Thoughts on Bounce Dives

[Akimbo]

... I don't obsess about names ... I'll judge someone's posts by the quality of the information they provide ... and by the attitude they present when they provide it...

I'm sure that is generally true but I (we?) do tend to make an effort to read posts by some members more than others. And then there are a few who consistently deride others or offer nothing worth reading and end up on my ignore list.

 

[Stoo]

In sum, don't bounce unless you go with an experienced diver who understands the physiology and has some emergency diving experience.

Quinn, by friend, this is a conversation to be had at the 'Nest or Squaller Holler!

I admit that I have not read every post in this thread, but my guess is that the situation you are asking about was widely condemned and/or mocked... As one of the old farts around here, let me toss in a couple of thoughts...

Back in the good old days, 200' dives on air were common place, even in chilly places like our home base of Tobermory. Sometimes these were done on single steel 72s. I am not aware of any serious accidents under this scenario. I'm not even aware of any "Come to Jesus" moments, although I'm sure it's possible and indeed likely that they occurred.

The dive you are "proposing" is what I would call an "if'n" dive. If'n nuthin goes wrong, you live. If'n sumthin goes wrong you die. There's not likely a whole lot of middle ground.

Also, narcosis affects people differently, and individuals are effected differently at different times. "Narcosis can be fun" some say, but as many will say that it's terrifying... the so called "Dark Narc". A certain Instructor that we both know and who may have just bought a dive shop readily admits that he is a mess at depth.

A single tank at that depth (200'ish) holds lots of gas from some people. For others, it's not enough to make it back to the surface. Depends on about a zillion factors.

As you probably know, there are lots of people routinely doing pretty deep dives (170' +) on air around Tobermory. I don't think you will find any doing those dives on single tanks and without redundant regs etc.

All in all, a 200' dive on air on a single tank is very doable. Or it might kill you. Either way, it's dumb.

I would also point out that the difference between a 200' dive and 300' dive is a lot more than 50% deeper... the risk is exponentially greater. On the right day, for the right reason, I would still pop down to 200' with doubles and all that stuff. I wouldn't give you the time of day to even "think" about a 300' dive on air.

And finally, I think geography plays a big roll... doing a 200' dive in a 3mm suit in Cayman is a lot different than doing it here, in a drysuit. The gas demands are greater due to the greater buoyancy change with depth, and the drag is greater, increasing gas use... and on and on...

 

[Kevrumbo]

(And resetting the thread back on topic). . .

It's generally accepted that at about 217 feet air becomes toxic. Go deeper on air and do so at your terminal peril. A fast descent to about 200 on air with controlled ascent using recreational limits is totally dooable on a normal size tank as a first dive (i.e. no residual Nitrogen). The risk in this dive is the narcosis that you WILL experience. Tunnel vision. Potentially magical thinking. Lack of focus. Now, on their own these things aren't that big of a deal and can be fun, but when you are dropping like a rock and supposed to be paying ABSOLUTE STRICK UNWAVERING attention to your depth gauge, narcosis can be deadly. In sum, don't bounce unless you go with an experienced diver who understands the physiology and has some emergency diving experience.

Without reference to personal beliefs vis a vis whether bounces are dumb... Thoughts or concerns of a technical nature would be great.

During a recent dive trip I had the opportunity to dive to a depth below the recreational limit. The person who took me has had experience diving to 225 ft and below on air. The dive itself was uneventful and nothing special except for the thrill of going deep. Narcosis was not that much more intense than at 135 ft. While I don't advocate diving to below 180 ft on air(I would not have done it without someone experienced) I am curious how many people push the limits of diving with air on a regular basis. I saw several people diving below the rec dive limit on this trip, something I have not seem much of in the past.

. . .I think everybody should be free to do the dives they want to do -- but I think the decisions to do them should be EDUCATED decisions, and I fear that, all too often, they really aren't.

I just wrote it off as the normal third Thurdsay of the month deep air bounce diving post.The OP may want to read some of those - especially the recent ones involving permanent paralysis and fatal injuries.In general diving with someone who has experience on deep air dives to 225' won't insulate you from being killed along with the "experienced" diver anymore than playing Russion Roulette with an "experienced" Russian Roulette player will reduce the odds of shooting yourself in the head.The general consensus of properly trained technical divers who make meaningful dives to those depths after proper training and using appropriate gasses and equipment configurations is that deep air bounce diving is pretty farking stupid.

This seems to be a pretty hot topic on ScubaBoard these days. I am sure you must know that a dive shop owner in Cozumel died recently after just such a dive. The plan was to go to 300 feet on air and then surface. I am sure that if she were alive she would tell you that the narcosis at 300 feet felt pretty much the same to her as it did at 135 feet. That's one of the problems with narcosis--you usually don't feel it enough to realize what it is doing to you. In her case, she apparently did not realize that she had reached the 300 foot depth and kept going. A buddy caught up with her at 400 feet and turned her around. That extra descent used up their air supply, so they ran out of air at about 200 feet. Fortunately, they had another diver with them who had not gone past 300 feet and who still had enough air to get them to the surface (no stops) with three person buddy breathing. She died. The buddy who caught her at 400 feet is still paralyzed. The other diver will probably recover, although we are not getting reports in him so I can't be sure.

Let's start with narcosis. It affects different people differently, and it affects the same person differently on different dives. Frequently it is not a problem on dives like that, especially if nothing out of the ordinary happens. If something out of the ordinary happens, you may not have the mental agility to deal with it effectively. You may also be numb to circumstances around you, missing vital clues, like the reading of your depth gauge. So it's a roll of the dice. Usually it won't be a problem. Sometimes it will.

You don't mention how much air you brought with you. You evidently had enough to reach your depth and reach the surface. Great! Did you have enough to deal with an unforeseen problem, like a free flow, a blown o-ring, or a buddy who unexpectedly descends an extra 100 feet and needs to be caught and returned to the surface? So taking just enough air to get you down and back is a roll of the dice. Usually it won't be a problem. Sometimes it will.

You did not mention any decompression planning. Did you use any established tables or computer algorithms, or did you take a good guess? Decompression sickness is hard to predict. Diving within recreational limits is extremely safe, and the incidence of DCS on such dives in extremely low. As you go deeper, the need to follow decompression schedules becomes more and more important, and the odds of having a DCS incident such as the one I described above increase. If you are following a computer, do you have a backup in case it fails? If you are following a plan or a computer and and are able to do the stops because nothing unforeseen happened and you have enough air, you are likely to be fine. So it's a roll of the dice. Usually it won't be a problem. Sometimes it will.

I am not sure how much experience you have dealing with equipment issues at depth. If nothing goes wrong, you will be OK. If something does go wrong with the equipment, do you have the ability to deal with it at depth? If not, it's a roll of the dice. Usually it won't be a problem. Sometimes it will.

Technical dive training results in divers using gas mixtures that minimize (but don't eliminate) the effects of narcosis. These divers calculate their gas needs carefully and carry enough in case of emergencies. They train for equipment emergencies. They have carefully planned decompression profiles and backup plans in case of emergency. They have well established protocols for dealing with every possible situation, and they know how to work effectively in buddy teams in case of emergency. In short, they have thought of everything that can go wrong and made plans to deal with those rare but anticipated problems.

So, yes, there are people who do this kind of diving regularly. They seem to enjoy that roll of the dice. Usually they don't have a problem. Sometimes they do.

 

[Nuribromanski]

IF he was to share some George table stuff...it could NEVER be shared on Scubaboard, but would have to be only to a couple of individuals like yourself, with actual "pedigree"

lol let dinosaurs' knowledge die off with them, no loss 4 ne1

 

[Akimbo]

lol let dinosaurs' knowledge die off with them, no loss 4 ne1

Ouch, that really hurts. That makes me primordial soup if these [-]newbies[/-] pioneers from the late 1980s are dinosaurs -- and I know a lot of divers that have been around a lot longer than me. Loosing hard-won knowledge is never a good thing.

 

[Germie]

I teach adv. nitrox, up to 42m on air as backgas. I am allowed to teach the technical diver course, that is 51m depth on air. But I don't teach that, I teach then normoxic trimix, to 60m/200 ft depth. As I am a human too, I will get narced too on such depths on air. And I am responsable for my students and myself. So that is why I don't teach deeper air than the 42m in the adv. nitrox course.
The psai level 6 course was offered to me, but I refused. My instructor has to dive trimix then, to conduct the dive safe. That is written in standards he said. I don't like to get narced on such depths where I cannot help myself anymore and an instructor needs trimix in case of emergency to help me.

When I was a sportsdiver, I have done bounces to 55m on single tank too. The problem was that I never mentioned narcoses, it did not happen to me I thought. But since I dive trimix I know I was narced too. And that is the danger.
The PO2 on a bounce to 1.6 is not where I am afraid for, it is a bounce, so just a short period on higher PO2. But being narced, that can be a problem. And most sportsdivers never feeled being narced because they don't know what to feel.

As CO2 is a contributing factor for getting narced. The breathing resistance of a regulator on air to 200 ft depth is really higher than diving on a 20/30 trimix. Calculate the eadd (equivalent air density depth), that is 42m on a 20/30 trimix on 200ft depth. That is a big difference from an eadd of 60m when diving air. When using more helium, the eadd will lower more (and the end of course). So trimix has not only the advantage of lowering the end, but the breathing resistance of your regulator too. And when diving trimix in a twinset, and take 1 or 2 decogases, you have time to stay on 200ft depth to see something

Then another problem of getting narced: the faster you go down, the more sensitive most people are for getting narced. This has to do with the partial pressure difference of the nitrogen you are breathing and the nitrogen in your body.

So yes, I dive to 200ft regularly, to 300-400+ ft sometimes (I don't have places to reach that depth in my backyard), but I do that on a twinset with decogases or I take a rebreather. 200ft is a depth where you need extra training for, on a single tank you never have enough gasreserves in a worst case scenario. Doing decodives is not directly dangerous, but you have to know what you are doing.

 

[Diver0001]

Without reference to personal beliefs vis a vis whether bounces are dumb... Thoughts or concerns of a technical nature would be great.

Seems to me that O2 toxicity is a function of more than just depth. The 1.4 or 1.6 line is kind of a black line through a grey area. Exposure times, personal physiology, breathing and water temperature are among the things I've heard can have an impact.

Seems to me that at 60m narcosis is the bigger issue. I still do quite a few dives to about 50m on air but 60m starts to become a whole other ball game in terms of narcosis.

R..

 

[Rhone Man]

Back in the good old days, 200' dives on air were common place, even in chilly places like our home base of Tobermory. Sometimes these were done on single steel 72s. I am not aware of any serious accidents under this scenario. I'm not even aware of any "Come to Jesus" moments, although I'm sure it's possible and indeed likely that they occurred.

etc., etc.

Brilliant post; 100% in line with my thinking (and experiences).

---------- Post added November 6th, 2015 at 08:34 AM ----------

Seems to me that O2 toxicity is a function of more than just depth. The 1.4 or 1.6 line is kind of a black line through a grey area. Exposure times, personal physiology, breathing and water temperature are among the things I've heard can have an impact.

It is also highly random. Two quick tales: when Bret Gilliam set the deep air record, he went past 500 feet. For the duration of his bottom time (which wasn't much) he had a ppO2 of around 3.6. That is so far past 1.4 or 1.6 you need a new zip code. And when you include ascent and descent time, he spent an awful lot of that time breathing a mix northwards of 2.0 ppO2.

Separately, I remember reading in one of my books about North Eastern wreck divers about one particular diver (I forget his name) who always set his bottom mix for a ppO2 of 1.8 (open circuit). For years he was fine, and thought he was smarter than everyone else. Until one day - for no reason that anyone could distinguish - he toxed and drowned. Why then and not previously? Who knows.

But broadly agree, for the brief exposures you would have on a "bounce" dive, your problem is unlikely to be oxygen toxicity. Either narcosis or CO2 retention is what is much more likely to kill you.

 

[LiteHedded]

Brilliant post; 100% in line with my thinking (and experiences).

---------- Post added November 6th, 2015 at 08:34 AM ----------



It is also highly random. Two quick tales: when Bret Gilliam set the deep air record, he went past 500 feet. For the duration of his bottom time (which wasn't much) he had a ppO2 of around 3.6. That is so far past 1.4 or 1.6 you need a new zip code. And when you include ascent and descent time, he spent an awful lot of that time breathing a mix northwards of 2.0 ppO2.

Separately, I remember reading in one of my books about North Eastern wreck divers about one particular diver (I forget his name) who always set his bottom mix for a ppO2 of 1.8 (open circuit). For years he was fine, and thought he was smarter than everyone else. Until one day - for no reason that anyone could distinguish - he toxed and drowned. Why then and not previously? Who knows.

But broadly agree, for the brief exposures you would have on a "bounce" dive, your problem is unlikely to be oxygen toxicity. Either narcosis or CO2 retention is what is much more likely to kill you.

I think i know the reason

 

[Akimbo]

... Two quick tales: when Bret Gilliam set the deep air record, he went past 500 feet. For the duration of his bottom time (which wasn't much) he had a ppO2 of around 3.6. That is so far past 1.4 or 1.6 you need a new zip code...

The same kind of anomalies exists with Nitrogen Narcosis. Here is another set of data points. Myself and two other divers on a project have repeatedly switched between air and HeO2 at 50M/165' over days and could not tell the difference. However, this was after a considerable number of close-succession air jumps.

This was 1973 and we doing a salvage project on a wreck that required a lot of prep work to lower a habitat to the port side. A Hyperbaric Doc on leave from the Navy proposed that we could switch between air and HeO2 on Scuba and double the number of dives we could make in a day to four -- plus 100% O2 in-water stops and Sur-D-O2 chamber stops on O2 at 60'. None of us were hit after weeks of this.

We had already made around 15 jumps on air at this point. I remember being on the bottom thinking "the gas king screwed up my tanks" because this was supposed to be an air jump and I couldn't sense any narcosis. We were pretty cold on every dive so it was hard to tell if the gas was conducting heat faster or not. Sure enough, the squeaky-voice check confirmed it was air after hitting the surface.

Later we were in sat inside the habitat. The atmosphere was HeO2, but we had a problem with our diver gas recirculation system so we switched to open circuit air for lockouts while burning an access hole at +/-165' (accounting for tidal changes). Same thing. We couldn't sense any Narcosis even though we were doing some pretty complex analysis. We would literally switch between air and HeO2 instantly by putting the mask on. Even the diving super manning the comms was surprised how clear-headed we were.

That is not to say that we had zero impairment, but it wasn't perceptable by the divers, the topside supervisor monitoring us over hardwire comms, or compromised performance during some pretty complex work. This was way before most sat divers wore TV cameras and lights on their hats, which might have detected some reduced efficiency or coordination.

There are several points to make here. One is that most divers who do a lot of successive deep air jumps notice an improvement in their Narcosis threshold. Bret Gilliam wrote about it and most commercial divers have experienced and witnessed it.

There was a small study at EDU (US Navy Experimental Diving Unit) that could not confirm a tolerance improvement but IMHO the study was badly flawed because the "buildup" dives on air were far too few and shallow.

This is the relevant part to Scuba divers: Very few recreational or even technical divers get the opportunity to perform enough deep air dives in close succession to develop the increased tolerance described. However, it is entirely possible that this apparent tolerance increase is a mental adaptation with no bio-chemical component at all. That mental adaptation IS narcosis management, whether it is done consciously or not.