Why build a watch capable of 300 metres when very few people will ever dive anywhere near that depth? It’s a fair question.
When we began developing the Siebe Gorman Type-D, 300-metre water resistance wasn’t simply a specification we wanted to print on the dial or add to a list of features. It represented something more important: the level of engineering we wanted this watch to achieve.
Because the difference between a 200-metre and a 300-metre watch is much greater than the numbers might initially suggest.
It isn’t just another 100 metres
At first glance, moving from 200m to 300m sounds relatively straightforward. After all, it’s only another 100 metres. But pressure increases with depth. A watch rated to 300 metres must withstand approximately 50% more water pressure than one rated to 200 metres.
And that additional pressure is acting upon almost every part of the watch simultaneously.
The sapphire crystal, case, caseback, crown, gaskets and seals all form part of one pressure-resistant system. Increasing the specification therefore isn’t simply a matter of changing a gasket or making one component stronger.
The entire architecture of the watch has to work together.
Think about the crystal
A useful way of visualising the forces involved is to consider the sapphire crystal.
Take a crystal approximately 30 mm in diameter. Its exposed surface area is around 707 square millimetres.
At approximately 20 bar of pressure, the force acting across that area is around 1,400 Newtons.
Increase that to approximately 30 bar and it rises to around 2,100 Newtons.
To put those numbers into more familiar terms, that’s roughly equivalent to increasing the load from the weight-force of around 144 kg to more than 216 kg acting across the crystal.
Of course, a watch case is an engineered structure rather than simply a weight sitting on a piece of glass, but it gives some idea of the extraordinary forces involved.
And that load has to be safely transferred through the sapphire, its gasket and retaining surfaces into the case.
The case itself is under pressure
Stainless steel feels completely rigid in your hand.
Under sufficient pressure, however, even steel can deform microscopically.
At 300 metres those tiny movements matter.
A case, crystal or caseback doesn’t need to visibly bend for deformation to become important. Very small changes can alter the compression of a gasket or the relationship between two sealing surfaces.
That is why engineering a serious dive watch isn’t simply about making something that looks strong.
It is about controlling how the complete structure behaves under pressure.
Then there's the crown
The crown presents another challenge.
You’re effectively creating an opening through the side of a pressure-resistant steel case and then placing a moving component through it.
The crown tube, threads, seals and gaskets therefore have to maintain the integrity of the case while still allowing the crown to perform its everyday functions.
The greater the pressure specification, the more important the design, tolerances and sealing arrangement become.
It’s one of the reasons a properly engineered screw-down crown is such an important part of a professional dive watch.
Every seal matters
Water will exploit the smallest available path.
The crystal gasket has to seal. The caseback gasket has to seal. The crown system has to seal.
And they have to continue doing so while the surrounding components are being subjected to enormous external pressure.
That is why water resistance is best thought of as a system, rather than an individual component or feature. If one part of that system isn’t doing its job, it doesn’t matter how strong everything else is.
The Helium escape valve adds another challenge
The Type-D also incorporates an integrated helium escape valve, or HEV.
Historically, helium escape valves were developed for professional saturation diving environments, where divers could spend extended periods living and working in pressurised habitats containing helium-rich breathing gases.
Because helium atoms are extremely small, gas can gradually enter a watch during prolonged exposure to these environments. During decompression, pressure trapped inside the watch can then exceed the rapidly falling external pressure.
A helium escape valve provides a controlled method of allowing that pressure to escape. But from a watchmaker’s perspective there is an interesting contradiction.
To create a pressure-resistant watch, you generally want as few openings in the case as possible. Then we deliberately introduce another one. So the HEV itself becomes another precision-engineered sealing system that has to maintain the water resistance of the complete watch.
Why not simply make the watch bigger?
There is, of course, an obvious way of increasing strength: make everything enormous.
A very thick case, thick crystal and substantial caseback can provide plenty of material with which to work. But that wasn’t what we wanted for the Type-D. We wanted a professional 300-metre dive watch that remained genuinely wearable.
The Type-D is approximately 41 mm in diameter and only 11.9 mm thick. Achieving the water-resistance specification within those dimensions required us to think carefully about the complete case architecture rather than simply adding unnecessary bulk.
For us, that is an important part of the watch. It should have the engineering credentials of a professional diving instrument without feeling like one on the wrist.
Manufacturing is just as important as design
There is another side to this that doesn’t receive enough attention: repeatability. It is one thing to create a prototype capable of surviving a pressure test. It is another thing entirely to manufacture watches consistently so that the production pieces achieve the same specification.
Components are manufactured to tolerances. Gaskets have tolerances. Cases, crowns, crystals and casebacks have tolerances. The challenge is ensuring that those tolerances work together repeatedly. That requires precision in manufacturing, assembly and testing.
And as pressure increases, those details become increasingly important.
So why did we choose 300 metres?
Because Siebe Gorman deserves to be more than a name attached to a watch. The history behind Siebe Gorman is one of diving equipment, engineering and people working in extraordinarily demanding underwater environments.
We wanted the Type-D to respect that heritage through its engineering as much as through its design. We don’t expect our owners to dive to 300 metres. That’s really not the point. The 300-metre specification is about engineering margin.
It’s about creating a watch capable of withstanding approximately 50% more water pressure than a 200-metre rating demands. It’s about engineering the sapphire, crown, caseback, gaskets, case and helium escape valve as one complete pressure-resistant system.
And it’s about achieving all of that in a watch measuring just 11.9 mm thick. After more than two decades working with dive watches, we know that 200 metres is already more than sufficient for almost every owner.
With the Type-D, however, we wanted to go further. Not because we needed another number on the specification sheet. Because if we were going to put the Siebe Gorman name on a professional dive watch, we wanted the engineering behind it to deserve that name.
Siebe Gorman Type-D. SUB AQUA VITA. Avaiable now with pre-order pricing, limited edition.