? for Doc Deco

[Sandworm 40]

Hey Steve,

Latest I've heard, this December. I appreciate your and everyone else's comments to this thread.

With my limited knowledge of decompression and the theory thereof, I'd like to make a few comments. We know that staying at a deeper depth will require more gas. We can agree that staying at a deeper depth will increase our CNS % if the time is roughly equivalent to that of 20'/10'. So this begs the question "Does staying at 20' on O2 better facilitate decompression of inert gasses because the partial pressure is higher or does stopping at 10' accomplish the decompression better/faster because the partial pressure is decreased once the appropriate time has been spent at 20'?". I think this may be where theory comes into play, maybe not. I do know that using VPM-B, configured for salt water and conservatism +2, deco gasses EAN50 and O2, the calculations have resulted with a marginally shorter run time doing 20' & 10' versus just 20' on some of the dives I've planned. And that, gentlemen, is why I asked the question. That was a great trimix class you taught Steve, best dive course I've had to date.

 

[DepartureDiver]

Does staying at 20' on O2 increase decompression of inert gasses because the partial pressure is higher or does stopping at 10' accomplish the decompression better/faster because the partial pressure is decreased once the appropriate time has been spent at 20'?

Staying at 20' should not increase the decompression once on O2. The gas elimination rate is based on the gradient between the tissue tension of the gas and the partial pressure of the gas being breathed. So once on O2, no inert gas is being breathed and the gradient is at it's maximum irregardless of the depth. However, if a nitrox mix is being breathed, then moving to a shallower depth will reduce the partial pressure of the inert gas being breathed and thus will increase the off-gassing rate of only the inert gas in the deco mix. So if EAN80 is being used, helium elimination is at its maximum irregardless of the depth, but the nitrogen elimination rate will be affected by the depth due to nitrogen now being in the deco mix with its partial pressure being affected by depth. But this assumes that a diver does not move to a shallower stop too quickly. If a diver ascends before they should, then bubbles could start influencing the elimination rate and slowing it down. So while the math may say a faster elimination rate at 10' if on EAN80 when compared to 20', the reality is that the elimination rate would be better at 20' as compared to surfacing too soon to 10'. So the small theoretical penalty a diver takes by staying deeper on EAN80 may be worth it.

 

[Sandworm 40]

Departure Diver,

I should have used the words "better facilitate" instead of "increase" to say what I mean. I understand that using O2 at 20' will not increase or lengthen the deco time required. And I'm not asking about EAN80, just 100% oxygen at 20' and above. What difference if any in total time does your software show for dives with O2 stopping at 20' only and stopping at 20' & 10' ?

 

[DepartureDiver]

... I should have used the words "better facilitate" instead of "increase" to say what I mean. I understand that using O2 at 20' will not increase or lengthen the deco time required. And I'm not asking about EAN80, just 100% oxygen at 20' and above. What difference if any in total time does your software show for dives with O2 stopping at 20' only and stopping at 20' & 10' ?

Sometimes it is easy to misread emails and posts ... sorry. I wasn't trying to limit my answer to my software and was trying to answer it in general. Regarding your question about Departure specifically, other than some rounding that may occur, there would be no difference in deco time between pulling all of your deco on O2 at 20' as compared to breaking it up between 20' and 10' or any other depth. My personal philosophy is to time the deco so that you leave 20' with a very slow ascent and then spend a few minutes at 10' to complete the deco and then a very slow ascent to the surface.

 

[Sandworm 40]

Departure Diver,

You read the unedited post correctly, I wrote it wrong.

I've been reading Bruce Wienke's "Second Edition Basic Decomprression Theory and Application" (most of what he discusses is so far above my head I can't understand it even with a telescope). He writes "To eliminate dissolved gasses, the diver is brought as close as possible to the surface. To eliminate free phases, the diver is maintained at depth to both crush bubbles and squeeze gas out by diffusion across the bubble film surface. Since both phases must be eliminated, the problem is a playoff in staging." So I think this means that once the "free phase elimination" time has been spent at 20', the best means of accomplishing the elimination of dissolved gasses is to ascend to 10' rather than staying at 20', whether you're breathing oxygen or some other deco gas. This makes sense to me, and V Planner software seems to follow this in its calculations in the tables I've run.

 

[DepartureDiver]

Departure Diver,

You read the unedited post correctly, I wrote it wrong.

I've been reading Bruce Wienke's "Second Edition Basic Decomprression Theory and Application" (most of what he discusses is so far above my head I can't understand it even with a telescope). He writes "To eliminate dissolved gasses, the diver is brought as close as possible to the surface. To eliminate free phases, the diver is maintained at depth to both crush bubbles and squeeze gas out by diffusion across the bubble film surface. Since both phases must be eliminated, the problem is a playoff in staging." So I think this means that once the "free phase elimination" time has been spent at 20', the best means of accomplishing the elimination of dissolved gasses is to ascend to 10' rather than staying at 20', whether you're breathing oxygen or some other deco gas. This makes sense to me, and V Planner software seems to follow this in its calculations in the tables I've run.

I’ll apologize for the length of this post in advance. The simple answer is the statement of bringing the diver as close to the surface as possible to eliminate the dissolved phase assumes the diver is on a gas containing inert gasses. Once on oxygen, coming closer to the surface does not increase the gradient since it will not change the partial pressure of any inert gas being breathed. For a more complete answer, at depth, gas is taken up (obviously). This gas is in the dissolved state meaning it is all in solution and not in any bubble form. If there were no nuclei or seeds, then the diver could come directly to the surface and not ever bend since bubbling would never occur due to the lack of nuclei causing them to form. So reducing the pressure as much as possible allows the dissolved gasses to be eliminated in the most efficient fashion due to breathing lower pressures of inert gasses on the way up. The problem is that the body does contain nuclei and these limit the ascent. So the give and take is to come us as shallow as possible, but not to come up too much where the nuclei grow or cause bubble growth. Bubbles will of course basically always occur, but you don’t want too many of them since this is what causes DCI issues. I would not term any of the deep stop style decompression as “crushing” bubbles. Crushing only occurs during pressurizations and then it depends on the descent speed. However, bubble shrinkage is a goal. “Bubbles” have a skin on them. This skin is the surface tension and it keeps trying to make bubbles smaller and shrink them. But for this to work, the bubbles must be kept at a small size. Once they grow too much, they keep growing past the skin tension being able to shrink them. When this happens, bubbles must be reabsorbed which is a time consuming process. An easy example is Eric Maiken’s balloon example. We all know that it is hardest to blow up a balloon when you just start blowing it up. Once it gets bigger, it is easier to blow it up. This is because the compression is less the bigger it is. The same holds true for a bubble. When it is small, the skin tension is squeezing in on it forcing gas out and making it smaller. If it gets too big, it will just keep growing and no squeezing in will occur. So back to the example (which is what Dr. Deco also stated), staying under pressure at 20’ keeps the microbubbles smaller for more efficient off-gassing for the squeezing g effect. This takes care of the free phase. Now to the dissolved phase. Since a diver is on pure oxygen, there is simply no reason to bring the diver closer to the surface since it will not reduce the partial pressure of the inspired gas and thus it will not cause the dissolved phase to be eliminated any faster. So on pure O2, staying at 20’ is better (neglecting CNS issues and gas supply issues). It only changes when one is not on pure O2. I hope this rambling helped.

 

[Sandworm 40]

Now to the dissolved phase. Since a diver is on pure oxygen, there is simply no reason to bring the diver closer to the surface since it will not reduce the partial pressure of the inspired gas and thus it will not cause the dissolved phase to be eliminated any faster. So on pure O2, staying at 20’ is better (neglecting CNS issues and gas supply issues). It only changes when one is not on pure O2. I hope this rambling helped.

I disagree with you on this part. Ascending to 10' from 20' most certainly does reduce the partial pressure of the inspired gas (O2, Ean80, whatever you're breathing) and it then follows that the dissolved phase gasses will be eliminated faster. Even though you may be breathing pure oxygen, you're still decompressing both phases of inert gasses which were inhaled prior to going on pure oxygen.

How long could we plan an air dive to 20' with no deco? A very, very long time, because we can handle 79% nitrogen at that partial pressure without incurring a deco penalty. So why are we worried about nitrogen or other inert gasses for deco at 20' and above? At this partial pressure, I believe that the inert gasses don't become "bad" inert gasses requiring additional deco, but they do become "getting in the way" gasses which inhibit (but don't completely stop) the offgassing of the "bad" inert gasses requiring deco. So by decreasing or eliminating the percentage of inert gas at this point (or earlier) in deco, we are lessening (EAN50, EAN80) or eliminating (O2) this inhibition to the offgassing. But we still have to deal with the "bad" inert gasses by making stops (high partial pressure) and ascents (lower partial pressure) at the appropriate times. That's my theory at this point.

 

[DepartureDiver]

Yes, anytime the pressure is reduced the partial pressure of the gasses are reduced. But the issue is the partial pressure of the inspired inert gas. It is the difference between the tissue tension and that of the inspired inert gas that determines the rate of off-gassing. When on O2, it is always the same whether at 20' or 10'. As I have stated, this example only applies to being on pure O2. So my question is when on pure O2 at 20' what is the gradient or driving force for nitrogen elimination when on O2 at 20' as compared to being on O2 at 10'? Pick any tissue tension or nitrogen pressure you would like for any compartment. The result is the same. If you get a different result, let me know. It may just be the way you are viewing it, but throw an example out and let's see if we can find where your thinking is going.