AquaTec,
I don't intend to criticize anyone. The only people I have problems with are those who try to convince other people to do the same dumb things they do.
So let's get back on topic, and hope that Ed can stay away long enough to allow a constructive conversation to happen.
Here's my take:
To me, "deep" means beyond 100 fsw. However, I prefer to use the umbrella term "technical," since even "shallow" dives can require decompression and advanced training and equipment. "Technical," IMHO, refers to any dive outside the accepted recreational limitations, i.e. max 130 fsw, no overhead, no deco. There is a gray area between 100 fsw and 130 fsw, because a 130 fsw dive could be classified as either technical (based on its depth), or recreational (based on its runtime and lack of task loading). It's generally ok to breathe air at 100-130 on a recreational dive -- but if you're doing deco, air is usually a bad idea.
I choose my definition of "technical" because it is simple. I choose my definition of "deep" because that is roughly where I believe the symptoms of nitrogen narcosis become significant (at least on SOME dives for SOME divers). I believe that air is basically contraindicted in technical dives, because there are better choices.
I also chose my definitions because they agree with the majority of experienced technical divers. It only makes sense to agree to some commonality to aid in discussions.
Physiologically, nitrogen is dangerous for a couple of reasons. First, as we all know, it causes narcosis. The severity, onset, etc. are highly variable, so one would do well to limit nitrogen exposure as much as possible. Second, nitrogen is dense, which translates to increased breathing effort at depth. This has been verified in laboratory conditions to predispose a diver to a positive-feedback carbon-dioxide cycle which is probably at least related to deep-water blackout. Third, nitrogen's slow diffusion results in lengthy decompressions. Fourth, there is conjecture that nitrogen stiffens the cell walls of red blood cells, which makes circulation less efficient, as red cells must deform to squeeze through capillaries. Air has a lot of nitrogen.
Air also has a lot of oxygen, which comes bundled with its own set of physiological hazards. Countless studies have indicated that exposure to high partial pressures of oxygen causes seizures and unconsciousness. This is called central nervous system (CNS) oxygen toxicity. It is theorized that the excess dissolved oxygen promotes exessive oxygen radical production. The enzymes (superoxide dismutase, catalase, and others) that specifically bind free radicals can become overwhelmed. When this happens, the free radicals begin to interfere with nervous conduction. Like nitrogen narcosis, the severity of toxicity symptoms and their onset is highly variable. A huge collection of laboratory tests and real-life Navy tests indicate that a PPO2 of 1.6 is tolerated in "most" subjects without harm. Most experienced divers choose to limit their exposure to 1.2 ATA or below to make CNS toxicity unlikely, and to ward off the lung damage caused by high PPO2s. This lung damage is thought to decrease respiratory efficiency, which translates to decreased decompression efficiency.
There is no PPN2 at which anything sudden occurs, which is why you should strive to minimize your nitrogen exposure. There are only rules of thumb. Oxygen is somewhat more straightforward, as a PPO2 of 3 ATA will cause most subjects to convulse in seconds. However, at more sane levels like 1.6 ATA, the conditions of toxicity are highly variable.
The Short List of Why Deep Air is Bad:
1. It has excessive nitrogen, leading to unnecessary narcosis.
2. It has excessive oxygen, leading to unnecessary toxicity exposure.
3. It is too dense, leading to increased breathing effort and possibly contributing to hypercapnia and deep-water blackout.
- Warren
I don't intend to criticize anyone. The only people I have problems with are those who try to convince other people to do the same dumb things they do.
So let's get back on topic, and hope that Ed can stay away long enough to allow a constructive conversation to happen.
Here's my take:
To me, "deep" means beyond 100 fsw. However, I prefer to use the umbrella term "technical," since even "shallow" dives can require decompression and advanced training and equipment. "Technical," IMHO, refers to any dive outside the accepted recreational limitations, i.e. max 130 fsw, no overhead, no deco. There is a gray area between 100 fsw and 130 fsw, because a 130 fsw dive could be classified as either technical (based on its depth), or recreational (based on its runtime and lack of task loading). It's generally ok to breathe air at 100-130 on a recreational dive -- but if you're doing deco, air is usually a bad idea.
I choose my definition of "technical" because it is simple. I choose my definition of "deep" because that is roughly where I believe the symptoms of nitrogen narcosis become significant (at least on SOME dives for SOME divers). I believe that air is basically contraindicted in technical dives, because there are better choices.
I also chose my definitions because they agree with the majority of experienced technical divers. It only makes sense to agree to some commonality to aid in discussions.
Physiologically, nitrogen is dangerous for a couple of reasons. First, as we all know, it causes narcosis. The severity, onset, etc. are highly variable, so one would do well to limit nitrogen exposure as much as possible. Second, nitrogen is dense, which translates to increased breathing effort at depth. This has been verified in laboratory conditions to predispose a diver to a positive-feedback carbon-dioxide cycle which is probably at least related to deep-water blackout. Third, nitrogen's slow diffusion results in lengthy decompressions. Fourth, there is conjecture that nitrogen stiffens the cell walls of red blood cells, which makes circulation less efficient, as red cells must deform to squeeze through capillaries. Air has a lot of nitrogen.
Air also has a lot of oxygen, which comes bundled with its own set of physiological hazards. Countless studies have indicated that exposure to high partial pressures of oxygen causes seizures and unconsciousness. This is called central nervous system (CNS) oxygen toxicity. It is theorized that the excess dissolved oxygen promotes exessive oxygen radical production. The enzymes (superoxide dismutase, catalase, and others) that specifically bind free radicals can become overwhelmed. When this happens, the free radicals begin to interfere with nervous conduction. Like nitrogen narcosis, the severity of toxicity symptoms and their onset is highly variable. A huge collection of laboratory tests and real-life Navy tests indicate that a PPO2 of 1.6 is tolerated in "most" subjects without harm. Most experienced divers choose to limit their exposure to 1.2 ATA or below to make CNS toxicity unlikely, and to ward off the lung damage caused by high PPO2s. This lung damage is thought to decrease respiratory efficiency, which translates to decreased decompression efficiency.
There is no PPN2 at which anything sudden occurs, which is why you should strive to minimize your nitrogen exposure. There are only rules of thumb. Oxygen is somewhat more straightforward, as a PPO2 of 3 ATA will cause most subjects to convulse in seconds. However, at more sane levels like 1.6 ATA, the conditions of toxicity are highly variable.
The Short List of Why Deep Air is Bad:
1. It has excessive nitrogen, leading to unnecessary narcosis.
2. It has excessive oxygen, leading to unnecessary toxicity exposure.
3. It is too dense, leading to increased breathing effort and possibly contributing to hypercapnia and deep-water blackout.
- Warren
