Why is 18m set for OW?

[boulderjohn]

This is just a guess, but from my experience in diving at those depths and working with students, if you had asked me to pick a round number for the suggested limit for new divers, I would have picked 60 feet. Why? Because 50 feet seems to be a little too shallow and 70 seems to be a little too deep.

 

[chrisch]

Ten fathoms....

 

[tbone1004]

like many things that are standard in SCUBA, I suspect it was a number picked by a bunch of old white dudes sitting around the PADI conference table and pulled it out of their a$$. Nothing wrong with it mind you, just that there are a lot of things that you may try to find a scientific backing for and there just isn't one.

Now, to be somewhat rational about it, IIRC they say that 60ft is the deepest effective depth for a CESA, so that's a convenient reason to choose 60ft over 50ft or 70ft, and I also believe there was a study done about nitrogen narcosis and that 3ata was a major tipping point in cognitive function and since we live in the imperial world, it was rounded to "Safety" which is 60ft not 70ft. Had that decision been made by an agency based on Europe or if the USA was on the metric system, then it would probably have been 20m and whatever archaic swine left on the imperial system would have had to deal with a non-round number

 

[Storker]

Had that decision been made by an agency based on Europe or if the USA was on the metric system, then it would probably have been 20m and whatever archaic swine left on the imperial system would have had to deal with a non-round number

As already mentioned, the BSAC equivalent limit is 20m. Same thing for CMAS. So you're more correct than it would seem at first glance.

 

[RainPilot]

RAID is 20m as well. 3 ATA

 

[Akimbo]

Now, to be somewhat rational about it, IIRC they say that 60ft is the deepest effective depth for a CESA

Nonsense, see: Max depth for CESA? and Max Depth for CESA.

Edit: Comment not directed at you personally.

 

[tbone1004]

Nonsense, see: Max depth for CESA? and Max Depth for CESA.

Edit: Comment not directed at you personally.

the "they" in that comment was PADI. If you can freedive to a depth, you can do a CESA from that depth. I know I can do an UW swim the length of a long course olympic pool *50m*, and I have freedove to 110ft before, so I know I can CESA from that depth, but I do believe that one of the agencies has said the effective depth of a CESA is 60ft and I thought it was PADI. Could be wrong. @boulderjohn any comment about PADI's current statements on that?

 

[boulderjohn]

the "they" in that comment was PADI. If you can freedive to a depth, you can do a CESA from that depth. I know I can do an UW swim the length of a long course olympic pool *50m*, and I have freedove to 110ft before, so I know I can CESA from that depth, but I do believe that one of the agencies has said the effective depth of a CESA is 60ft and I thought it was PADI. Could be wrong. @boulderjohn any comment about PADI's current statements on that?

Padi's rules about CESA have evolved over the years, and I do not believe that evolution has anything to do with the 60 foot standard for scuba, which existed long before current thinking. Those standards existed before the UHMA workshop that changed the CESA standards for just about all agencies a couple of decades ago. They are two different things, and I have never heard them connected.

I also do not believe there is any connection between the CESA and freediving depths. The ability to hold a single breath of breath of air for a period of time is quite different from the ability to exhale slowly to allow a rapidly expanding air volume to escape the lungs. I do not practice freediving, and have no notable skills in that area. I am quite confident that I could do a CESA from 100 feet or more; I cannot freedive to that depth. I am also quite confident that the average OW student cannot freedive to the depths from which they are required to do a CESA in OW training.

 

[boulderjohn]

Just to add to this from memory because I am in vacation in France and far from any of my materials....

PADI makes a distinction between the CESA and the buoyant emergency ascent, but their wording is interesting (although I can't quote it now). It essentially comes down to this: if you are confident you can make it on a CESA, that's what you should do. Not confident, drop your weights and continue doing the CESA. The only real difference between the two is whether or not you have your weights.

 

[Remy B.]

For your own safety, that is the main reason

A novice diver tent to have events of uncontrolled ascents, for what I remember reading an old article don't know if it was NOA or US Navy divers, the probability of a DS ( decompression sickness event ) is less likely at that depth exposure, "note" less likely, not that you wouldn't get DS. because it depends of each persons physiology, and if you are 30min at that depth you are more likely to have some sort of DS than if you are there for 5min in the event of an uncontrolled ascent.

And if you took diving because you like to see sea creatures, you don't need to go deeper than 7m, beyond that colors begin to fade away specially red.

Safe diving.