It’s officially summer, which for watch lovers means it’s dive watch season. One of the most persistent questions watch owners seem to have is what watch water resistance ratings really mean, and more precisely, what dive watch depth ratings really mean. There are several sides to this question. Among the issues are whether or not a dive watch can or should be used to its official rated depth; whether or not a given depth rating is sufficient to protect a watch under a variety of real-life circumstances; and of course, the evergreen and often repeated-as-fact notion that a depth rating is only the “static” rating and that motion causes a dramatic increase in actual water pressure, which may defeat the seals of a water resistant watch.
Let’s look at each of these issues in turn. Can a dive watch that is rated to 200 meters, or 300, or what have you, really be taken to that depth with an expectation it will live to tell the tale? That seems to depend a bit on what dive watch you are talking about, but in general, you’re probably fine. Let’s take two examples: Seiko and Rolex.
Back in May 2015, HODINKEE had an unprecedented opportunity to visit all four Rolex factory locations in Switzerland and as you can imagine, came away with a treasure trove of information. Especially relevant to our discussion of dive watch depth ratings is the fact that Rolex static pressure tests all of its watches, after assembly, and before they are shipped out.
As it turns out, all Oyster-cased watches are static tested to 10% greater than their stated maximum depth rating; dive watches (like the Submariner) are tested to 25% greater than their stated max depth rating. That this would have to be the case seems obvious from a common sense engineering standpoint, as you would naturally want to design something that would fail at least somewhat in excess of a stated spec the maximum “test depth” for a modern submarine, for instance, is typically anywhere from one-half to two-thirds the “crush depth” (at which the hull is expected to fail under pressure) and there are reports of submarines under combat conditions exceeding their crush depths without implosion.
How much deeper can you take a dive watch than its rated depth rating before mechanical failure occurs? Again the answer is probably some variation on “it depends,” however this video produced by Seiko is thought-provoking.
In September 2014 Seiko put two 1000m Marinemaster watches on the hull of an ROV and started lowering the whole thing deeper and deeper into the Sea of Japan. These are 1,000 meter rated watches, so reasonably, or perhaps naively, one might expect failure around the 1,000 meter mark. Surprisingly the hand of the quartz model did not stop until the ROV had reached a depth of 3,248 meters, and the mechanical model didn’t stop until an incredible 4,299 meters, at which the water pressure on the case was 6,263 lbs. per square inch (water pressure increases by about one atmosphere, or 14.6 lbs. per square inch, every 10 meters).
It would seem clear then that if static testing of water resistance is the norm for a dive watch, as it must be for any ISO 6425 diver’s watch, then at the very least dive watch ratings can be trusted to the official rating and at least in the case of Rolex and Seiko, probably well in excess of that rating (I have always wondered just what you could get away with,with an SKX-007).
The next question is whether a given dive watch is adequately protected against failure (that is, entry of water into the watch) for real life use. Let’s look again at the ISO 6425 requirement.
Here is an Oris Diver’s Sixty-Five, a watch with a 100m depth rating. Watch enthusiasts sometimes point to such a depth rating and decry it, saying that it is inadequate. While certainly for professional use one always likes to build in as much of a safety margin is possible, the fact remains that most recreational scuba diving takes you down to only about 40 meters, which is the maximum recommended for recreational diving by most training agencies. Deeper than this you start to enter the realm of technical diving, where specialized equipment and further training are necessary.
40 meters is less than half the depth rating of a 100m dive watch, so even if it were to be built to fail at exactly its rated depth (which it almost certainly is not) you still have a very comfortable safety margin built in. This is even more true for 200 meter diver’s watches. Considering how few dive watch owners will every actually dive, and how few of that number will ever do any +40 meter technical diving and you begin to see how objections to a 100m dive watch are almost entirely academic.
At this point in the discussion someone usually mentions the notion of static vs. dynamic pressure on a watch. The claim is that you should never use a dive watch in real life anywhere near the official depth rating because the rating is only for “static” pressure and that movement of the arms during swimming, for example, can dramatically increase the actual pressure.
The Internet being what it is, someone got curious about this issue and ran the actual numbers. As it turns out, movement does indeed increase actual pressure, but to a trivial degree. The relevant forum post on Watchuseekincludes this remark: “Without repeating all the calculations here (they involve denominators and the Greek alphabet and are PITA to type out), at a depth of 330ft(100 m) and moving your arm at 3 ft/sec, the dynamic pressure is in the order of magnitude of 0.14 feet of head or 0.04% of the depth. Even assuming you could move your arm at 20 ft/sec (14 mph!) the dynamic pressure is only about 6.2 feet of additional depth (
In summary therefore we can see that except for specific cases: (a) 100 meters is more than enough; (b) your watch may be a lot tougher than just the numbers tell you; and (c) you can’t swim fast enough to defeat your dive watch depth rating, not by a long shot. In the last case I assume that hitting the water fast enough would of course at a certain point cause enough of an increase in pressure to be an issue (one wonders about jet skis) but at that point, you probably have other problems.
To look at what happens to the human body, and a watch, as you go deeper and deeper check out our story on the IWC Aquatimer 2000, right here.