Tough Tech: Extreme Water Resistance

“Tough Tech” is Worn & Wound’s series that takes a deep dive into the technologies that protect our watches, covering their use throughout watchmaking history, functional benefits, and the watches they’re used in. Check out our previous installments on antimagnetic watches and shock resistance.

For a lot of watch fans, the value and utility of a timepiece is related entirely to its water resistance rating. It doesn’t matter if you’re a diver, either as a hobby or by profession, or you can barely swim: water resistance is often seen as a signal for overall quality. 

To be fair, sometimes it is. A watch with a solid water resistance rating is going to be constructed in a robust way that you’d expect to keep it protected from elements other than water. If it can keep moisture out, it can surely keep lots of other undesirable things out as well. And a watch with a caseback that screws down and a thick sapphire crystal is likely to fare better in a dropped-on-the-floor situation (but please don’t test this at home – I’m not a scientist). 

Still, there’s something impressive about water resistance specifically, as ingress of moisture poses a unique and traumatic risk to a mechanical watch movement. Over the years, the drive to go deeper and print larger and larger “WR” numbers on dials has led to something of an arms race among watch manufacturers to produce ever more resistant dive watches. 

We’ll talk about some pretty extreme dive watches here, so it’s worth mentioning right up front how truly over-the-top many of these watches are when it comes to using them in the real world. These watches are showcase pieces that flaunt a brand’s R&D and manufacturing capabilities – they are hardly necessary tools, even for people who spend a lot of time underwater. Most divers agree that a watch that provides a solid 100 meters of water resistance and working gaskets is going to be more than capable for almost everyone. There are even different schools of thought on the effectiveness of a screw-down crown, which is now standard equipment on a diver, but wasn’t always. 

Most modern dive watches have a water resistance rating of 200 or 300 meters. The Rolex Submariner, perhaps the most iconic dive watch of them all, carries a 300 meter rating, as do many of its closest competitors, such as the Omega Seamaster. Once you get above that, you start entering into the so-called “Professional” realm of dive watches, tools that are purpose built for, well, professionals. Even if in practice they are most often worn by regular folks who simply enjoy watches are well made, and perhaps a bit over engineered. The technology that allows these watches to exist (and the specs it bestows upon them) is a huge part of the appeal, even if they’re never actually used for diving. 

Before we get into some of the watches that can descend to depths that are frankly unimaginable for human beings, it’s worth taking a look at what exactly makes a watch water resistant in the first place. 

The first line of defense against water are the various gaskets that are fitted to watch cases at their points of weakness. These gaskets are often made of rubber or a synthetic material, and are designed to create seals at the crystal, caseback, and crown. The crown gasket is particularly important, as it’s likely to be exposed to the elements more often than the other gaskets. Everytime you unscrew the crown to wind your dive watch from a dead stop, or adjust the time, you’re exposing the gasket to the outside world, and over time the synthetic material can dry up and become brittle, making it less effective.  

Most modern tool watches have casebacks that screw into the case, providing a level of security that old-school tension-fit cases simply can’t, particularly when paired with a gasket made from modern, long lasting materials. The crown, of course, is expected to screw into the case as well, and some brands, Rolex among them, have devised proprietary systems that use multiple gaskets working together through the length of the crown tube to create a series of sealed watertight “zones” that can keep water from getting into the case even when the crown is unscrewed.

One of the more exotic bits of tech to emerge recently in the dive watch arena is the oil filled case. In the case of Sinn’s UX line of watches with their HYDRO technology, the (quartz) movement, dial, and hands are held in a fluid bath. This has several benefits, according to Sinn. The most obvious and visible is that it creates a completely reflection free view of the dial when underwater. The fluid has the same refractive index as the sapphire crystal itself, so when light is reflected off the dial (which is what allows you to see it) it isn’t refracted through the crystal at all, creating a view that is completely free of distortion, and eliminating the “mirror” effect you’ll get with a crystal when viewed underwater (see the photo below for a great example of this). Ressence makes an oil filled dive watch that works on a similar principle. 

The other advantage of an oil filled case is that the fluid is incompressible, and effectively neutralizes the external pressure on a dive watch at any depth that a diver is likely to achieve in the real world. The case, of course, still needs to be structurally sound, but fluid used inside of the case offers an enhanced level of protection in addition to being distortion free. There aren’t many brands that use this technology, and it’s more of a curiosity than a necessity, but then again, most of what we’re discussing here falls squarely into that category.

The most important piece of the water resistance puzzle as it relates to watches that are designed for extreme depths is case construction. At a certain depth, it’s not so much about keeping water out, although that’s absolutely important and necessary to keep the watch functioning, but about the structural integrity in the case itself, particularly in how the case and crystal attach to one another. This is just basic physics, but the enormous pressure exerted by water at depth can wreak havoc on a case if it’s not well made. Consider that at a depth of 100 meters, a watch has about 146 psi pushing down on it. At 300 meters, you’re up to 437 psi. At 3,900 meters, the depth rating of the Rolex Deepsea Sea Dweller, pressure is calculated at a staggering 5,678 psi. This is a depth that, short of being in a submersible, isn’t survivable by a human being, but watches have been developed to withstand this pressure (and beyond) to prove a certain level of engineering mastery. 

Thicker watches made of stainless steel and titanium are needed if they’re going to come close to managing these insane amounts of pressure. You begin seeing trade-offs in the comfort and wearability metrics once you start getting over about 500 meters of water resistance, as you simply need a watch case to be sturdier and more robust to theoretically withstand such a depth. 

It’s silly to talk about deep divers without discussing what is likely the best known early example of a watch meant for absurd depths, the Rolex Deepsea Special. This was an experimental watch made in very limited numbers and not for general resale, but of course a small handful have found their way into the hands of private collectors. The Deepsea Special was designed by Rolex to accompany the Bathyscaphe Trieste on its expedition to the literal bottom of the ocean, Challenger Deep, near Guam in the middle of the Pacific. Total depth? Around 10,911 meters. That’s nearly 36,000 feet, close to 7 miles down. 

Rolex began development of the Deepsea Special in 1953, with the expedition to Challenger Deep taking place in 1960. Over the course of seven years, Rolex refined and made adjustments to the Deepsea Special to handle the unusual, almost extraterrestrial environment it would encounter at the bottom of the ocean. As reported by Rolex Passion Report, the key changes made throughout the testing process were primarily to the size and shape of the crystal, and the protection offered to the large screw down crown. 

The Deepsea Special’s crystal deserves some special attention, as it’s very much the watch’s calling card, and was used frequently in Rolex marketing of the period to highlight the brand’s commitment to engineering. The dramatic bulb shape evolved from a flatter, but still highly domed crystal that Rolex used early on in the prototyping process. The laws of physics dictate that a dramatically curved crystal will do a better job of resisting pressure, as it will spread the force of the water weighing down on it over a greater surface area. To manufacture a crystal that was adequately thick and curved with 1950s technology must have been an incredible challenge for Rolex designers, and it’s one of the great feats of watchmaking that they were able to make a watch capable of descending to the lowest points in the ocean at such an early moment in dive watch history. Rolex strapping the Deepsea Special to the outside of the Trieste as it sank to the bottom of the ocean, and observing that the watch kept time throughout the trip is, in retrospect, as significant a moment in watch history as any other. The echoes of this adventure have been felt in the design of other extreme dive watches in the ensuing decades, with Rolex mirroring their own accomplishment years later as a sponsor of James Cameron’s Deepsea Challenge, with another experimental watch attached. 

As underwater exploration gained steam in the 1950s and 60s, new and very practical problems began to emerge among the undersea explorers who lived in submerged habitats for scientific research (like SEALAB) or to do commercial diving work. The helium rich environment that these divers and scientists lived in for extended periods of time posed a unique problem for the watches on their wrist. While the depth ratings of these watches were substantial, they couldn’t keep out the tiny helium molecules being pumped into the pressurized atmosphere. When divers eventually decompressed as they returned to the surface, escaping helium would be likely to pop a watch crystal right off, sometimes violently. This is obviously not just an inconvenience requiring a trip to a watchmaker to resolve, but could potentially be a safety hazard.  

The solution, a helium escape valve, is now fairly common in professional dive watches. We see it in the Tudor Pelagos, Omega Seamaster, the Rolex Seadweller (a direct descendant of the Deepsea Special) and many other watches. The HEV is often maligned as an unnecessary waste. That’s a fair criticism – only a fraction of a fraction of people who own a watch a helium escape valve are likely to use it in any meaningful way. As the owner of a Tudor Pelagos, I count myself among those who are walking around with a “serious” diving instrument whose full potential is unlikely to ever be tapped. 

But these watches are a technical marvel, and that’s really the key to their appeal for the vast majority of us who are not currently employed as saturation divers. While the implementation of helium escape valves differ somewhat from brand to brand, the general principle is shared. These valves only work in one direction – when pressure within the watch case is greater than the ambient pressure outside of it (during decompression), the valve opens and allows helium that has built up inside the case to escape gradually. The valve is closed from the outside at all times, so water ingress is never a concern. The technology is completely passive and doesn’t require input at all from the wearer of the watch, but these valves do typically use a gasket that would need to be replaced along with those that protect the case at the crown, crystal, and caseback. 

The HEV is a curious design solution to a problem that is incredibly niche, but it’s not the only way that watch brands have built watches to meet the demands of saturation divers. Seiko has been making watches that are suitable to deep diving and saturation applications for years, but have eschewed the HEV in favor of a case design that they claim makes the valve superfluous.

The deep diver as imagined by Seiko puts a focus on the case, and making it strong enough to keep its structural integrity at the deepest of depths, and to, effectively, show those helium atoms who’s boss. The monocoque cases used in iconic Seiko references like the 6159-7010 (the “Grandfather Tuna”), the 6215-7000 (a precursor to the Marine Master), and so many watches derived from these early Seiko divers were made with a single steel block with access to the interior of the case through the dial side. At a time when multi piece cases were standard, Seiko saw the introduction of a removable caseback as a potential weak point. Their approach was to concentrate on building the best single piece case possible and attaching the crystal and bezel securely. The results speak for themselves – Seiko dive watches have a sterling reputation in the dive community, and were well ahead of their time. The Grandfather Tuna, for example, was introduced in the 1970s with a depth rating of 600 meters, which is impressive today and would have been downright rare nearly 50 years ago. Modern Seiko divers are known to exceed their lofty specs, and the brand is happy to test their newest watches publicly, with a somewhat theatrical touch.

Since the Trieste’s expedition in 1960, Rolex held the record for making the watch that had sunk to the deepest depth. That changed last year, when Omega unveiled the Seamaster Planet Ocean Ultra Deep Professional, a modern answer to the Deepsea Special, which went a total of 12 meters deeper. Those with horses in the race might argue that Rolex’s achievement sixty years ago was more significant, but Omega’s project, similarly experimental, is worth taking notice of. 

We’re obviously not talking about huge differences in depth, here, so we’ll agree that these watches are both, essentially, capable of going as deep as the planet will allow. With similar technical capabilities, then, the two watches side by side represent a unique study in contrasts, both featuring technology that was absolutely state of the art at the time they were made. In the case of the Omega, that means utilizing the latest in materials science to incorporate a case of forged titanium (28mm thick) with their own Liquidmetal technology to bond the crystal to the case. This effectively eliminates the need for traditional gaskets, and is the reason Omega’s watch uses a crystal that, while much thicker than what you find in your run of the mill diver, doesn’t have the over-the-top bulb-like appearance that you see on the Deepsea Special. 

Omega’s Ultra Deep is somewhat comically rated to 15,000 meters, which is a depth that is not possible on Earth. At this point, it’s unknown if the Ultra Deep is capable of reaching a depth of 15,000 meters on any other planet in our solar system, or indeed the galaxy. I imagine atmospheric conditions and the force of gravity on another heavenly body could impact the depth rating somewhat, but it seems like we’re a ways off from being able to test that. It does make you wonder what’s next, though. 

Presumably, the technology in Omega’s Ultra Deep will trickle down into consumer products (the watches Omega made for this project were not intended to be sold). It’s hard to say what practical purpose this level of water resistance has in a watch, even one designed for people who dive professionally or live in underwater habitats, but we know that everyday usefulness isn’t the threshold for these types of watches to gain a following. It’s a safe bet that if Omega were to serially produce some version of the Ultra Deep, or a watch that makes use of just some of the engineering that went into it, it would be a popular novelty for watch lovers who don’t so much as swim. At a certain point, it’s not about practicality, or tool watch pedigree, it’s all about celebrating technology, innovation, and ingenuity.