From SSI
My question : Why the higher EAN for shallower dives vs low EAN for deeper dives?
Seeking to understand
View attachment 471762
Thank you
Im going to give you an answer that is not number sup-ported, but is a process explanation. O2 is a poison. O2% times depth == poison affect on diver. Much like drinking affects your driving. you can drink many low concentration drinks and have the same effect as fewer higher concentration drinks. So equate depth to number of drinks and alcohol content as O2 content. So to maintain a functional alcohol level in your body the more drinks you plan to have the lower the alcohol content in them has to be,,,,,,, to still be able to walk. You can get just as plowed on 3% beer as you can on 151 rum. beer just takes more of it. ............ Although O2 like N2 is toxic on the surface the body can deal with it,,,,, but take that O2 concentration down to deep water where the depth multiplies the affect of the O2 and you will have problems. The lower the O2% the deeper you can go. It is a trade off.
After a prescribed safety margin it is accepted to use an O2 level of 1.6 or less as a safe level for the body to work with. Rec divers have added a bit more safety margin and use 1.4 as the continuous limit. The body reactions are pretty linear up to about 1.65 where the effects become more exponential rather than linear. General thoughts is that at a level of 2 in most divers will start to feel the effects of the O2 poison. by a level of 3 things are not good at all loss of body conro and by 4 you are dead. Not precise numbers but good enough for explaination purposes. So you take the O2 content of the gas you are breathing (air is 20%) times the depth in Atmospheres. 100 ft is 4 ATM so the level our body is at is 4 times .2 or .8 VERY MUCH WITH IN THE MAX LEVEL OF 1.4. now go to 32% and at one hundred feet you now have a little less than 1.3 .32 x 4 ATM or 1.28. If you were breathing 40% at 100 ft you would be at 1.6. Still technically safe and below the exponential increase amount of about 1.65 for the NORMAL person but it is accepted practice that you do not tempt fate by doing so. Think of the avg person as the football jock warm water on no meds and no cold at the time , not stressed etc.
Your class will or use to teach you to calculate MOD (max operating depth, for a given O2 content) at depth in ATM divided into your limit of 1.4 So that if you are going to 132 feet that is 5 ATM divided into 1.4 and that gives you .28 which means if you go to 132' to do your dive you should have no more than 28% O2 to not exceed the 1.4 limit. That percentage allows you to drop a tool to say 140 ft allow you to go after it and not exceed the 1.6 limit. These limits are not a monster that will kill you at the moment you hit the level. There is a time factor involved, so hypothetically you can go to say 2.0 for a couple of minutes because it takes time for the body to catch up to the new level.
I one time on the Oriskani found my self at 1.85 with 32% because iw as not watching my depth while looking for a land mark on the ship. When i figured out that i was seeing the deck at 150+ feet or so i beat feet to about 70 ft to not just stop the increasing level but to reduce the level on my body to a depth of 70-100 ft quickly. That incident was not pushing my luck it was a result of not paying attention and implementing an immediate response to the problem. since I could not change gas to control the level i had to change the depth. The rest of my dive remained well less than 100 ft.
