Underwater off-gassing equivalent to a surface interval on air

[rx7diver]

While "off-gassing" at 6m on O2 is happening at the exact same rate as it would at 3m or on the surface if still on O2 there are other reasons to stay lower. Some people like to avoid the swell if the surface is not dead flat but from a deco point of view it does limit the further growth of any bubbles that are present (which could themselves impede the off gassing process or cause DCS directly).

Dean

Yes, my understanding (from 20 years ago) is we prefer to deco out at on 100% O2 at 20 ffw because:

1. This helps keep inert gas micro-bubbles from forming, and this helps keep any inert gas micro-bubbles that might already be in circulation, from becoming larger. Keeps the blood circulating, without obstruction, until it arrives at the lungs where inert gas can be off-gassed safely.

2. This provides the highest practical, and safe, PPO2 (1.6 ATA) for wet deco. This PPO2 "washes out" the inert gas extremely efficiently while allowing a long enough time on 100% O2 for deco, assuming cycling on and off 100% O2, given the relatively high oxygen exposure that has already occurred both at depth and each time a new deco gas is switched to. (Oxygen limit for a non-repetitive dive is 45 min at PO2 = 1.6 ATA. From ancient memory.)

3. The only thing more practical, and just as safe (perhaps), would be dry deco (chamber/habitat deco) on 100% O2 at 30 ffw.

4. There is no compelling reason to complete the deco on 100% at 10 ffw--since 1.6 ATA is a stronger "dose" of oxygen, and it is administered while bubbles/micro-bubbles are kept smaller.

Again, I am recalling this from 20 years ago. If thinking has changed about this, I hope members here will let me know.

rx7diver

 

[lowwall]

N2 clearance is based on the percentage of N2 in the mix, not partial pressures. So from the perspective of clearing N2, the depth at which you switch to a high O2 mix does not matter. But simply decreasing N2 load is not the point of deco stop. What really matters is minimizing inert gas bubbles of the size and volume and location that could potentially cause DCS.

That's why you must make stops at various depths even with less than 100% O2 instead of going straight to the surface and switching to 100% O2.

 

[lowwall]

Yes, my understanding (from 20 years ago) is we prefer to deco out at on 100% O2 at 20 ffw because:

1. This helps keep inert gas micro-bubbles from forming, and this helps keep any inert gas micro-bubbles that might already be in circulation, from becoming larger. Keeps the blood circulating, without obstruction, until it arrives at the lungs where inert gas can be off-gassed safely.

2. This provides the highest practical, and safe, PPO2 (1.6 ATA) for wet deco. This PPO2 "washes out" the inert gas extremely efficiently while allowing a long enough time on 100% O2 for deco, assuming cycling on and off 100% O2, given the relatively high oxygen exposure that has already occurred both at depth and each time a new deco gas is switched to. (Oxygen limit for a non-repetitive dive is 45 min at PO2 = 1.6 ATA. From ancient memory.)

3. The only thing more practical, and just as safe (perhaps), would be dry deco (chamber/habitat deco) on 100% O2 at 30 ffw.

4. There is no compelling reason to complete the deco on 100% at 10 ffw--since 1.6 ATA is a stronger "dose" of oxygen, and it is administered while bubbles/micro-bubbles are kept smaller.

Again, I am recalling this from 20 years ago. If thinking has changed about this, I hope members here will let me know.

rx7diver

1 is true.

2-4 are mostly false. The accurate part is that starting 100% at the deepest safe PO2 will result in faster overall decompression. But there is no advantage to staying at that depth once the 20' deco obligation is met. Moving up on schedule reduces oxygen stress without increasing deco time.

 

[rx7diver]

... The accurate part is that starting 100% at the deepest safe PO2 will result in faster overall decompression. But there is no advantage to staying at that depth once the 20' deco obligation is met. Moving up on schedule reduces oxygen stress without increasing deco time.

@lowwall,

I am not sure I understand your statement. My first deco schedules cut on a laptop were generated using "Dr. X" (Sheck's software), and then "Abyss." In both cases you could specify your shallowest deco stop. If you specified 20 fsw, you got one schedule (and you ascended, slowly, directly to the surface from 20 fsw). If you specified 10 fsw, you got a different schedule (and you ascended, slowly, directly to the surface from 10 fsw). And in both cases you're using 100% O2 beginning at 20 fsw, and clearing your deco obligation at your specified shallowest deco stop. This is how things should work, I think.

You seem to be describing something different. Maybe I am misunderstanding you.

rx7diver

 

[lowwall]

@lowwall,

I am not sure I understand your statement. My first deco schedules cut on a laptop were generated using "Dr. X" (Sheck's software), and then "Abyss." In both cases you could specify your shallowest deco stop. If you specified 20 fsw, you got one schedule (and you ascended, slowly, directly to the surface from 20 fsw). If you specified 10 fsw, you got a different schedule (and you ascended, slowly, directly to the surface from 10 fsw). And in both cases you're using 100% O2 beginning at 20 fsw, and clearing your deco obligation at your specified shallowest deco stop. This is how things should work, I think.

You seem to be describing something different. Maybe I am misunderstanding you.

rx7diver

Before I posted, I did check my understanding of the theory with a dive planner and it gave the same total time for a deco dive with a final stop (with 100% O2) at 20 as at 10. For example 12 minutes at 20' or 4 minutes at 20' + 8 minutes at 10'. It also shows identical tissue loadings at surfacing. The only differences were in OTUs and CNS% which were obviously higher for the 20' final stop.

I used DiveProMe Scuba Technical Diving Planner - Apps on Google Play as the dive planner based only on it being the first one I found that let you for free specify the final stops at 20' and 10' or just 20' using 100%.

If you don't/can't constrain the penultimate stop to 20' on a plan that ends at 10', it's not a direct comparison.

 

[inquis]

There is no compelling reason to complete the deco on 100% at 10 ffw--since 1.6 ATA is a stronger "dose" of oxygen, and it is administered while bubbles/micro-bubbles are kept smaller.

It's the nitrogen gradient (tissue minus inspired ppN2) that controls the off-gassing rate of nitrogen, not ppO2. (If ppO2 was the driving mechanism, why not stay at the 70 ft stop on EAN50 until it was safe to surface since ppO2 is also 1.6 atm? Answer: tissues would never progress beyond the inspired ppN2 -- also 1.6 atm -- and it will never be safe to surface, in spite of the strong "dose" of oxygen.)

One reason to NOT stay at a high ppO2 (i.e., 20 ft when on O2) after cleared higher is there's greater margin for depth changes, facilitating deeper breathing and more efficient gas transfer. For example, hanging out between 12 and 14 ft when cleared to 10 ft. The reasons to hold a tight stop are simply not present when on O2 since the off-gas gradient is the same anywhere between MOD and the ceiling.

I wouldn't go so far as to call that a "compelling" reason, but if I'm not in washing machine conditions I think it's a good idea.

 

[lowwall]

Cropped screenshots from planner. This is for a 99' ffw (you said Great Lakes ) with 40 minutes of bottom time, a GF of 50/75 using the ZHL-B algorithm. Only gasses are air and 100%.

 

[inquis]

Staying at 20' takes more O2. Fitting your gas usage within the capacity of your cylinder could be a very compelling reason in some cases.

 

[Jacob BV]

As inquisit has said off-gassing is not defined by ppo2. But the inert gas gradient, i.e. the difference between inert gas tension and the inspired inert gas pressure. Oxygen has no inert gas pressure so this won't change in the discussing of 20' and 10'. The inert gas tension will. Any tissues that are still supersaturated at 20' will become more supersaturated at 10' and hence offgass quicker.

Now we are only talking 10' so the effect is small, in your examples you don't see it as you have it set to round to minutes. But put a deeper longer dive in and you will start to see the 10' be quicker by a minute or 2 at some point.

Moreover, with the OP's logic this means the actual optimal place to off gass is on the surface on 100%...or at higher altitude (this is why on days of very low pressure you do need to be more conservative as when you surface the inert gas tension will be higher than normal - in the UK we collect a lot of accident data and you can see cases of DCI rise when particular low low-pressure systems pass)

 

[dmaziuk]

As inquisit has said off-gassing is not defined by ppo2. But the inert gas gradient, i.e. the difference between inert gas tension and the inspired inert gas pressure. Oxygen has no inert gas pressure so this won't change in the discussing of 20' and 10'. The inert gas tension will. Any tissues that are still supersaturated at 20' will become more supersaturated at 10' and hence offgass quicker.

The model's not that sophisticated. If you're breathing pure O2, ppN2 in your lungs is 0. At any depth.