kaylee_ann
crazy diver
I'm kinda confused on the last part- do you mean if the diver stays at the bottom for a while and then ascends, they'll on gas while ascending? even at a normal ascent rate?
thanks for the explanation!
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thanks for the explanation!
Here's what he means.
Your body is comprised of multiple tissues, and they absorb and release nitrogen at different rates. When we say that the tissues in your body is off-gassing as you ascend, the reality is that SOME of the tissues are off-gassing. The ones that are slower at absorbing and releasing nitrogen are still on-gassing. In an NDL dive, they should not be able to on-gas to a level of concern. In a decompression dive, that on-gassing is more important.
Here is a quick treatise on oxygen use, both during decompression during the dive and on the surface after the dive.
The rate at which nitrogen leaves the tissues depends upon the difference between the amount of nitrogen in the air being breathed and the nitrogen in the tissues. It will help to understand why. The reason is the combination of prefusion and diffusion. Perfusion is the rate at which blood circulates, carrying the gases picked up from the lungs into the body and carrying the gases leaving the tissues back to the lungs. Diffusion is how those gases get into and out of the body. Diffusion is pure chance--the gas molecules easily go through blood vessel walls and tissues randomly.
That means that during perfusion, gas molecules are both entering and leaving the blood and the tissues. When you are on the surface before the dive, you have been there long enough so that everything is about equal, so just as many molecules enter the tissues as leave it. Descend to 99 FSW, and you have 4 times as many molecules entering as leaving. As the tissue molecule levels grow, things start to even out again. When you ascend and the air has less nitrogen in it, some tissues now have more molecules leaving than entering, and those tissues are off-gassing.
Oxygen has no nitrogen in it, so when you breathe it, there is no new nitrogen entering the body, meaning the nitrogen leaving the body is not being replaced. This greatly speeds up off-gassing, whether you are under water doing decompression or on the surface between dives. When a tech diver surfaces after doing a long deco stop on oxygen, the amount of nitrogen in the fastest tissues will have dropped so far that those tissues actually on-gas during the surface interval.
Yes, you will benefit from oxygen during the surface interval, and it is more than some people seem to think. NOAA used to have a chart telling how far on the US Navy tables a diver would drop after breathing oxygen on the surface. (I can't find it now.) It was pretty significant. The problem for divers is the computer does not know you are breathing oxygen. It assumes you are breathing air, and it plans the next dive accordingly.
We had a thread on this a while back, and IIRC, there is a computer that can be set to track changes when using oxygen during a surface interval, but I don't recall which.
As for me, when I finish a day of diving in New Mexico and begin my drive home to Colorado, I breathe oxygen for at least the first hour as I drive.
What @boulderjohn said, and also: not at a normal ascent rate. Too slow an ascent rate. I didn't want to get too deep (so to speak) into tissue compartments as it's beyond what the OP was asking.
Best regards,
DDM
It depends upon the individual tissues. There will always be more nitrogen in the air being breathed than in some of the tissues, so those tissues will continue to take on nitrogen. Only the fastest tissues are off-gassing during that initial ascent. The rest will still have less nitrogen than the air being breathed, so they will still be on-gassing.
My last post should explain why that is true.
Believe it or not, some of your tissues are on-gassing during your safety stop. It all depends upon how quickly they absorb and release nitrogen.
Read my treatise in post #33. Then check out the following.
Read my treatise in post #33. Then check out the following.
- To be safe, the amount of nitrogen in the tissues (tissue pressure) cannot be too much greater than the pressure around us (ambient pressure). We need to get all tissues to a safe level before ascending. We need to be sure the pressure is not too great when we reach the surface.
- As we stay at safety stop depth, all tissue pressures move toward the ambient pressure at that depth.
- The faster ones are above that pressure, and they drop toward that level, hopefully reaching a safe level before we surface.
- The slower tissues are still on-gassing, so they are rising to that pressure level.
- When a tissue is at the same pressure as ambient, we say it is saturated.
- Research indicates that any tissue saturated at safety stop depth can then go safely to the surface.
- That means that it is helpful for the faster tissues to drop to that level, and it is not harmful for the slower tissues to rise to it.
From the NOAA Dive Manual, 1991, 3rd Edition, page 14-31:
"To shorten the necessary surface interval before flying,
oxygen may be breathed instead of air. Table 14-13 lists for the various Repetitive Dive Group classifications,the length of oxygen breathing time necessary before flying is allowed."
Here is why you should NOT use these numbers:
1) NOAA has taken table 14-3 out of its dive manual, which is now in its 6th edition; the 3rd edition was the last appearance of Table 14-3, shown above.
2) Not all talbe use the same repetitive groups, so (for example) you cannot say your are in group G from a PADI RDP, and then use the Navy or NOAA tables; Group G means very different things.
3) If you are using a computer, you don't get any repetitive group labels anyway, so you can't trasnfer you computer into to ANY tables.
The "NOAA, 1979" reference for the table above is actualy the 2nd Edition of the NOAA Dive Manual, dated 1979. In that edition, the table is Table 6-6 on p6-17 and has no source citation with it. I do not have a copy of the 1st edition of the NOAA Dive Manual so cannot check if that table is there also, perhaps with a source citation.
So if you're ascending slower than the normal ascent rate, you can actually on-gas?
1. What are you considering the normal ascent rate to be sure?
2. Is this the harder, more dense body tissue (slower absorbing & releasing) that is still absorbing nitrogen as you ascend very slowly or all body tissue (whether slow or fast absorbing)?
3. Let's say you ascend slower than the normal rate for the entire ascent. When the tissues are still on-gasing nitrogen, does that on-gasing occur during the entire ascent or just during the first part of the ascent?
As I explained above, some tissues (the slowest ones) will be on-gassing all the way through the ascent. If you stay within NDLs, none of them will on-gas to the point that you cannot go directly to the surface (with a suggested safety stop). The ones that matter are the faster tissues, and they are off-gassing.
Most people today recommend an ascent rate of 30 FPM. It is OK to ascend more slowly than that. Many people doing multi-level dives end up ascending much slower. The key is staying within No Decompression Limits.
If you violate no decompression limits, things change. In that case, one or more decompression stops will be required. In that case, ascending too slowly does make a difference. If you ascend more slowly than the algorithm you are using requires (usually 30 FPM), you will have more required decompression time.
Most people today recommend an ascent rate of 30 FPM. It is OK to ascend more slowly than that. Many people doing multi-level dives end up ascending much slower. The key is staying within No Decompression Limits.
If you violate no decompression limits, things change. In that case, one or more decompression stops will be required. In that case, ascending too slowly does make a difference. If you ascend more slowly than the algorithm you are using requires (usually 30 FPM), you will have more required decompression time.
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