Is conventional wisdom just conventional?

[knotical]

Conventional wisdom has said:
- bounce dives,
- saw-tooth dives,
- "backward profile" dives, and
- reverse profile dives
are all bad.

But are they really bad? How much science is behind those assertions?

I suggest that as long as we stay within the limits of whatever model we are using, these practices are no more dangerous than the degree to which they put us near those limits.

Bounce dives are just dives with rapid descents, short bottom times, and short ascents. As long as the ascent is sensible, plug them into your model and carry on.

Saw-tooth dives: Is deep-shallow-deep really bad? As long as we follow our model's ascent rules, it should be OK shouldn't it?
And what defines a saw-tooth? How many feet of re-descending does it take to qualify as a saw-tooth?

"Backward Profile" (dive that starts shallow, but gets deep toward the end of the dive): Shouldn't a proper ascent be sufficient to make this safe?

Reverse Profile (deeper dive after shallower dive): This one has some studies that say the earlier prohibition was unfounded. See:
Divers Alert Network : Alert Diver Articles

A remaining issue with reverse profiles is the reduction of bottom time.
To see this, consider (for example) the following dives on PADI tables:
Dive A: 90 feet for 21 minutes
Dive B: 60 feet for 37 minutes
Surface interval: 60 minutes
If you do Dive A before Dive B, no problem.
But if you try to do Dive B before Dive A, the model says you shouldn't do the second dive.
The model would let you do the deeper dive after the shallower dive only if you reduce your bottom time for Dive A to 14 minutes, or extend your surface interval to 144 minutes.

I welcome the usual opinions and anecdotes, but it would be nice to see some citations of authoritative sources, too.

thanks,

k

 

[captain]

Pretty much have done all of them at one time or the other when spearfishing at oil rigs without problems.

 

[JeffG]

Bounce dives are just dives with rapid descents, short bottom times, and short ascents. As long as the ascent is sensible, plug them into your model and carry on.

"ascent is sensible" - Define
"short bottom times" - Define

If you bounce dive after diving, you could recompress a bubble in the blood allowing it to cross to the arterial side.

If you ascend before that bubble can be adsorbed back into you tissues, it (the bubble) can cause issues on ascent.

So I do not happen to agree with your assertions on bounce diving (in regards to bounce diving after diving)

If the bounce dive occurs first. (like setting an anchor) It shouldn't be an issue.

 

[MXGratefulDiver]

A well-known line from a Dirty Harry movie comes to mind here ...

The question isn't whether it's safe or unsafe ... all dives carry a certain amount of risk. The question is how risky is "risky enough" ... and how lucky do you feel today?

... Bob (Grateful Diver)

 

[ligersandtions]

I think I can answer some of these questions....I've been reading the book "Deco for Divers" and it is really fantastic. If you're interested at all, I'd highly recommend it.

Regarding sawtooth dives (and presumably a bounce dive followed by either a dive that's less "bouncy" or another bounch dive)....when you descend, you start to on-gas. When you ascend, your tissues may go from under-saturated to supersaturated....this may cause the formation of "silent bubbles". Silent bubbles generally do not cause any symptoms of DCS and are normally filtered out and dispersed by the lungs (i.e. they cause no problems when you're on the surface after a dive). The problem with silent bubbles comes if you decide to make another dive, or continue your sawtooth profile....the silent bubbles remain in your body and can act as a catalyst for further bubble formation, and also slow down the efficiency of off gassing during your ascent. This can result in a formation of more bubbles than you would have doing a square profile, or at least a profile that doesn't include a number of ascents and descents.

As for reverse profile dives, some analysis has been done and it shows that doing these types of dives is not an issue - the restriction on reverse profile diving has no basis. In fact, no one seems to know why this guideline is in place or where it came from. So, given that you do a proper ascent and stops, a reverse profile should not be an issue. I would imagine the same would go for a backward profile....as long as you do a proper ascent and set of stops (whatever that may be for your particular profile), you should not have an issue.

 

[Anti-Hero]

"Saw tooth profile" is a pretty ambiguous term. Most profiles have a degree of sawtoothedness (new word score!) it's the amplitude between the peaks and valleys: Say the difference of going from 60fsw, to 40fsw, to 65fsw - versus going from 125 fsw, to 40 fsw, to 90fsw is where one *may* run into problems.

There are simply too many variables to say yes it's dangerous, or no you'll be fine. Factors like bottom time, repetitive dives, N2 load, etc. all play a roll.

 

[rstofer]

All of the guidelines have been around for a very long time and many millions of dives have been made using them. As soon as these new 'models' have an equivalent level of testing, perhaps they will become mainstream But a thousand, or even a hundred thousand, dives isn't anywhere near equivalent..

In the meantime, there is not one good reason to intentionally violate these well tested guidelines. I must say, I have never heard of the 'sawtooth' admonition. Most dives do have some aspect of a sawtooth and we don't have divers getting bent doing them. But, as Anti-Hero said, if the amplitude is high enough there may be an increased probability of DCS. I suppose the degenerate case of a sawtooth dive is a repetitive bounce dive.

This whole DCS thing is just voodoo. At best it is a guess with a bit of data; it certainly isn't science.

A long time ago, very bright people thought that the Sun revolved around the Earth. I think some of them have been reincarnated and are working on deco theory. They're very bright but may ultimately be proven wrong.

Richard

 

[Rhone Man]

Certainly in relation to reverse profiling - the algorithms on which tables and computers are based are predicated upon there being an element of off-gassing when the diver ascends to increasingly shallower depths. If you turn that on its head, then you completely undermine the premise upon which the algorithm is intended to work (whether that is simple Haldanean tissue analysis or some complicated computer model).

If you undermine the basis on which something is supposed to work, there is obviously a greater chance that it won't work.

The whole science of decompression contains too many unknowns to definitively state an answer, but if you don't play by the rules by which the procedures were drawn up, the scope for something going wrong has got to get larger.

 

[Rhone Man]

Additional: I remember reading an article (might have been the DAN website, can't recall for sure) about a test that the Royal Navy conducted. It involved divers going down to 180 feet for 5 minutes, then a 1 hour surface interval; going back down to 180 feet for 5 minutes, then a 1 hour surface interval; then going back down (3rd time) to 180 feet for 5 minutes. And they all got bent. But under all of the various algorithms that they were working off they shouldn't have. The algorithms are based on the idea that people will dive a certain way. Not do stupid things like a series of "bounce" dives.

I'll see if I can find the article again and post a link.

 

[knotical]

"ascent is sensible" - Define

Following the rules of whatever algorithm is in use.

"short bottom times" - Define

I don't have a good definition. Which means I don't have a good definition of a bounce dive. But we are admonished to not do them, whatever they are.

If you bounce dive after diving, you could recompress a bubble in the blood allowing it to cross to the arterial side.

I've never understood this. If a small bubble can cross to the arterial side, why didn't it cross when it was small the first time? Do you have a reference that discusses this?

So I do not happen to agree with your assertions on bounce diving (in regards to bounce diving after diving)

Sorry, although some sentences are worded as assertions, I'd intended the entire post as suggestions/questions/fodder for discussion.

I've been reading the book "Deco for Divers" and it is really fantastic. If you're interested at all, I'd highly recommend it.

I have it on order, but in the meantime, I thought I'd raise these questions here.

....the silent bubbles remain in your body and can act as a catalyst for further bubble formation, and also slow down the efficiency of off gassing during your ascent. This can result in a formation of more bubbles than you would have doing a square profile, or at least a profile that doesn't include a number of ascents and descents.

I hadn't heard this before. Does Mark Powell propose it as a theory, or does he have evidence? (either at the bubble level, or from studies of saw-tooth or bounce dives?)



Thanks to all for your responses. I'm sure we haven't exhausted these topics.

k