200m or 20ATM


Active Member
Hello everybody,
Casio says Frogman watches can resist to 200m pressure under water. fine!
On the other hand, most of other G-Shock watches are able to resist up to 20ATM.
According to my understanding 200m deepth and 20ATM are quite similar.
So, what is the real difference between these two capabilities ?


My understanding is that all (or most) G-Shocks are water resistant to 200 metres, 20ATM or 20 bar. 20 bar, 20 atm and 200 metres indicate the same level of water resistance, often they are denoted differently depending on intended market of the watch. For example some DW-6900 show Water Resist 200m, some show Water Resist 20 bar - I forget which is for which market.

The Frogman are slightly different, they are the only G-Shock watches which are ISO certified to 200 metres. The Frogman are certified to ISO 6425 > http://en.wikipedia.org/wiki/Water_Resistant_mark#ISO_6425_divers.27_watches_standard .


Ok - my take on this (as a former BSAC diving instructor)

Air at the surface of the earth presses on your body uniformly at a pressure of one atmosphere - 1 ATM -1000millibars - 1 bar - 14.7lbs per square inch - 1 kg per square cm

Going under water pressure increases quite markedly as water is much denser than air so that although a column of air from the surface of the earth to the limits of the atmosphere exerts a pressure of 1 kg per sq cm a column of water only 10m high exerts thar same pressure.

So normal pressure of 1 bar at the surface becomes 1 bar + 1 bar = 2 bar at 10 metres below the surface, 3 bars at 20, 4 bars at 30 and so on.

This would all be easy if the watch manufacturers weren't, like hi-fi manufacturers, lying little s***weasels. The depth rating of a watch is measured as a static pressure and is done in controlled laboratory conditions. Real world conditions involve movement with different pressures to front and rear of the watch, rather more pressure than the static test. (And amplifier power outputs can be 5w per channel RMS or 60w PMPO - roughly the same power but looks much different to the naive consumer)

Manufacturers will of course plump for the highest figure they can get away with for their advertising, I was always asked why people needed a 200metre watch when they never would dive below 50 metres, the reason is that the 50m / 5 ATM watch is good for the deep end of the bath and the 200m / 20ATM is just about ok for normal scuba diving. (I used my 6600 down to 45 metres with no problems)

If you look at a watch instruction book it may tell you what the ratings actually mean - 3 bar / 30m / 3 ATM to me is a watch that can be left on the bathroom sink while you take a shower ... :D

This is what I pulled off a site when googling for an answer:

The different levels of water resistance as expressed in meters are only theoretical. They refer to the depth at which the watch will keep out water if both watch and the water are perfectly motionless, says Scott Chou, technical director at Seiko Corp. of America. These conditions, of course, are never met in the real swimmer’s or diver’s world. In real life, the movement of the wearer’s arm through the water dramatically adds the pressure on the watch as well, so it is advised that, to be safe, the watch should be worn to the depths less than what is indicated by lab testing machines.
What are the various levels of water-resistance?
Watches with the lowest level of water resistance are labeled simply “water-resistant.” They can withstand splashes of water but should not be submerged.
The following usage recommendations are suggested by industry standards.
  • Water-resistant to 30 meters (100 feet): Watches with this rating will withstand splashes of water or rain but should not be worn while swimming or diving.
  • Water-resistant to 50 meters (165 feet): Watches with this rating are suitable for showering or swimming in shallow water.
  • Water-resistant to 100 meters (330 feet): Watches with this rating are suitable for swimming and snorkeling.
  • Water-resistant to 150 meters (500 feet): Watches with this rating are suitable for snorkeling.
  • Water-resistant to 200 meters (660 feet): Watches with this rating are suitable for skin diving.
  • Diver’s 150 meters (500 feet). Meets ISO standards and is suitable for scuba diving.
  • Diver’s 200 meters (660 feet). Meets ISO standards and is suitable for scuba diving.
Nice Mr. rutteger has already referred you to Wikipaedia regarding ISO certified watches, personally I would wear a cheap secondhand G-Shock with a new battery I had fitted myself and with a properly greased O ring. (Froggies are too nice to bash against wrecks and rocks). I would also use a decent dive computer as my main source of information with the dive plan worked out previously from BSAC decompression tables, the G would be a backup for when the computer failed.

I have also just had a look at the Wikipaedia ISO watches article which gives a slightly different interpretation of the various water resistances, all I can say is that, from experience, a DW-6600, 200m non-ISO rated put up with many years of poking around the Solent, fighting giant octopi, chasing mermaids and raising sunken treasure with no ill effects apart from resin rot.


Nice post macspite. Have read elsewhere the watch companies are a little sharp when describing water resitance given the quoted values refer to static pressure.


I've read that post, very good and well worth a read.

As for the commercial site link, well, I'll let it slide ;)


New Member
@ Macspite - very interesting reading! As a kid I could never understand why a watch that was supposedly rated at 50m couldn't even be worn in the bath - and now I know!


Active Member
@Olivier - Je vous remercie, monsieur.

Are you sitting comfortably? Then take a deep breath (as if you were preparing to dive to 200metres without scuba gear) and take a look at this:


Apologies to the moderator if linking to commercial sites is against forum rules :D

Well... How to say....? Thank you very much Mr Macspite for bringing me such a (very) long and detailed information.
As you can easily guess it, it took me around twenty minutes for reading the whole post because English isn't my primary language. (And I don't want to use any automatic translators, making me lazier and sometimes with other mistakes too).
Such information totally answer my main request (and even much more).

What an excellent idea when I decided to come on this forum :cool: Thank you again.