Nitrogen absorption and reverse profiles

[iainwilliams]

Hello

I would like someone with more knowledge on this topic subject to make a comment.

I have been reading with interest a number of articles regarding reverse profiles and profiles that do not correspond to diving the deepest depth first, followed by a progressively shallower depth.

SCENARIO: If we are diving below 10 m (less than 10 m is where the greatest pressure change occurs) and alter our depths during the dive saw tailing to a shallower then deeper depth, but not going outside a 10-15 m depth envelope. Then surely a computer (ie UWATEC) should be able to accurately track the N2 within your tissue compartments at the various depths (I know this what a computer does).

The different depths cause N2 to diffuse into different theoretical compartments. The shallow portion of the dive will cause N2 to accumulate in slow to medium tissues whilst the deeper dive portion will cause N2 to diffuse into faster tissues. As the depth is below 10 m and outside the greatest pressure change, and provided ascent rates and bottom times were kept in order, micro bubbling would not occur.

Therefore, altering your depth (deep, shallow then deep again) should be OK. Likewise, if we did a shallow dive in the morning (ie 15 M) and a deeper dive in the afternoon (ie 20m), then this should also be OK – provided we used a computer to track N2.

Does anyone have any thoughts on this controversial issue which is the oppsoit to what all divers have been taught? Thanks………Iain

 

[Saturation]

iainwilliams once bubbled...

SCENARIO: If we are diving below 10 m (less than 10 m is where the greatest pressure change occurs) and alter our depths during the dive saw tailing to a shallower then deeper depth, but not going outside a 10-15 m depth envelope. Then surely a computer (ie UWATEC) should be able to accurately track the N2 within your tissue compartments at the various depths (I know this what a computer does).

The different depths cause N2 to diffuse into different theoretical compartments. The shallow portion of the dive will cause N2 to accumulate in slow to medium tissues whilst the deeper dive portion will cause N2 to diffuse into faster tissues. As the depth is below 10 m and outside the greatest pressure change, and provided ascent rates and bottom times were kept in order, micro bubbling would not occur.

Therefore, altering your depth (deep, shallow then deep again) should be OK. Likewise, if we did a shallow dive in the morning (ie 15 M) and a deeper dive in the afternoon (ie 20m), then this should also be OK – provided we used a computer to track N2.

Does anyone have any thoughts on this controversial issue which is the oppsoit to what all divers have been taught? Thanks………Iain

Hi Iain:

The deeper dive practice suggests that after a SIT some bubbles remain, and that a subsequent deeper dive may shrink them enough to pass through the lungs.

However, its known that a SIT of 1 hour or more reduces bubbling substantially, if not eliminates them altogether, and this should be a practice exercised rather than diving with reduced bottom times to compensate for short SIT.

A 2000 Smithsonian panel concluded from accident data that reverse profiles do not have higher DCI incidences. [ see another post by Dr. Wienke expressing concern that it must be stressed, the differences between both dives be under 40'.]

A dive computer isn't material to what you propose, and the UWATEC microbubble algorithm is a different issue for discussion.

See-sawing, computer or not, should not be standard practice since dives inevitably generate non-critical bubbles that are trapped by the lungs and off gassed by 30 fpm ascents and safety stops, but on return to a deeper excursion, squeezes some bubbles enough to pass through the filtering capacity of the lungs. On return to shallow depth, these bubbles grow and you now have bubbles in the arterial side of your circulation.

If one is faced with a see-saw dive, a 5 min safety stop or formal decompression with 02 should desaturate one enough to compensate for the risky behavior on short dives, say < 1/2 of the maximum NSL time at depth.

In the example you make, tissue uptake does not depend on the depth, but on depth _and_ time.

See-sawing or yo-yo diving become increasingly a concern as you build up inert gases enough to stimulate bubbles into growth. Thus, it depends on how large the shallow-deep-shallow-deep see-saws you do.

Instructors are at risk for these types injuries because of the number of classes to teach, and the number of dives associated with shallow yo-yo dives. DAN data over years show recreational scuba instructors as a risk group for DCI or barotrauma. Th 2003 stats suggests this is because they dive so many times a year. However, anyone who has seen a OW class knows how much work an instructor must do to keep students, particularly bad ones, from injuring themselves by unfortunately intervening early in bad student diving practices, that put instructors at risk for injury.

 

[Charlie99]

iainwilliams once bubbled...
The different depths cause N2 to diffuse into different theoretical compartments....

Saturation answered the main question, but I just wanted to point out that the above statement is incorrect. The different theoretical compartments just have different time constants or rate that they fill. All compartments will take on N2 provided the inspired ppN2 is higher than the current N2 pressure in that compartment (the only exception is in serial type models, like DCIEM, where slower compartments are assumed to fill from faster compartments rather than directly from the gas being breathed).

If you were to say that the faster compartments are generally the ones that determine deep NDL's, while the slower compartments set the shallower NDL's, that would be correct.

Charlie

 

[iainwilliams]

If you were to say that the faster compartments are generally the ones that determine deep NDL's, while the slower compartments set the shallower NDL's, that would be correct.

Yes you are correct Charlie here - I probably did not explain myself correctly

 

[iainwilliams]

Thank you saturation. I would like to ask a further question if I may.

I agree with your overall comment re: sea sawing and that instructors are at greater risk to DCI due to multiple ascents to the surface.

You state: See-sawing or yo-yo diving become increasingly a concern as you build up inert gases enough to stimulate bubbles into growth. Thus, it depends on how large the shallow-deep-shallow-deep see-saws you do.

My scenario suggested alterating depths (sea sawing) below 10 m, and not sea sawing where you surface then decend again.

What depth contraints do you beleive are OK? I recon that a change in depth of say 5 -10 m as long as you are below 10 m (greatest pressure change is 10 m to the surface) is probably OK, provided your ascent rate is kept in check and you are not at the upper end of an NDL. IE: Max depth 30 m; ascend to 15m, then back down to 20 m. Also of importance is the number of times you do it.

Why do I ask this. I have seen many divers who dive to say 35 m then rise to 15 m, only to see a fish and decend again to 27 m before once again ascending to a shallower depth. They do not seem to get bent! I have always thought this as a unsafe diving practice, but after reading some reports I am wondering if it is not that bad after all.

Your opinion / concerns?

Thanks.............Iain

 

[BRW]

If you extrapolate the SI RP Wkshp recommendations
about surface RPs, then deltas of 30 fsw for yo-yos
and see sawing are certainly OK down to 130 fsw.
That is a lower limit point. And conservative, because
it's based on repets from surface ambient pressure of 1 atm.

With increasing pressure, of course, that delta becomes larger.
How much larger is the question. Bubble mechanics fits to
data, as encoded into RGBM meters, suggest that the permisible
deltas scale as the change in absolute pressure divided by
the change in excitation radii of bubbles for the yo-yo or see
saw. At the surface on air, delta is near 35 fsw, while at
100 fsw, delta is something like 70 fsw.

Some of this is covered in Technical Diving In Depth,
and published papers (Wienke, Comp. Biol Med 22,
389-406, 1992, Int J Biomed Comp 26, 237-256, 1991)
and also posts to the Net (RB List, Techdiver List, Deco
List). Also check out RGBMdiving.com.

 

[Saturation]

BRW once bubbled...
If you extrapolate the SI RP Wkshp recommendations
about surface RPs, then deltas of 30 fsw for yo-yos
and see sawing are certainly OK down to 130 fsw.
That is a lower limit point. And conservative, because
it's based on repets from surface ambient pressure of 1 atm.

With increasing pressure, of course, that delta becomes larger.
How much larger is the question. Bubble mechanics fits to
data, as encoded into RGBM meters, suggest that the permisible
deltas scale as the change in absolute pressure divided by
the change in excitation radii of bubbles for the yo-yo or see
saw. At the surface on air, delta is near 35 fsw, while at
100 fsw, delta is something like 70 fsw.

Some of this is covered in Technical Diving In Depth,
and published papers (Wienke, Comp. Biol Med 22,
389-406, 1992, Int J Biomed Comp 26, 237-256, 1991)
and also posts to the Net (RB List, Techdiver List, Deco
List). Also check out RGBMdiving.com.

Wow, very interesting BRW. I don't recall this distinctly in your book but assuming a ratio exists between ATA/excitation radii such that deltas of 70' are allowed, for clarity will add past 100' not before it, otherwise it suggests RP dives are possible.

I'll hit your book once again with this delta in mind.

This produces some interesting points if I have this right. A RP diver can dive anywhere in the 100-170' zone were delta is 70' versus staying within 100' where a delta of 30' is operant?

Otherwise, Iain the expert panel suggests problems with see-saw or yo-yo diving can be minimized if the difference in depth between two successive descents is kept about 30', 10m or 1 ATM. If Dr. Wienke's theory is right, its less a concern if you dive deeper!

See another thread where I comment on the Suunto Vyper:
http://www.scubaboard.com/showthread.php?threadid=27942&goto=newpost

For clarity, yo-yo diving assumes diver surfaces then submerges again, in see-sawing the diver is still fully submerged. What is critical in these action is the presumed change in volume of bubbles references to absolute pressure, rather than whether diver surfaces or not.

Violating yo-yo diving or RP rules is not the best practice and doing so increases a risk of getting bent, but it doesn't mean the risk is as a dive to 200' for 30min and surfacing without decompression stops. By comparison, this is a lower risk rule, like driving without a seat belt versus driving without breaks. Risk depends also on how fast you are going, or how deep and long your are diving.

 

[Dr Deco]

Dear Readers:

Reverse Profiles

I do not have any personally experimental experience with these dives. The way to maximize dives times is to perform the deeper dive first. If one simply wishes to move about a bit in the water column to look at fish, I do not see any particular problem with this.

A subsequent, full-blown dive to differences in depths of greater than 40 fsw from the previous dive as indicated by Dr Wienke should not be done if one wishes to minimize DCS.

Bubble Dynamics?

The claim is made that bubble dynamics will explain this type of diving. Without having seen the models and the data, I cannot say. It is possible, however, that the difference lies essentially in the type of dive. Maybe they are more vigorous on the bottom (for some reason) and the gas loads increase.

One should always recall that larger micronuclei appear to be created essentially from musculoskeletal forces. The nuclei formed are from “stress-assisted nucleation.” These processes are not incorporated into bubble models but are very important. Many researchers since the 1940s have recognized these processes.

The formation of nuclei can be avoided by avoiding heavy physical activity post dive. It is up to the individual and is not something that can be done by an outside “watchdog.”

Threading the Needle :mean:

Divers believe that decompression is always performed on the knife-edge of bas bubble formation. Nothing can be farther from the truth. Decompression is not like “threading a needle” or rushing downhill between the gates of a slalom course.

For some reason, divers appear to prefer a mechanical approach to safety rather than one that is directly in their control.

Dr Deco :doctor:

Please note the next class in Decompression Physiology
http://wrigley.usc.edu/hyperbaric/advdeco.htm