? for Doc Deco

[Sandworm 40]

I have heard proponents for two methods of decompression use of 100% O2. One method recommends all time be spent at 20', the other method recommends scheduled stops at both 20' & 10'. I'd like your take on this.

 

[Dr Deco]

Hello Jim:

My quick answer on this would be decompressing at the deeper depth. It will keep microbubbles in the microbubble form. Off gassing of free-nitrogen [i.e., gaseous] is faster at the deeper depth.

This is of course assuming that you do have sufficient oxygen gas for the 20 foot stop. Less gas is inhaled per unit of time the shallower you go.

Dr Deco :doctor:

 

[Sandworm 40]

Thank you.

 

[Doppler]

I have heard proponents for two methods of decompression use of 100% O2. One method recommends all time be spent at 20', the other method recommends scheduled stops at both 20' & 10'. I'd like your take on this.

Hi Jim... Doc has given you the simple answer as he stated; however, as you know, decompression is not simple and nor is it a science... more like alchemy.

There are several refinements one might throw into a comprehensive answer to this question -- and it would take about 2 000 words so I ain't gonna do it -- but there is one seriously important point I will make... for the general readership here...

Executing final deco obligation at 20 feet on oxygen delivers an oxygen partial pressure of 1.6 ata (slightly more in salt water) and therefore CNS loading is high. One must have a clear understanding of how to balance the potential risks of poor bubble management and poor CNS management when employing this deco strategy.

 

[DA Aquamaster]

Hello Jim:

My quick answer on this would be decompressing at the deeper depth. It will keep microbubbles in the microbubble form. Off gassing of free-nitrogen [i.e., gaseous] is faster at the deeper depth.

This is of course assuming that you do have sufficient oxygen gas for the 20 foot stop. Less gas is inhaled per unit of time the shallower you go.

Dr Deco :doctor:

Let's extend the original question to using deco gases other than 100%. How about 80% with a PPO2 of 1.28 at 20 ft?

To extend further, in a single deco gas dive using 50%, does the lower PPO2 acheived of 0.80 with 50% warrant remaining at the 20 ft stop rather than ascending to 10ft?

Executing final deco obligation at 20 feet on oxygen delivers an oxygen partial pressure of 1.6 ata (slightly more in salt water) and therefore CNS loading is high. One must have a clear understanding of how to balance the potential risks of poor bubble management and poor CNS management when employing this deco strategy.

My thoughts are that 80% may offer a nearly ideal compromise for completing the last stop at 20 ft in that it offers nearly as much bang for the offgassing buck at 10 ft and 20 ft, but with less impact on the CNS clock than 100%. Plus being able to start on 80% at the 30 ft stop would in many profiles appear to make it more efficient than 100% O2 overall.

 

[Doppler]

Let's extend the original question to using deco gases other than 100%. How about 80% with a PPO2 of 1.28 at 20 ft?

To extend further, in a single deco gas dive using 50%, does the lower PPO2 acheived of 0.80 with 50% warrant remaining at the 20 ft stop rather than ascending to 10ft?

My thoughts are that 80% may offer a nearly ideal compromise for completing the last stop at 20 ft in that it offers nearly as much bang for the offgassing buck at 10 ft and 20 ft, but with less impact on the CNS clock than 100%. Plus being able to start on 80% at the 30 ft stop would in many profiles appear to make it more efficient than 100% O2 overall.

Once again a question requiring a multi-part answer, but again one which brings up one point which needs to be addressed first.

Doing all one's deco at 20 only "works" when there's 0% inert gas in the deco mix -- that's to say it's 100% oxygen. If you opt to breathe something which contains N2 for example, you should NOT in my opinion execute all your deco at 20 feet. Move up. You have a partial pressure of inert gas in what you're breathing and need to wean off it.

In partial answer to the other questions... As stated earlier, deco is a balancing act where we have to consider bubble management and CNS avoidance; however, most experienced technical divers -- especially those with experience with helium mixes -- will try to keep the partial pressure of oxygen during deco between 1.0 and 1.6 ata. a 50% oxygen mix is excellent between 70 and 30 (inclusive) for this reason. If one wishes to use it alone for all one's deco, the operational strategy should be the same as with an 80%... do 20 and 10 foot (or 20, 15, 10 and a slow ascent from there).

 

[DepartureDiver]

Doing all one's deco at 20 only "works" when there's 0% inert gas in the deco mix -- that's to say it's 100% oxygen. If you opt to breathe something which contains N2 for example, you should NOT in my opinion execute all your deco at 20 feet. Move up. You have a partial pressure of inert gas in what you're breathing and need to wean off it [snip] ... (or 20, 15, 10 and a slow ascent from there).

Several things have been brought up. I agree with doing more deco at 20 feet due to keeping microbubbles smaller, which keeps a higher surface/skin tension squeezing in on microbubbles. Of course a very slow ascent from 20' to the surface as Doppler also stated is advisable and I also agree with using 100% O2. But if one uses a final deco gas with some nitrogen in it such as EAN80, I still think that pulling the last stop at 20' is beneficial due to being under greater pressure. Of course, the amount of deco might be slightly increased due to being on EAN80 instead of O2. But when doing deco after a Trimix dive, the slight amount of nitrogen does not affect the rate of helium off-gassing. Upon surfacing from any dive, there is still excess nitrogen in the body from the dive. So the fact that EAN80 is being used does not add to any risk in my opinion since the O2 does not fully rid the body of the nitrogen anyway. It also allows you to get on a low inert gas blend earlier at 30'. When comparing profiles using either EAN80 starting at 30 and doing the final deco stop at 20' compared to using O2 all at 20', the difference in deco time can be small. Upon surfacing, you also “slam” the body with 79% nitrogen so while the slow ascent is very helpful, I would not call it a weaning off nitrogen process - even though the point is well taken.

 

[Rick Murchison]

Doing all one's deco at 20 only "works" when there's 0% inert gas in the deco mix -- that's to say it's 100% oxygen.

Wellll...... sort of... you can't just combine the stops unless you're on 100% oxygen, true, but you can certainly finish deco at 20' (or deeper, for that matter) any time your PPN2 is .8 or less at your final stop depth. (To be completely accurate, you could do it with a PPN2 up to about 1.2, but anything over .8 gets impractical pretty fast)
For example, (I'm using VPM-B, but you'll get similar if not exactly the same results no matter whose algorithm you use) on a dive where the last 3 stops (30/20/10) are 7, 10 and 17 minutes respectively on EAN80, if you do 2 stops with the final one at 20', those stops will be 7 and 28 minutes - an increase of only one minute. If you make your entire deco at 30', that stop will still be 35 minutes total - so if you have high seas, for example, for a penalty of one minute deco time over what's required at 30/20/10, you can complete your entire deco using EAN 80 at 30'.
The same schedule using oxygen would require a stop of 16 minutes at 30' on back gas (in this case I'm using 16/50, but the principle is the same for any typical back gas) and still require 27 minutes at 20' - 8 more minutes deco than required on EAN80 - with no reasonable option to finish deco at 30' for rough weather on that back gas.
Bottom line, EAN80 does work with a final stop at 20 or even 30 feet, with a very small deco time penalty at 20 feet or even 30'. Additionally, there is often a significant advantage (reduced total deco time) when compared to using pure oxygen. And, as an added bonus, using EAN80, even with the final stop at 30', uses less oxygen clock than oxygen with a final stop at 20'.
On the down side, EAN80 is "stroke gas" they say...
Rick

 

[ScubaDadMiami]

One of the guys with whom I have been diving with showed me some profiles run on V-Planner with the last stop at 20 and then moving up to 15 (part of the time at 20, end of the stop conducted at 15). It made virtually no difference in the outcome, however, it did have a much lower oxygen exposure. I find this interesting but I will admit that I am still doing my stops at 20.

 

[Doppler]

Wellll...... sort of... you can't just combine the stops unless you're on 100% oxygen, true...

SNIPPED

Rick

Well unc, now you're starting to write stuff from the 2000 word answer and I didn't want to go there...

By the way Jim, when are they going to put that bloody aircraft carrier on the bottom?!