All About Water Resistance in Watches – Part One of Two

Resistance factors, depth ratings, practical guidelines, and age considerations

Virtually all modern watches claim some degree of water resistance, but the terms are often misleading to consumers and collectors, occasionally leading to damaged timepieces that “should have been able to handle” the depth of water they were exposed to. In this post, I do a deep dive (haha?) into water resistance as it relates to watches.

What does water do to watch movements?

First off, we’re going to discuss why we care at all about water resistance. What happens if water enters your watch case?

For a quartz watch that’s powered by a battery, this may be obvious; water enters the watch, seeps into the inner workings, and causes a short circuit in the electronic components of the movement, damaging them irreversibly. But what about mechanical movements? If springs and gears get wet, what’s the big deal?

Water can harm a mechanical watch in a number of ways. The most immediate is that water can slow down gears and cause some parts to stick together, hampering accurate timekeeping and proper function. The water can also carry away the very small traces of oil needed to lubricate the high-friction areas of a mechanical movement, leading to those parts wearing out more quickly. Water entering the watch can also carry small particles of dirt and debris that can jam the watch or abrasively wear down the parts. The worst effect of water though is corrosion. Watch parts can corrode from even trace amounts of skin oil and sweat (which is why watchmakers always wear gloves or finger cots when handling movements), and water is a very corrosive agent. Because watch parts are so small, even a little corrosion (which can occur in less than a week) could compromise the integrity of, or even eat right through, some of the components of a movement.

Aside from the movement, most watch dials are also very delicate (sometimes even more delicate than the movement). They are also notoriously hard to restore, and even small imperfections can sometimes be impossible to fix.

In other words, small amounts of water entering the case can damage the inner workings and defining visual elements of a watch irreversibly, requiring those parts to be replaced for the watch to be restored to its original condition.

Resistance Factors

Many design features influence a watch’s resistance to water. These all affect how well the case can isolate the movement from the outside world. Unsurprisingly, the areas most vulnerable to the watch’s environment are the ones designed to interact with it, i.e. the ones a wearer uses to operate the watch such as the crown and any pushers or buttons.

The Crown and Pushers
The crown is attached to the end of the stem which operates the movement’s keyless works and allows the wearer to set the time and wind the mainspring of the watch (assuming a mechanical movement that’s hand-wind-able). The stem fits inside the stem tube which sticks out of the side of the case, and which the crown fits over and seals against when pushed or screwed in. The seal is created by a small rubber gasket in the crown. This gasket experiences more wear and tear than many parts of the watch, and is also very sensitive to crown misalignment, a bent or cracked stem tube, and a bent stem. All these factors make the crown and crown tube the most likely place for leaks to occur.
Any buttons or pushers on the watch will have similar sensitivities, though often with less chance of getting bent or cracked.

Screw-Down Crowns and Pushers
Screw-down crowns and pushers fix many of the potential issues discussed above by helping to ensure proper alignment of the crown with the stem tube, and by allowing better compression of the gasket when properly tightened (don’t over-tighten though, as this can damage the gasket and the crown threading). Most diver’s watches utilize screw-down crowns and pushers to increase water resistance.
It should be noted that whether a watch has a screw-down crown and pushers or not, they usually shouldn’t be manipulated while the watch is submerged or very wet, as this can temporarily break the gasket seal and allow water into the watch.

The Caseback
A watch’s caseback allows the case to be opened to access the watch’s movement. Since it allows access to the inside of the case, this is another potential weak point for water to penetrate.

Snap-In Casebacks
Snap-in casebacks have ridges that are slightly deformed when the caseback is being pushed into the case, and snap back to their original shape (nearly) when fully seated, causing a tight fit against the case. This type of caseback is removed using a case knife to pry away one side until the caseback pops off. As any watchmaker will tell you, it is very easy to scratch, chip, or otherwise damage the case, caseback, or gasket when doing this. These small imperfections can allow water to penetrate the watch, making this a poor design for water resistance.

Screw-In Casebacks
Most modern watches of decent quality have threaded case backs that screw into the case and receive the same water resistance benefits as screw-down crowns and pushers.

Screw-On Casebacks
Screw-ON casebacks are held onto the case with small screws at regular intervals around the edge of the caseback. These are better than snap-in casebacks in terms of water resistance, but uneven screw tightness can potentially cause the caseback to lift in one corner slightly, resulting in a poor seal, thus this design is usually less resistant to water than screw-in casebacks. They are also less common.

Front and Rear Crystals
The edges where the watch crystal meets the case are another potential point of water entry. However, since the fit of the crystal in the case is usually very snug and gaskets are often used to create an even better seal, this is not a particularly weak point. The rear crystal in an exhibition or open caseback is somewhat more prone to leakage than the front crystal and is an unnecessary additional potential point of water entry. This is why many diver’s watches with high depth ratings do not have exhibition casebacks.

Crystal Shape
An often ignored (or at least rarely discussed) factor in water resistance, is the actual shape of the crystal, both in terms of its curvature or thickness and the shape of its outer edge.

Double-domed crystals (with an inner and outer dome to maintain uniform thickness and reduce optical aberrations) can be flexed more than crystals with flat faces when fitting them into a case. This means that the crystal can fit more snugly and can create a better seal.

It’s also easier to get a more uniform seal with a round crystal than with those that have a rectangular or oblong shape. Seals on crystals with non-circular shapes can become uneven with temperature changes if the case material and crystal expand at different rates. Without diverting into a materials science lecture, this is because objects tend to expand more in the direction of their longer dimensions. So a circular case or crystal will want to expand evenly about its center, but a rectangular case or crystal will likely expand or contract more in certain directions, resulting in less uniform seals.

Material Thickness
When pressures get extreme, such as in cases of very deep diving, the watch’s crystal could shatter or the case itself could be crushed. Because of this, specialty watches designed for these depths utilize extra thick crystals and cases. An iconic example of this is the Rolex Deep Sea Special which was tested to a depth of more than 10,000 meters underwater in the Marina Trench back in 1960.

Depth Ratings and Practical Considerations

Depth ratings are usually listed in meters (m), feet (ft), atmospheres (ATM), or bar (BAR). Though the first two are length measurements and the second two are pressure measurements, they all relate the same information because we assume we’re talking about length in terms of depth underwater, which results in a corresponding pressure.

Pressure Basics
One atmosphere of pressure is approximately equal to the average atmospheric pressure on Earth at sea level. This pressure is caused by the combined weight of all the air molecules in the atmosphere above the location being discussed pressing down on that spot. The same thing occurs in water, where the weight of all the water molecules above a certain point press down on that point and create pressure. Since water is denser than air, the pressure builds up more quickly, and an additional atmosphere’s worth of pressure is experienced with every additional 10 meters (approximately 33 feet) or so of water above a given point. So, 1 atmosphere is equal to 10 meters, which is equal to 33 feet. A bar is defined in terms of a different unit of pressure called Pascals (Pa), but what you need to know is that 1 bar of pressure is very close to 1 atmosphere of pressure, and when we talk about water resistance in watches, we treat them as exactly the same value.
So to summarize the equivalencies in the depth measurements we’ve discussed:
1 ATM = 1 BAR = 10 m = 33 ft

Depth Ratings
In the watch industry, there are a few standard depths that most watches are rated for. These standard depths, and the common types of watches rated for them, are listed below. The typical watches rated for each depth are broad generalizations, so expect variation. These are also not the only depth ratings you’ll see on watches.
– 20 m (2 ATM, 2 BAR, 66 ft), usually just labeled “Water-Resistant” with no depth or pressure specified – Lower-end watches, especially cheap quartz watches
– 30 m (3 ATM, 3 BAR, 99 ft) – Entry-level watches, both mechanical and quartz
– 50 m (5 ATM, 5 BAR, 165 ft) – Watches from all levels not meant for diving, especially dress watches
– 100 m (10 ATM, 10 BAR, 330 ft) – Watches from mid-level and up, especially sports watches not meant for diving, also older diver’s watches (this is also the minimum depth rating allowed for ISO 6425 certification – more on this in Part Two)
– 200 m (20 ATM, 20 BAR, 660 ft) – Mid-level watches meant for diving, or high-end older diver’s watches
– 300 m (30 ATM, 30 BAR, 990 ft) – Higher-end watches meant for diving
– 500 m + (50 ATM, 50 BAR, 1650 ft) – Professional-level diver’s watches

Practical Considerations
Now, you may be thinking “I’m never going to dive more than 50 meters underwater, so why would I ever need a watch rated to 300 meters?” The reason is that depth ratings are not measured by a diver taking armfuls of watches down to 300 meters to see if any of the watches leak, they are measured in a laboratory setting in ideal conditions with brand-new watches in static water (more on testing procedures in Part Two). This means that in real life, depth ratings are very unrealistic. In fact, many experts would say that you shouldn’t even take that 50-meter rated watch in the swimming pool.

We’ll get into this more in Part Two, but the (semi) recent changes to the international certification standard governing watches marked “water-resistant” (ISO 22810), made in 2010, were meant to ensure that the depth rating on said watches would cover “any form of the aquatic environment,” i.e. all reasonable activities performed at that depth. This should, in theory, mean that you can safely surf, snorkel, and jet ski with your 10-meter water-resistant watch. However, the interpretations, freedom in testing practices, and responsibilities left to the manufacturer as defined in the standard mean that this assurance of water resistance is very unlikely, and is heavily dependent on the manufacturer.

So what are the practical recommendations for getting your watches wet? Here are some conservative suggestions:
– 10 meters: Hardly resistant to water, keep the watch dry
– 30 meters: Fine if the watch gets splashed or rained on, do not submerge or expose it to hot water or steam (such as in the shower)
– 50 meters: Ok to submerge briefly, and should be ok to shower with
– 100 meters: Swimming and submersion are fine (such as snorkeling)
– 200 meters: Fine for most water activities such as scuba diving, surfing, and other water sports
– 300 meters: Good for commercial diving activities
– 500 + meters: Will handle serious deep water diving and saturation diving (helium-enriched environments – more on this in Part Two)

Age Considerations

No, I’m not talking about how old you are, I mean how old your watch is, and specifically how long it’s been since the watch was tested for water resistance. Since most of the water resistance a watch has is thanks to its gaskets, and gaskets are made from rubbers that degrade over time, the watch you bought 15 years ago with a depth rating of 200 meters may not be able to handle a shower now.

Gasket degradation occurs during normal use and can be accelerated by many factors, including exposure to saltwater, oxygen, and chemicals like chlorine. Luckily, you can do some very basic maintenance to slow the degradation process: rinse your watch with fresh water after it’s been in the pool or ocean, or if it’s been exposed to any chemicals. Even if your watch has only been hanging out on your dry wrist for a while, it could probably use a rinse to remove the sweat, dead skin particles, and dirt that inevitably build up after many hours strapped to a body.

In any case, to be certain that your watch is ready to handle the water, you should get it tested regularly and have gaskets replaced as needed, especially if you know you’ll be exposing the watch to more water than it usually sees (for instance if you’re planning a beach vacation where you expect to participate in some water sports). The frequency of these checks should be determined by your watch’s lifestyle. For professional divers and watches that are in harsh environments often, gaskets should be checked and replaced (if needed) every year. For casual swimmers, it would be wise to have gaskets checked every couple of years. For those that hate the water, but wash their hands vigorously and live in rainy climates, a check every 3 to 4 years should be adequate.

Any good watchmaker should be able to test your watch for water resistance, and we’ll discuss these tests in Part Two.

What’s Up Next Time

In this post, I’ve covered the practical side of water-resistance ratings and have given some background on how pressure and water resistance work.

In Part Two, I’ll get into the details of how watches are tested, the certifications and standards that relate to water resistance, and a bit about one of the interesting technologies that are used in many professional-level dive watches.