Bounce diving?
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When I was in HI, I was on the boat with some recreational divers who were bragging about having "bounced" to 200'. It's tempting, in that warm clear water, and can feel somewhat safe, looking back up from 100' and seeing snorkelers in the sun above you. The water is clear and bright, and 200' just doesn't seem that far away.... just go down and touch 200' and then come right back up...shood have plenty of gas for that even with these AL80's...
And there are people who have survived such a dive.
And there are people who have not. Even one young man in the PNW area who went with his instructor, but didn't come back. In fact, it was some months before they found his body.
Some good questions to ask yourself before attempting such a dive might be:
- What's my experience at planning and executing deep dives in the area of gas management?
- What is my min gas, or rock bottom turn pressure at 200'?
- What is my experience at task management while extremely narced at 200'?
- What will the PPO2 be at 200' on 21%, and what are the signs and symptoms of O2 toxicity, and am I trained to deal with them in the event of an emergency?
- In the event of a possible delay at depth, what are my back-up decompression plans? And does my team have enough gas to execute those plans, based on known team sac rates and emergency sac rates?
- Has my equipment been serviced and tested for 200'?
- And on and on...........
Personally, I've always hated the answer, "If you have to ask, then you shouldn't do it," but in the case of a 200' bounce dive on air, your question applies.
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Rick M, I'm not convinced that Bounce diving is any less safe than ordinary bottom times, given adequate, conservative deco schedules and adequate SI time. I recall a study a few years back describing the crushing pressure of bubbles, possibly included with the RGBM docs, or by Erik Baker. Crushing pressure was described as being very great. This is also the reasoning behind the USN treatment tables 5 and 6, and 6a. The depths are not incredibly deep, the aim is to open the O2 window rather than re-compressing (crushing) the bubbles. Maybe you can explain a bit further....
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Let's give the OP some good info:
Air diving is considered by some to be incredibly stupid, while to some diving trimix at depths shallower than ~60m is considered wasteful and over complicated. The concern is gas narcosis for some, but the real limiting factor needs to be the partial pressure of oxygen. Most agencies recommend bottom mix oxygen pressure to be below 1.4 bar, meaning breathing 1.4 times the amount of O2 one would breath at the surface if breathing 100% O2. This is a wise limit for one who does not have further studies into the processes behind CNS toxicity due to an oxygen "overdose".
OK, so we know that 1.4 is a "safe limit" for our oxygen exposure. Now, how do we know what the partial pressure of oxygen in our breathing gas is at a given depth? We have a nifty equation that we can use.
PP
-------
Fg x P
where PP = Partial pressure , Fg = Fraction of gas, and P = ambient pressure.
Knowing two of the variables will give us the third....
1.4
-------
.21 x P
In this example, we are using air (21/100, 21%, or .21 Oxygen... all are expressing the same value) and we have chosen 1.4 to be our "CNS Limit"
We find that our equation gives us something like 6.666666
6.6666 represents the maximum absolute pressure that our gas is safe to use at, given our chosen limit of 1.4
From this, we can find that a depth of 56 meters corresponds conservatively to our MOD, or Maximum Operating Depth.
So, we found that by choosing 1.4 as a CNS Limit for our breathing mix, we have a maximum depth of 56 meters.
I strongly recommend further training, but for a no decompression, or very short decompression dive, this information will suffice to stay safe. Keep in mind, there are time limits for Oxygen Exposure, but staying within NDL will ensure that you are well with conservative times. It's very normal for sport divers in many parts of the world to hit these depths without thinking twice on a single AL80, but I don't recommend following this advice blindly.
Air diving is considered by some to be incredibly stupid, while to some diving trimix at depths shallower than ~60m is considered wasteful and over complicated. The concern is gas narcosis for some, but the real limiting factor needs to be the partial pressure of oxygen. Most agencies recommend bottom mix oxygen pressure to be below 1.4 bar, meaning breathing 1.4 times the amount of O2 one would breath at the surface if breathing 100% O2. This is a wise limit for one who does not have further studies into the processes behind CNS toxicity due to an oxygen "overdose".
OK, so we know that 1.4 is a "safe limit" for our oxygen exposure. Now, how do we know what the partial pressure of oxygen in our breathing gas is at a given depth? We have a nifty equation that we can use.
PP
-------
Fg x P
where PP = Partial pressure , Fg = Fraction of gas, and P = ambient pressure.
Knowing two of the variables will give us the third....
1.4
-------
.21 x P
In this example, we are using air (21/100, 21%, or .21 Oxygen... all are expressing the same value) and we have chosen 1.4 to be our "CNS Limit"
We find that our equation gives us something like 6.666666
6.6666 represents the maximum absolute pressure that our gas is safe to use at, given our chosen limit of 1.4
From this, we can find that a depth of 56 meters corresponds conservatively to our MOD, or Maximum Operating Depth.
So, we found that by choosing 1.4 as a CNS Limit for our breathing mix, we have a maximum depth of 56 meters.
I strongly recommend further training, but for a no decompression, or very short decompression dive, this information will suffice to stay safe. Keep in mind, there are time limits for Oxygen Exposure, but staying within NDL will ensure that you are well with conservative times. It's very normal for sport divers in many parts of the world to hit these depths without thinking twice on a single AL80, but I don't recommend following this advice blindly.
Previous post explains how to determine a safe depth for a given breathing mix in relation to CNS Oxygen Toxicity, only one of the factors that must be weighed when making dives in the range that this is an issue.
Other factors to put into play include SAC and gas narcosis. If your buddy is relatively efficient with his air, breathing an RMV of 12-15 l/min, diving to 55 on an AL80 on a multilevel dive is feasible and safe. Do the math yourself before making these dives.
Narcosis, at these depth, does not impair most divers if conditions are good (no/light current, clear, warm waters). As conditions deteriorate, narcosis becomes stronger and (in some cases) more dangerous. Though if conditions are good, I don't know anyone (cept' a few americans on this board) who would dive trimix at these depths.
It takes time and experience (short term and long term) to get up to these depths (50 - 55m) on air. In my humble opinion, your friend won't be pushing any dangerous limits, only the limits imposed by an agency that is geared toward a larger diving public that really has no business diving here. With a bit of education, everyone can dive here, on air, with an acceptable level of risk (CMAS and BSAC divers, anyone?). It's wise to learn about narcosis in somewhat controlled conditions, with a diver who has experienced severe narcosis, using narcosis checks throughout the dive, and by remaining very close to the dive partner. I wouldn't want someone to be making their first dives sub-40 solo... that's just unwise.
I think we also need to clear something up... what do you mean by "bounce dives"? Many would assume that your friend will be hopping in the water, dropping to his chosen depth, and returning to the surface. I am reading it like a typical liveaboard diver might use "bounce"... in that your friend plans to drop to his chosen depth and gradually move shallower, only spending a very short time at maximum depth. Please give us some insight.
Other factors to put into play include SAC and gas narcosis. If your buddy is relatively efficient with his air, breathing an RMV of 12-15 l/min, diving to 55 on an AL80 on a multilevel dive is feasible and safe. Do the math yourself before making these dives.
Narcosis, at these depth, does not impair most divers if conditions are good (no/light current, clear, warm waters). As conditions deteriorate, narcosis becomes stronger and (in some cases) more dangerous. Though if conditions are good, I don't know anyone (cept' a few americans on this board) who would dive trimix at these depths.
It takes time and experience (short term and long term) to get up to these depths (50 - 55m) on air. In my humble opinion, your friend won't be pushing any dangerous limits, only the limits imposed by an agency that is geared toward a larger diving public that really has no business diving here. With a bit of education, everyone can dive here, on air, with an acceptable level of risk (CMAS and BSAC divers, anyone?). It's wise to learn about narcosis in somewhat controlled conditions, with a diver who has experienced severe narcosis, using narcosis checks throughout the dive, and by remaining very close to the dive partner. I wouldn't want someone to be making their first dives sub-40 solo... that's just unwise.
I think we also need to clear something up... what do you mean by "bounce dives"? Many would assume that your friend will be hopping in the water, dropping to his chosen depth, and returning to the surface. I am reading it like a typical liveaboard diver might use "bounce"... in that your friend plans to drop to his chosen depth and gradually move shallower, only spending a very short time at maximum depth. Please give us some insight.
Why not simply remember the definition of partial pressure, which for a given component gas in a given gas mixture at a given temperature (normal surface atmosphere conditions) is:-
[partial pressure] = [total (ambient) pressure] * [fraction of the component gas in the mixture]?
No need to remember any jargon formulae - everything follows from the definition.
As to the original question, it depends just what a bounce dive is. There is no fixed or clear definition, and Rick's attempt at giving one is as good as any. But what did the OP mean when he referred to a "bounce dive"? That is surely the key question.
I have known severe cases here of DCS following a "bounce dive" just after a normal dive. Rick's warning (DON'T DO IT) should be remembered. In fact, they're the only DCS cases I've ever come across.
Always remember that ANYTHING you've taken into the sea with you is expendable, by definition. I don't care if it's a $40,000 TV-quality video camera - LET IT GO. If you're really worried about taking expensive gear out with you, either don't or get good insurance.
[partial pressure] = [total (ambient) pressure] * [fraction of the component gas in the mixture]?
No need to remember any jargon formulae - everything follows from the definition.
As to the original question, it depends just what a bounce dive is. There is no fixed or clear definition, and Rick's attempt at giving one is as good as any. But what did the OP mean when he referred to a "bounce dive"? That is surely the key question.
I have known severe cases here of DCS following a "bounce dive" just after a normal dive. Rick's warning (DON'T DO IT) should be remembered. In fact, they're the only DCS cases I've ever come across.
Always remember that ANYTHING you've taken into the sea with you is expendable, by definition. I don't care if it's a $40,000 TV-quality video camera - LET IT GO. If you're really worried about taking expensive gear out with you, either don't or get good insurance.
I tend to agree - the problem comes with figuring out what's "adequate."
For example... the Navy tables allow an immediate bounce to 80' after a 40 minute dive to 60', so, by the tables, chasing after that light you dropped while climbing into the boat should be alright... but I don't think it is. I think that'd be dangerous, and I won't make such a bounce.
I would do it after two hours, however.
Would I do it after one hour? Probably, with a very conservative ascent profile... maybe even after 30 or 40 minutes, with a very conservative ascent profile.
My point is that the tables and dive computers aren't designed for bounces, and just simply do not adequately cover the hazards of bounces, so depending on them to define what's "adequate" or "conservative" w/r/t bounces is a dangerous practice.
Don't do it.
Rick
I will confess that I have done it from time to time, mainly when the boat is in say 50' of water and somebody boarding at the end of a dive drops something. I always stay in the water fully kitted until everyone else is out, so if something does go like this I stand a decent chance of catching it before it reaches the bottom. But I'm generally quite prepared to follow it to the bottom if necessary, but then the boat will have a long wait for me, as I'm going to take a minimum of 15 minutes coming back up again. If I've already used my SMB for the original ascent then I'll just have to make a free ascent and hope the boat sees me!
I'm sufficiently experienced to have an idea of what I'm doing. I don't recommend this behaviour to anyone else.
I'm sufficiently experienced to have an idea of what I'm doing. I don't recommend this behaviour to anyone else.
I didn't notice the units in your post but I hope you are not saying diving to 55 METERS on a a single AL 80 is a good idea?? Do you mean 55'? (180') If not I would like to see your math for the profile (time and depths) for a 55 m dive on a single AL 80 with the sac rate you are using.
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