Understanding Water Resistance in Watches Without Drowning
Waterproof watch: Are you being lied to? Or is the international watchmaking industry miscommunicating?
The famous “20 m” engraved on your watch is not a sesame to set off in pursuit of the Kraken.
In practice, this “20 m” refers to a static pressure test carried out in a laboratory. In concrete terms, a 20 m watch will resist a pressure of 2 bars.
But under a jet of water (shower, tap) or in motion (swimming, diving), we’re talking about dynamic pressure. For the record, water pressure in a domestic installation is generally between 2 and 3 bars. So a jet of water directed at a sensitive area of your watch – such as the crown or case-back – could be fatal if it’s not equipped to withstand it.
For our diving friends, the confusion is even worse! When diving (in the real world, not in a laboratory), at a depth of 20 meters, water pressure is added to atmospheric pressure. So you feel a total pressure of 3 bars. In other words, the water resistance of a “20 m” watch won’t suffice at all.
Unfortunately, this information is all too often poorly communicated. The average customer reads “20 m” and thinks he can take a dip. But the brands cover themselves discreetly… in the instructions. Yes, that little leaflet folded like a Michelin map that you never read: it contains lines written in small print, specifying that the wristwatch is not actually designed for prolonged exposure to water, let alone in motion. The result: you take a shower with your beautiful brand-new automatic, it gets wet, and the after-sales service politely tells you that it’s “inappropriate use”. And there’s no recourse: it was written. In black and white. In the ten pages that nobody reads.
So why aren't all watches water-resistant to 1000 m?
The answer is simple: because it’s complicated.
Achieving true watertightness at 100 m (i.e. 10 bar pressure) requires real engineering work. Thicker gaskets are needed to withstand this pressure. This takes up a lot of space, without interfering with the internal mechanics. The watch also needs a thicker sapphire crystal, cut to resist crushing. And all this in a case that remains elegant, comfortable on the wrist, and not as thick as a chopped steak.
This explains why this is not a priority for all brands. Many prefer to aim for style, finesse or a controlled production cost, rather than take on the serious waterproofing headache.
As a result, we sacrifice a little safety for a simpler design. It’s a choice, but you have to understand it if you don’t want any nasty surprises.
A bit of technique:
When we talk about water-resistance, we think of a seal, a bit like a faucet gasket: it prevents water from passing through, and that’s that.
But in watchmaking, it’s a little more complex. Take O-rings, for example – those little rings of nitrile (a synthetic rubber) found on watch backs and crowns. Yes, even the crown can hide one, two, sometimes three gaskets… in just a few millimetres. And not all are created equal. Nitrile comes in several hardnesses, measured in shores, and the choice of hardness is crucial: it determines how the gasket will deform once compressed in its housing. Too hard, and it won’t crush enough. Too soft and it leaks.
It’s all a question of dosage, which I define at the design stage.
And that’s just the beginning. Ice seals are often made of Hytrel, a hard plastic capable of deforming just enough without breaking. But making a quality seal here is a different kettle of fish: we’re talking about a manufacturing tolerance of less than 50 microns.
Its shape, height, cross-section and compression calculation are a mixture of greasy formulas and years of experience. And don’t forget: high compression is all well and good, but the sapphire crystal mustn’t break during assembly. For these glasses, however hard they may be, remain fragile. And frankly, it’s not much fun picking up pieces of sapphire crystal.
Let’s talk about sapphire crystals. They’re scratchproof, but not unbreakable. A crystal 1 mm thick and 38 mm in diameter has no chance of holding up at 10 bars.
To withstand the pressure, the thickness of the glass must be adapted to its diameter and to the maximum pressure targeted. And that’s where the design drama begins. Because a watch that’s water-resistant to 200 m is all well and good. But it also means that the crystal will be 3 mm thick. So, aesthetically, it’s more complicated to make the watch slim and elegant.
Tested, approved... and to be reviewed in 5 years.
Once the watch is assembled, it’s not over. Each part I make undergoes a water-resistance test at a higher pressure than the guaranteed one. Carried out using a special device, this test simulates the pressure build-up that would occur if the watch were submerged, without getting it wet. This is known as a dry air test, and enables us to detect the slightest deformation and therefore the slightest leak. We even go so far as to test your watch under negative pressure (as in an airplane depressurization test).
But water-resistance is not eternal, even with the best materials. Gaskets, however good they may be, age with time: they settle, crack and lose their elasticity. That’s why a check-up every 5 years is essential – and not just for intensive use.
And if in doubt, please don’t immerse your watch. It’s better to take it off for a bath, a shower or a paddle than to find it fogged up from the inside.
A final piece of workshop advice: some chemicals are devious enemies. Nail polish remover, perfumes, certain soaps, even insecticide sprays… all of these can attack seals without warning. So avoid putting perfume directly on your wrist.
Water-resistance is a delicate balance between precision engineering, quality materials and a little common sense.
A few recommendations:
To avoid water-resistance problems, adopt a few good reflexes: always make sure the crown is fully pushed in or screwed down before any contact with water. Never use the chronograph pushers when the watch is submerged.
Avoid contact with aggressive products – perfumes, solvents, insecticides, sprays – which can invisibly attack the seals. And finally, if in doubt, or if the watch has not been inspected for several years, it’s best to avoid immersion altogether.
When in doubt, contact us!
All my watches are tested to withstand 7 bars of pressure, the equivalent of 60 m diving. But to avoid confusion between meters, bars, immersion and water jets, I’ve decided to indicate the pressure directly in bars on future watches.
It’s clearer, more honest, and avoids many misunderstandings.
And if one day you have the slightest doubt about the water-resistance of your watch – because it hasn’t been looked after for a long time, or it’s suffered a shock, or simply because you’re keen on it – contact us to have it checked.
