Ownership & Care
Water Resistance on a Pre-Owned Watch: What the Rating Can't Tell You
The depth on the dial describes a watch as it left the factory. On a pre-owned watch, the number that matters is the pressure it held at its last test. Here is how to read both.
In this article
The figure printed on a dial or caseback, whether 30 metres or 300, describes what the watch was designed and tested to withstand when it left the factory. On a pre-owned watch, the more useful figure is a different one: the pressure the case held the last time someone actually tested it. The two can be far apart, and knowing why is the difference between swimming with confidence and sending a fogged-up watch to a service centre.
What the rating actually certifies
The reference point is ISO 22810:2010, the international standard for water-resistant watches. It sets the test methods (an overpressure test, a test at shallow depth, a test with force applied to the crown and pushers, a thermal-shock test and a condensation check) and governs the marking a manufacturer may apply. The marking is a pressure expressed as an equivalent depth: the standard notes that 1 bar corresponds to 10 metres of water, so "10 bar" and "100 m" mean the same thing.
What the standard does not do is promise that a watch marked 100 m is suitable for any activity at 100 metres. ISO 22810 leaves it to the manufacturer to state which activities fall within a particular watch's field of use. Its introduction is also unusually candid about durability: how well and how long the protection lasts depends on how soundly the case was built and on "the product's history (service actions, shocks, etc.)." Watches sold specifically as divers' watches fall under a separate standard with additional requirements, ISO 6425.
Manufacturers translate their ratings into plain guidance, and the translations are conservative. Seiko's published water-resistance table is a clear example:
| Seiko marking | Uses Seiko lists as acceptable |
|---|---|
| 3 bar | Accidental splashes and rain only |
| 5 bar | Adds swimming, yachting and showering |
| 10, 15 or 20 bar | Adds bathing and shallow diving |
| Diver's watch, 200 m | Adds scuba diving |
| Professional diver's watch, 1,000 m | Adds saturation diving |
Read the first row again: in Seiko's guidance, a watch marked 3 bar, the equivalent of 30 metres, is not meant to go swimming at all. Other brands set their own thresholds, so check the maker's guidance for the specific watch. Rolex, for its part, states that its Oyster Perpetual models are waterproof to at least 100 metres and its Perpetual 1908 models to 50 metres.
Why a pre-owned watch drifts from its rating
A watch case keeps water out with gaskets: under the caseback, inside the crown and its tube, around the crystal and at any pushers. Those seals are not permanent. Seiko says plainly that "a watch's water resistance is not permanently guaranteed," naming the ageing of gaskets and the deformation of parts after accidental shocks.
A secondhand watch has had more time for both. It may also have been opened since it left the factory, for a service, a battery change or a dial swap, and every opening is a chance for a seal to be reseated badly, reused when it should have been replaced, or pinched. None of that is visible from the outside. A Submariner from 1999 and one from 2023 carry the same 300-metre rating on the dial; only a test says whether the older one still lives up to it.
One failure point is also the simplest. A screw-down crown seals only when it is fully screwed down. Rolex's own care guidance calls it "extremely important" to screw the crown down "until it can't turn any further" after setting or winding. Threads that are worn or cross-threaded, or a crown tube that has taken a knock, can leave a crown that feels closed but isn't.
How a pressure test works, and what to ask about it
A common first step is a dry test. Rather than submerging the watch, the machine places it in a sealed chamber, applies pressure or vacuum, and measures how much the case flexes. A leaking case behaves differently from a tight one. Witschi's ProofMaster bench tester, for example, describes its method as an "analysis of the deformation of the watch case under vacuum and/or pressure," with test pressures from –0.7 to 10 bar, according to the manufacturer's product page. ISO 22810 also includes a condensation check: the watch is warmed on a plate at 40–45 °C and a drop of cool water is placed on the crystal. Fogging that does not clear within about a minute indicates moisture inside.
Note the top of that ProofMaster range. Ten bar is the 100-metre mark. A Submariner rated to 300 metres that passed a test at 10 bar is tight at a third of its rated pressure. That is a meaningful result, and more than enough for swimming, but it is not a confirmation of the full rating. Verifying a dive watch to its full depth takes equipment built for higher pressures.
That is why "it's been pressure tested" is the start of the conversation rather than the end. Before buying a watch you intend to get wet, it is worth asking:
- At what pressure was it tested, and when? A pass at 3 bar and a pass at 10 bar say different things.
- Dry, wet or both? A dry test with a condensation check is a sensible baseline.
- Has the case been opened since the test? If so, the test no longer describes the watch.
- Were the gaskets replaced at the last service? Service paperwork sometimes lists this; often it does not.
- Does the crown engage cleanly? It should thread in smoothly and seat firmly, without grinding or catching.
- Is the rating realistic for the watch's age? A decades-old watch is best treated as splash-resistant until a current test says otherwise, whatever its original specification.
Habits that keep a good result good
Screw the crown fully home every time you set or wind the watch, and never pull out the crown or press a chronograph pusher underwater unless the manufacturer specifically allows it. ISO 22810's own test of those components applies force at a depth of about 10 centimetres, so a 100-metre rating says nothing about pressing a pusher at depth. Rinse the watch in fresh water after the sea, as both Rolex and Seiko advise, and after a chlorinated pool. Be wary of heat: the standard's thermal-shock test cycles the watch between 40 °C and 20 °C water, and a sauna is far outside anything that test simulates. Have the watch tested again after any case opening, after a hard knock, and before a season of regular swimming.
Where Swiss Watches Dot Com stands
Every watch Swiss Watches Dot Com sells is pressure tested before sale. Under our 1-Year Limited Warranty, if a watch fails to hold its tested result during the first year of normal use, with the crown and pushers fully secured, it is covered. The warranty excludes water damage from use beyond the tested result or with an unsecured crown or pushers, and damage caused by third-party opening, repair or service, so a quick battery change at a kiosk is worth avoiding during that year. If a watch you are considering will be worn in water, ask us for the pressure it was tested to before you buy. For the rest of a pre-purchase inspection, our guide to buying a pre-owned Rolex Datejust covers what to check on the case, dial, bracelet and movement.
The rating on the dial shows what the watch was built for. The test result shows what it can do today. On a pre-owned watch, swim to the second number.
