the transition between temperatures taking no longer than one minute apiece.
Finally, the over-pressurization test takes the watch down to its 125% depth rating within one minute, leaves it there for two hours, and then reduces the pressure to 0.3 bar, again in just a minute, and maintains that density for a further hour.
Before and after all these separate tests, the watch is examined for any moisture having penetrated inside the case. Placing the model on a plate, it is then heated up to between 40 and 45°C, and a single drop of water at between 17 and 25°C is dropped onto the crystal. After one minute, the water is wiped off and any watch which has formed condensation on the inside of the crystal will have been deemed to have a leak and, therefore, failed.
To the Extremes
For those models that go beyond the recreational diving world, such as the Rolex Deepsea, the tests are ramped up in severity.
As they are designed for professional saturation divers who commonly use mixed breathing gases at incredible depths, the watches themselves are exposed to trials that recreate those conditions and then some. In the case of the Deepsea, the 125% of its pressure rating means taking it down to the equivalent of some 16,000ft, which puts about 4.5 tons of weight on the watch.
It is left in that helium-rich gas mixture for 15 days, after which it is brought back to normal pressure in just three minutes. The tiny helium molecules will have seeped into the case during that time, so the repressurization is the ultimate test for the Rolex-developed Helium Escape Valve, the one way regulator in the side of the watch that allows the gas to escape without damaging the crystal.
The standards drawn up in ISO 6425 are designed to ensure those models designated dive watches can withstand the underwater environment. However, the majority of the high-end models in circulation are likely to get no closer to diving for sunken wrecks than the office water cooler.
The dive computer has long since replaced them as essential equipment for both hobbyists and professionals alike. Yet, with the inherent extra toughness needed to pass and exceed the ISO’s requirements already built in, they remain some of the most robust and durable of any watch type, able to brush off the worst that everyday life can throw at them.
Resistance to magnetic fields is genuinely useful. In this day and age, there are myriad magnetic forces at play all around us. Our cellphones, laptops, helicopters, whatever… They are all pumping out harmful gauss that can negatively affect the timekeeping of your ticker.
In fact, the requirement that all divers’ watches are able to withstand immense physical shocks is also pretty useful for those of us that have to cram our bodies into small spaces on underground train systems that take many of us to work. If your watch can take a shock of up to 5,000 Gs, you can be confident that swinging your way freely through rush hour will no lasting damage to your daily beater. It also helps that strap affixation is tested as well. This is done by applying a force of 200 N to each point of affixation in opposite directions. In order to pass, quite simply, the watch must remain undamaged.
Coupled with the fact that more than half a century of refinement has produced some of the finest looking models of any sort, and the dive watch will likely to continue to be the first choice for many people from any walk of life.