How Water Resistant Is Your Watch?

Few subjects in the world of watches create as much confusion as water resistance.

Over the years, I have had variations of the following conversation more times than I care to remember:

“Hans, my watch is water resistant to 1,000 metres. I dropped it on a stone floor and now the timing is off. How can that happen?”

The answer is actually quite simple.

Water resistance tells you how well a watch is designed to resist water pressure. It does not tell you how well it will survive being dropped onto a stone floor.

U.S. Navy Virginia-class nuclear-powered attack submarine (SSN)

A submarine can withstand enormous pressure hundreds of metres below the surface of the sea. Drop the same submarine from a sufficiently great height onto a concrete car park and you will encounter an entirely different set of problems.

Your watch is no different.

So what do those numbers on the dial or caseback actually mean? What is an ATM? Is a watch marked 30 metres actually safe at 30 metres? Can you swim with a 50-metre watch? And if your watch says 300 metres, does that really mean you can take it 300 metres beneath the surface?

Let's see if we can make some sense of it.

First things first: what does water resistance actually measure?

A watch marked 100 m / 10 ATM / 10 bar has been designed and tested to withstand a certain level of pressure.

The three measurements are simply different ways of describing roughly the same thing. One atmosphere, or ATM, is approximately the atmospheric pressure we experience at sea level. In watch terminology, 10 ATM or 10 bar corresponds approximately to the pressure associated with 100 metres of water.

So, very roughly:

3 ATM / 3 bar = 30 metres
5 ATM / 5 bar = 50 metres
10 ATM / 10 bar = 100 metres
20 ATM / 20 bar = 200 metres
30 ATM / 30 bar = 300 metres

Simple enough.

Except this is where much of the confusion begins.

A water-resistance rating is primarily a pressure rating. It isn't a general toughness rating for the watch.

A 300-metre rating doesn't mean the watch is three times stronger than a 100-metre watch. It doesn't mean its movement is better protected against shocks. It doesn't mean you can drive over it, hit it with a hammer or drop it onto the bathroom floor.

It means the case, crystal, crown, caseback and seals have been designed to keep water out at the specified pressure.

Nothing more. Nothing less.

An Enoksen Deep Dive E01/E with its characteristic Helium Escape Valve

What actually keeps the water out?

A watch case isn't a solid lump of steel.

There are holes everywhere.

The crystal has to meet the case. The caseback has to come off. The crown needs to turn and, on a chronograph, pushers have to move in and out. Every one of these places represents a potential route for water to enter the watch.

The clever bit is sealing them.

That job is largely performed by small gaskets – normally made from rubber or another flexible material - which create watertight seals between the various components.

A screw-down crown is useful because it helps keep the crown firmly seated against its seals. A properly fitted caseback does the same thing at the other end of the watch.

When everything is correctly assembled and the seals are healthy, remarkably high levels of water resistance can be achieved.

But those seals don't live forever.

They age. They dry out. They can become compressed or damaged. And a sufficiently hard impact can deform components or compromise a seal even though the watch looks perfectly fine from the outside.

This is why water resistance is not a permanent property of a watch.

If you regularly take your watch into the water, having its water resistance checked periodically is cheap insurance.

So. Can I actually swim with a 100-metre watch?

This is where things become unnecessarily confusing.

Spend five minutes on the internet and you will find charts telling you that a 30-metre watch must barely be allowed near a glass of water, 50 metres means you can wash your hands, 100 metres means swimming and 200 metres means diving.

Manufacturers themselves tend to be conservative too.

As a practical rule, I would think about it like this:

30 m / 3 ATM	Everyday protection against rain, splashes and washing your hands. I wouldn't deliberately take it swimming.
50 m / 5 ATM	A useful level of everyday water resistance. Some manufacturers permit bathing and similar water exposure, but if swimming is going to be a regular activity, I would personally choose something with more margin.
100 m / 10 ATM	Now we're getting serious. Swimming, holidays, boats and normal recreational use in water should present no problem for a properly maintained watch.
200 m / 20 ATM	More than enough water resistance for almost any normal owner. Swimming, snorkelling and serious use around water are comfortably within its territory.
300 m / 30 ATM and beyond	We are firmly into serious dive-watch territory - assuming, importantly, that the watch has actually been designed and tested as a dive watch rather than simply having an impressive pressure rating printed on the dial. And that last distinction matters.

A PROOFMASTER pressure tester

Water resistant doesn't automatically mean dive watch

There is an international standard specifically for diver's watches: ISO 6425:2018.

To qualify, a watch needs to do more than merely survive water pressure. Among other requirements, a diver's watch must be designed for diving at depths of at least 100 metres and have a secure means of measuring elapsed diving time that can be read in darkness.

That rotating bezel on a traditional dive watch isn't there just because it looks good.

A proper diver is an instrument.

This also explains why two watches can both say “200 m” on the dial while being quite different propositions. One may simply have excellent water resistance. The other may have been designed specifically as equipment for diving.

The number alone doesn't tell the whole story.

Please don't touch the crown

There is another very simple rule which prevents a surprising number of problems:

Don't operate the crown or pushers underwater unless the watch has specifically been designed to allow it.

Your watch might be perfectly capable of resisting 200 metres of water pressure with the crown properly secured. Pull that crown out while the watch is submerged and you may have just opened a route straight into the case.

The same applies to chronograph pushers.

There are watches specifically engineered to operate underwater, but unless the manufacturer tells you otherwise, leave everything alone until the watch is dry.

And if it has a screw-down crown, make sure it is actually screwed down before jumping into the pool.

You would be amazed.

Up and close with the classic 300m Enoksen Dive E02/A

Water is not always the biggest enemy

Heat deserves a mention too.

Taking a watch from one temperature extreme to another isn't ideal, and saunas, very hot baths and steam introduce conditions quite different from simply swimming in cool water. Soap and chemicals aren't particularly friendly towards seals either.

Salt water presents another challenge – not because a properly sealed dive watch can't handle it, but because salt should not be left sitting around the case, bezel and crown afterwards.

After swimming in the sea, rinse a suitable water-resistant watch in fresh water and dry it.

Simple.

And back to the stone floor...

Which brings us back to our supposedly indestructible 1,000-metre watch.

A watch is a remarkably tough little machine, but it is still a precision instrument.

Inside it are tiny gears, pivots, hands and other components manufactured and assembled to extremely small tolerances. A severe impact can disturb those components without allowing a single drop of water through the case.

Equally, a watch can remain completely undamaged mechanically yet lose some of its water resistance because an ageing gasket is no longer sealing properly.

Water resistance and shock resistance are two different things.

Perhaps the simplest way of putting it is this: Water resistance tells you how well your watch keeps water outside. It doesn't tell you how difficult the watch is to break.

Use it - but understand it

I have always believed that a good watch should be worn and used. If you own a proper dive watch, take it swimming. Take it into the sea. Get it wet. That's what it was designed for.

But understand what the numbers mean - and what they don't mean.

Keep the crown secured. Look after the seals. Have the water resistance checked occasionally if the watch spends a lot of time in water. And if you've given it a particularly hard knock, don't automatically assume everything is fine simply because the dial says 300 metres.

Most importantly, don't be frightened of your watch.

A good tool watch is designed to accompany you into the real world.

Just don't use it as a hammer.

 

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