View Single Post
Old 03-28-2009, 01:08 AM   #33
Mullins
Registered User
 
Join Date: Jan 2005
Location: Wellington
Posts: 224
Re: Three more blackouts!

Quote:
Originally Posted by Ray Odor View Post
DO NOT: Think this has to be a deep water dive for it to happen.
DO NOT: Take too many deep breaths. One atmosphere, (14 ½ ft) of water, can exert enough pressure to cause black out.
Test on dry land: Take ten deep breaths, then, have someone standing behind you, put their arms around you and squeeze your chest. Most people will black out. Now test 4 or 5 deep breaths, while exhaling as much CO2 as possible from each breath. This should be plenty for your deep dive.
Ray, breathing deeply to remove CO2 is exactly what causes divers to black out. Unfortunately, there is no way of 'safely' removing CO2. The more you remove, the more comfortable you will be and the earlier you will black out, which is a bad combination. The 4-5 breaths you're advocating WILL in fact reduce time to blackout. Many people accept this tradeoff because of the increased comfort, which is fine, but I think it's quite important not to pretend that there is a line between 'safe' and 'dangerous'. All forced breathing reduces time to BO by some degree.

Also, to clarify something: a shallow water blackout is exactly the same as a normal surface blackout (e.g. in the pool), which is caused by cerebral hypoxia. The only difference is that the high partial pressure on the bottom temporarily protects the diver from losing consciousness. Then when they return to 1atm at the surface, they have to instantly pay back the extra time they got on the bottom. It is not that the "water exerts enough pressure to cause black out." Instead it is the end-of-dive reduction in water pressure that makes you pay back your effective oxygen debt. Your main point is right though: divers can BO at any depth, and if you push hard enough you can BO even before you ascend. The raised ppO2 will only delay BO for so long.

Last edited by Mullins; 03-28-2009 at 01:26 AM.
Mullins is offline   Reply With Quote