Depth Rating

Watches come with a variety of levels of waterproofing and claim to be watertight at anything from 10m to 1000m depth.  It is important to bear in mind that these depth ratings have been achieved by placing a brand new watch in a fresh water chamber pressurised to the rated depth, and not by exposing a regularly worn watch to real life conditions at the relevant depths.  Real life is distinctly different to the controlled conditions of a pressure chamber, so with the OFFSHORE Professional chronographs I have a tendency to simply state suitable for swimming but not for scuba-diving.  

Just out of interest though the following can be used as a go-by for actual performance:


30m, 3 atm, 3 bar, 100ft

Atmospheric pressure is what we have on the surface of the earth from the air all around us pushing in a little on our bodies.  In outer space, where there is no air, the pressure is zero.  The difference in pressure between outer space and on the surface of the earth is handily called 1 atmosphere. When you start diving under the water you very quickly expose your body to much greater pressure than on the surface.  The pressure goes up with depth at a steady rate of 1 additional atmospheric pressure for every 10m of depth (it’s actually every 981cm in fresh water and 952cm in sea water).  So diving to 30 meters water depth will exert an extra pressure on your body and your watch of 3 atmospheres, or 3 atm for short.  Incidentally bar is just another term for atm in case your watch is rated in bar.  So a watch rated to 3 atm should theoretically be fine for the majority of recreational diving.  However, as you may have discovered, it most certainly is not fine.  I've put a few 3 atm watches into my pressure chamber and only rarely do they actually get up to 3 atm before leaking.  '3 atm' has become the generally accepted phrase for a watch that has been made with some rubber seals, and has nothing at all to do with its actual pressure rating.


A 3 atm rated watch is really only suitable for protection against rain.  If you're going to take a shower with your watch on then you need to be careful that you don’t get soap on it as soap has a much lower surface tension than fresh water and so can more easily penetrate the watches defenses, and a 3 atm watch will most likely only have single sealing. 
If you do get water into a watch the glass will mist up.  The quick solution is take the back off and leave it in the airing cupboard for a week and it should be fine.  Don't try and heat the watch to drive the water out any quicker as this will reduce the viscosity of the lubricating oil and allow it to disperse away from the jewels.  If you badly flood a watch then you'll have to send it to a watchmaker for a strip down, clean and re-lubrication.  It's best not to use a local jeweler for this as you really want it to be done properly.

50m, 5 atm, 5 bar, 200ft

This is the minimum depth rating that should be considered for swimming or taking a shower.  A lot of chronographs, including most Omega Speedmasters, carry a 5 atm depth rating to enable them to be worn without concern in a rainy or wet environment.  In watch makers terms '5 atm' should be read as 'its been sealed properly and quite possibly is good enough for 5 atm of static pressure'.


A VERY important thing to remember though is not to use the chronograph buttons when wet, as pushing the buttons will open a water ingress path.  So, remember, no using the stopwatch to time how fast you can swim a length or hold your breath under water.


100 m, 10 atm, 10 bar, 330 ft

Probably double sealed so that each leak path has 2 barriers against water ingress, 10 atm watches are completely suitable for general purpose use in a wet environment including swimming in fresh or salt water and showering with soapy water.  
A 100m rated watch is useful even if you never go swimming because, by having proper sealing mechanisms, it is also protected against dust and sand ingress.  Nothing will kill the movement in a mechanical watch as quickly as fine dust entering it. 
It is for the double sealing and dust resistance that I personally recommend a 100m watch as the minimum you should consider for everyday use.


200 m, 20 atm, 20 bar, 660 ft

At 200m depth rating we get into the real divers watch territory.  

A 20 atm watch can be worn without concern when swimming or scuba diving.  Even on a watch that has never been serviced the seals should happily withstand water depths up to 30 or so meters and have enough residual strength to resist knocks, scrapes and sand jamming at that depth.


300 m, 30 atm, 30 bar, 1000 ft

An interesting feature of many 300m watches is the inclusion of a helium relief valve, the purpose of which is to vent off excess internal air pressure from the watch during depressurizing.  This has always puzzled me a little as it’s an entirely useless feature unless you’re a professional saturation diver who decompresses from a helium atmosphere in a decompression chamber.  The nitrogen that makes up 70% of the air we breath becomes toxic at high pressure (nitrogen narcosis, feels a bit like getting drunk only it'll kill you), so 'deepsea' divers replace it with helium, and consequently talk to each other in squeeky voices because helium unfortunately tightens the vocal chords and makes you sound like Donald Duck.  Helium is a lighter gas than nitrogen with greater molecular mobility and hence the molecules can diffuse through a watches seals, allowing the internal pressure to slowly increase to the same as the diving chamber.  By slowly I mean over a number of days, which is plenty of time if you're stuck in a sat chamber for a four week rotation.  When you reduce the outside pressure during decompression, the internal overpressure trapped in the watch can't escape quickly enough and so blows the glass crystal off.  Hence the helium relief valve which bleeds off any excess internal pressure.  Sounds pretty cool.  However, if you're a scuba diver then you won't ever have gone anywhere near a helium sat chamber, and you're very unlikely to have spent several days at depth.  So what then is the point of the helium relief valve?  Sounds to me like it's generally put there by the marketing department.


1000 m, 100 atm, 100 bar, 3300 ft

A few watch makers offer 1000+m rated watches, which I’ve always found a bit optimistic considering the human body, even under pressure chambered, nitrogen free and helium saturated commercial diving, can only reach 350m before your brain turns to clotted cheese. 
'Did you just sneeze into your helmet?'

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