When a construction must be perfectly waterproof and transparent, the problem of how to handle the assembly between its different parts is an important technical subject, especially if the pressure difference between the inside and outside is high.
In architecture, this subject is a fundamental theme in facade design that everyone knows more or less, but there is another area of design in which it is just as important, although less obvious: that’s the design of watches.
The beginnings of Rolex
Watch designers have not always been aware of the importance of waterproofing: the first portable watches were long held in elegant vest gussets and it would not have occurred to any user to swim with them.
One of the first entrepreneurs to have understood that the modernization of the society would inevitably lead to the wearing of watches on the wrist, directly exposed to all possible human activities, was Hans Wilsdorf, the creator of Rolex.
He was also one of the first to understand the technical implications of this new use, which included a perfect waterproofing of the case so that the fragile mechanism of the watch is protected not only from humidity, but also from dust that is so harmful to it. Indeed, it sticks with the oil on the critical gears to form an abrasive paste that slows down movement and accelerates wear.
The technical solution he implemented for his famous “Oyster” model, whose the very name evokes sand tightness, is an elastomer O-ring which is compressed by a cover screwed into the case thanks to a thread on its outer edge and by means of flutes on the periphery of its outer face, facilitating the application of the tightening torque via a specific tool.
This principle has proved very effective and, as an accomplished entrepreneur, Hans Wilsdorf made it his privileged communication subject that has enabled him to become the most important watch manufacturer in Switzerland.
Blancpain’s 50 fathoms
However effective it may be, this principle is not perfect, in fact the more the water pressure increases, the more it is necessary to tighten the seal so that it can be correctly sealed and, as a result, it is inevitably sheared parallel to its plane due to the helical movement of the cover imposed by the screw during tightening. This induces high stresses in the joint which significantly reduce its service life.
The early 1950s saw the emergence of recreational diving and it turns out that Jean-Jacques Fiechter, the director of Blancpain, one of the oldest watchmaking companies in the world, was himself a passionate diver. He therefore naturally had the idea of developing a watch for this specific use, anticipating the emergence of a new market.
In 1953, the “Fifty fathoms” watch was launched, it was the first real diving watch which, as its name suggests, could reach the depth of 50 British fathoms, or about 90 meters.
This model represents an important advance because it is a real change in sealing strategy: although the O-ring is retained, it is no longer directly compressed by the cover because the latter is no longer screwed, it is a peripheral threaded and notched steel ring that is screwed on to the housing and thus pressing the cover against it.
The cover no longer shears the joint because it simply translates when the ring is screwed on, but it is free to move if the water pressure acts on its surface. It is therefore no longer necessary to compress the joint strongly permanently because the deeper you go, the more the water pressure will compress it via the cover, which is now free to move, and will automatically strengthen the seal. The strategy of brute force is therefore replaced by the more flexible strategy of dynamic adaptation to the environment.
This watch was a great success, including with the military who created combat diving alongside recreational diving, and is still a flagship model of the nearly 300 year old company.
Vostok at the service of the Soviet fleet
The intelligence of this strategy certainly caught the attention of engineers Mikhail Fedorovich Novikov and Vera Fedorovna Belova, working for Vostok in 1967, who were given the task of producing a diver’s watch that could reach -300m. They took up the idea, but improved it as much as possible: the seal is no longer an O-ring but flat and can therefore withstand higher pressures, and the cover has also 2 grooves which, when applied to the seal, create a double sealing barrier. Two shear keys are also introduced to ensure that the movement of the cover remains perfectly normal at the joint.
A similar strategy was applied to the watch glass, which is in fact not in glass because engineers preferred Polymethyl methacrylate (PMMA), which can be about twice as resistant (bending strength 80 MPa for 45 MPa for the glass) but is also about 20 times more flexible (modulus of elasticity of about 3 GPa for 70 GPa for the glass). This PMMA bezel rests radially on the steel case and has a domed shape so that the water pressure induces a radial deformation which also improves watertightness.
After some difficulties with the development of the seal, whose springback had to be adapted to the pressure reduction of the lift to avoid leaks in this phase of the cold water diving cycle, their model worked perfectly and showed great robustness, allowing it to be adopted by the Soviet fleet.
While all the watches described here often exceed $5,000, whether new or second-hand, it is interesting to note that it is still possible today to buy from Vostok, who survived the post-Soviet crisis, a watch with almost the same functions for around 100 times less (around $50)!
The ultimate evolution from Omega
When diving became more professional in the late 1960s, thanks in particular to Henri Germain Delauze, former teammate of Commander Cousteau and creator of the COmpagnie Maritime d’EXpertise, which was a great success in industrial diving, the major watch producers saw an opportunity. They have therefore created specific high-performance models including the famous Rolex Submariners.
But it is Omega that holds the palm of the most waterproof watch with the famous Seamaster 600 model called “PloProf”, for Plongée Professionelle in French. This impressive piece weighing more than a quarter of a kilogram combines raw power with great finesse in the treatment of the waterproofing, in fact the problem of the rear cover gasket is solved simply by elimination: the case back is a massive block of stainless steel whose waterproofness is of course total, the movement is mounted from the front and it is the crystal that seals the case alone. Here also it is not made of glass but sapphire, pure aluminium oxide in crystalline form, which is one of the hardest materials known after diamond and about 7 times more resistant than PMMA (bending failure stress of 570MPa). This is sealed with a principle similar to those of Blancpain and Vostok watches: the crystal has a conical edge that is pressed against the joint by a steel threaded ring with an inner edge that is also conical. The very low friction of stainless steel on the sapphire prevents the ring from dragging the crystal and shearing the seal. The seal is also protected from excessive pressure by a groove on the edge of which the crystal can rest directly.
The waterproofing principle of the watch winder is also remarkable, most other watches such as those of Rolex use screwed crowns which can thus compress joints, but of course present the same shear wear problem as described for the covers.
The Panerai brand is well known for having improved this waterproofing by introducing a lever with a cam that tilts 180° to press the winding joint without shearing it, but this system requires high precision in assembly and is therefore sensitive to the natural ageing of the joint, which will tend to settle.
The waterproofness of the PloProf watch winder has a system that surpasses them all in which the crown no longer allows the movement to be wound but is in fact a thumb wheel that is pressing the gasket through the rod that is also threaded, this thumb wheel is trapped in the case that surrounds it so that the clamping force can be transmitted to the rod. Once the system is unscrewed, the adjustment is made with the head of the rod that has a square section. This principle allows the seal to be ideally pressurized, even if it has aged and its thickness has been reduced.
The PloProf had probably the best water resistance of all existing diver’s watches, it was guaranteed at -600m, and a test to a depth of -1370m was carried out without failure of the water resistance, so it was the ultimate tool for deep diving because the water diving record is still today at -675m, held by a COMEX diver.
But there is a tightness that is more difficult to obtain than the one against water, it is the one against helium, a gas that is used to replace nitrogen for dives down to -300m: the atoms of this element are the smallest existing after hydrogen and, in the form of gas, it infiltrates very easily through the joints. Rolex experimented with it at its own expense because COMEX divers who stayed long enough in helium hyperbaric atmospheres had the inconvenience of seeing their watches explode when they were going up because the accumulated helium overpressure had no time to reduce. The Ploprof were insensitive to it, proving their effectiveness, and therefore have never been equipped with helium valves, while Rolex had to do so for all its diving models.
Unfortunately for Omega, despite this technical superiority, COMEX chose Rolex as its official supplier and thus contributed to further increase the reputation of this brand at the expense of its best competitor.
The abyssal monsters of Rolex
In 1960, the Swiss scientific adventurer Auguste Piccard, who would have inspired Hergé for the personage of Professor Tournesol, was about to entrust his bathyscaphe to his son Jacques so that he could explore the greatest depths known. Always on the lookout for publicity, Rolex wanted to be part of the adventure and designed a watch capable of withstanding extreme pressures: hung outside the bathyscaphe, it would go down with it to -10,916m, so under a pressure of about 110MPa, and rise intact.
The steel rear cover has a significant thickness of 7mm, but the PMMA glass is enlarged to 17mm thick, it is so domed that the resulting magnifying effect strongly distorts the vision of the dial.
Rolex may have been directly inspired by the bathyscaphe porthole, also made of PMMA, as previously described, this material is interesting for its resistance which supports pressure, but also for its flexibility which makes it a sealing element in itself because under such pressures, it crashes against steel which is 70 times more rigid and thus prevents the passage of water.
This watch is only a demonstrator without any use since it is of course unthinkable for a diver breathing a gas to descend to such depths.
52 years after this achievement, thanks to his enormous commercial successes, director James Cameron wants to reissue it and finances a new bathyscaphe. Rolex could not miss this opportunity and joined this initiative, manufacturing the “Deep Sea Challenge” model for the occasion.
For this one, the PMMA is abandoned for sapphire, which significantly reduces the thickness of the crystal and avoids the magnifying effect. But the enormous pressure applied to the entire surface of the crystal and the back of the watch creates great stresses on their supports and Rolex engineers had to design a special system to solve this difficulty.
Patented under the name “Ring Lock”, an independent ring of very resistant and rigid metal (made of Titanium Alloy or Austenitic Stainless Steel) allows this force to be directly bridged between the crystal and the bottom so that it does not pass through the housing, which can therefore be made of more conventional metal. To ensure that the external pressure is transmitted directly to this inner ring, Rolex has adopted Blancpain and Vostok’s principle of allowing the crystal and bottom discs to move freely.
The sealing of the crystal is obtained by a radial seal which is compressed by a metal tension ring, again the Vostok principle which consists in that the radial deformations of the crystal under pressure reinforce the sealing is reused.
Thanks to this original design, like its older sister, this special watch will travel intact from the deepest ocean.