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Wow! That's deep!

Updated: Apr 22

I am something of a watch fan, and like many watch fans, I find YouTube a rich source of information and entertainment. However, there is one subject that absolutely infuriates me, because it is constantly and repeatedly misrepresented - water resistance.


Even one of my favourite watch channels, Just one more Watch propagates the most common myths. Jody is a star but he does have a tendency to make comments along the lines of “At 100m of water resistance, you’d never go diving with it”. Why not? Fortunately YouTube can also address this misconception.


As far as I can see, the answer lies somewhere between physics and the reality of recreational diving. Let’s exclude commercial diving as this is a much more specialised endeavour. Information of what constitutes recreational diving comes from the PADI (Professional Association of Diving Instructors) website.


The Basic PADI open water course takes you to dives of 18m - 60ft. Anything beyond that is considered a deep dive, and for recreational purposes 40m/130ft is deemed the limit. Beyond 50m (or 60m depending on source), you are into the realms of the technical diver and the use of mixed gasses.


We can reasonably assume that 50m of water resistance is adequate for the vast majority of recreational divers, therefore 100m should be more than enough. In theory a wrist watch with basic 30m water resistance would be plenty for basic recreational scuba diving.

Since I’m interested in watch water resistance, we will ignore the fact that most divers use dedicated dive computers to monitor their dives.


This raises a couple of further questions which can, but all too often, aren’t answered; the myth of static pressure and whether or not that 100m rating on your watch means 100m.


The redoubtable Mark over at Long Island Watch did an excellent video on static versus dynamic pressure. Have a look here, and yes, there are equations! The upshot of his calculations are that to double the pressure on your watch you would have to be travelling at 50 miles per hour. Underwater. Good luck with that! His far more reasonable calculation assuming movement of 3 metres per second only increases the pressure on your watch by 450 Pascals. Given that at 100m, it is subject to 980,000 Pascals, this is insignificant.


Whether your watch can actually withstand the stated pressure is dealt with over at Beyond the Press. They’re very proud of their hydrostatic testing rig and have destroyed a number of watches in it. The most useful video however is here, demonstrating as it does that two 100 Euro dive watches, rated at 100m will continue to function far beyond that limit.

Interestingly the failure of the watches is not caused by water ingress but by the deformation of the case back putting pressure on the movement (both watches are mechanical). The Citizen watch, rated at 100m, stops working at just short of 300m, and restarts as the pressure is relieved. Yes, they run both watches to 3km and both implode, but in fairness I suspect I would at that depth.


I have a couple of ISO rated 200m divers watches in my collection. That’s 655 feet. Even the lesser 100m rating equals 328 feet of depth. At either of those depths, I have far, far greater concerns than whether or not my watch will remain waterproof. The only people regularly operating at those kinds of depths are highly trained, specialised divers, using Trimix and decompression chambers.


The problem with numbers is that we can take them to represent things that they don’t, especially if we don’t attempt to understand both the physics and engineering which underlie them. This is further obscured online by a failure to put those numbers into context. If you are going to make pronouncements about a watches suitability of diving, you need to present that in the context of what constitutes diving. Otherwise the numbers are simply that - numbers - and have little meaning in and of themselves. So, watch reviewers, a little context please so that those numbers actually mean something and are not just a bit of dial decor!



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