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 generally 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

Suitable for a rainy day..

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 between outer space and on the surface of the earth is handily called 1 atmosphere. When you dive under the water the pressure goes up at a steady rate of 1 additional atmospheric pressure for every 10m of depth (it’s actually every 9.81m in fresh water and 9.52m in sea water).  So 30 meters water depth will exert an extra pressure on your watch of 3 atmospheres, or 3 atm for short.  Incidentally bar is just another term for atm.  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.  

 

The watch industry takes a 3 atm rating as meaning, 'it has a nylon or rubber rubber seal on the back plate and around the crown stem so we think it's water resistant'.  You should take it as meaning, 'suitable for protection against rain, but don't wear it in the shower'.  A shower is quite an aggressive environment as hot water expands components and soapy water has a low surface tension so can more easily penetrate any defenses. 
 

50m, 5 atm, 5 bar, 200ft

OK in the shower..

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 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 adjust the time or use the chronograph buttons when wet, as  doing so can 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

Swimming pool and surfing..
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 with proper sealing mechanisms, it is also protected against dust and sand ingress.  Water evaporates, but dust sits by the seals and  can work its way slowly past them every time you wind the spring, adjust the time or push the buttons.  Dust in the movement can affect timing and cause wear. 
 
OFFSHORE Professional are double sealed on the chronograph buttons and triple sealed on the crown (the impact resistant floating crown being more susceptible to leaking than a screw down crown).  Case and crystals will take way in excess of 10 atm pressure, but overall it's 10 atm rated as that's what I test them to.  I use a floating crown design as the average field engineer is more likely to knock his Field Engineer off a pool side table whilst reaching for a G&T than free-dive for oyster pearls when he has some off time 'in the field'.

 

200 m, 20 atm, 20 bar, 660 ft

Scuba diving..
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 more meters and have enough residual strength to resist knocks, scrapes and sand jamming at that depth.


This class of pressure protection should see a screw down crown and chronograph buttons that lock in position so can't accidentally be pushed underwater.

 

300 m, 30 atm, 30 bar, 1000 ft

Scuba and mixed gas diving..
An interesting feature of many 300m watches is the inclusion of a helium relief valve, the purpose of which is to vent off internal pressure during depressurizing.  This has always puzzled me a little as it’s an entirely useless feature unless you’re a 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 Donald Duck voices because helium tightens the vocal chords.  Helium is a light gas with high molecular mobility, which allows it to diffuse through a watches seals and causes 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.  During decompression, the internal pressure trapped in the watch can't escape quickly enough and so blows the glass crystal off; hence the helium relief valve to  bleed off internal pressure.  Sounds pretty cool.  However, if you're a scuba diver then you won't have gone anywhere near a helium sat chamber and if you're a professional sat diver then sitting on your wrist is a Casio dive computer.  So what then is the point of the helium relief valve?  

 

1000 m, 100 atm, 100 bar, 3300 ft

Pub rated..
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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