Another Deco profile post.

[DevonDiver]

The aim is to lower ambient pressure and maximise the tissue-lungs gradient. The shallower, the better, up to whatever maximum gradient you, or your algorithm, have identified (m-value or GF-Hi).

Obviously, using O2 creates the most efficient gradient. As others have mentioned, that demands shallower stops for CNS toxicity management reasons.

As ending from 12m would also see a significant spike in gradients. In terms of pressure... 2.2ata is still a long way from the surface pressure. Regardless of model, every deco approach shows a curve to the surface. Longer stops at 12m would dictate even longer stops at 6 and 3m.

12m is neither here, nor there. It isn't a lucrative depth for maximising gradients, nor does it permit access to the most efficient deco gas. On the other hand, it isn't at a sufficient depth where fast-tissue bubbles would theoretically get resolved on technical dives.

As Simon Mitchell said, the technical diving community was "oversold" on bubble management. Off-gassing dissolved gas from tissues is the tried, tested and best proven approach. A 12m stop isn't algorithmically relevant as a bubble stop, and certainly wouldn't take precedence over tissue gradient benefits from shallow stops.

 

[Remy B.]

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

Once your compartments are saturated the bubbles will form in the different compartments at different sizes while the diver is ascending correct ?, while some in the faster compartment will diffuse faster as they reduce in size faster than others,( fast compartments ), will it not constrain the ability to diffuse the bigger ones in the blood stream ? looking it like at a conveyor belt in regards of size, the lungs can exchange so much at a moment in time and the smaller bubbles will be the 1st to do so correct ?

If you give time for the smaller bubbles in the lungs to get exchanged, mean while giving time for the bigger ones and the slow compartment ones to reduce size and surface tension, will they not be exchanged faster once the diver go a little higher, reducing the possibility of DCS type 2

Don't get confuse about deep stops, I'm only talking for the 12m, 9m and 6m stops, anything below 12m you keep it at your GF recommends.

For example the last portion of the Deco profile tells you the following.
with 50% O2 with a GF of 45/75

12m for 1min
09m for 4min
06m for 17min

what will be the possible or negative effects to redistribute it like this ?

12m for 3min
09m for 6min
06m for 13min

 

[KenGordon]

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

Once your compartments are saturated the bubbles will form in the different compartments at different sizes while the diver is ascending correct ?, while some in the faster compartment will diffuse faster as they reduce in size faster than others,( fast compartments ), will it not constrain the ability to diffuse the bigger ones in the blood stream ? looking it like at a conveyor belt in regards of size, the lungs can exchange so much at a moment in time and the smaller bubbles will be the 1st to do so correct ?

If you give time for the smaller bubbles in the lungs to get exchanged, mean while giving time for the bigger ones and the slow compartment ones to reduce size and surface tension, they will be exchanged faster once the diver go a little higher, reducing the possibility of DCS type 2

Don't get confuse about deep stops, I'm only talking for the 12m, 9m and 6m stops, anything below 12m you keep it at your GF recommends.

For example the last portion of the Deco profile tells you the following.
with 50% O2 with a GF of 45/75

12m for 1min
09m for 4min
06m for 17min

what will be the possible or negative effects to redistribute it like this ?

12m for 3min
09m for 6min
06m for 13min

The second profile will obviously leave you with more inert gas when you surface since the off gasing rate is faster shallower. If the first profile was 'just enough' then the second will not be enough.

 

[Patoux01]

Agreed with Ken.


It's also my understanding that this

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

(bold under italic added by me) is just plain wrong.

 

[elmo]

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

Once your compartments are saturated the bubbles will form in the different compartments at different sizes while the diver is ascending correct ?, while some in the faster compartment will diffuse faster as they reduce in size faster than others,( fast compartments ), will it not constrain the ability to diffuse the bigger ones in the blood stream ? looking it like at a conveyor belt in regards of size, the lungs can exchange so much at a moment in time and the smaller bubbles will be the 1st to do so correct ?

If you give time for the smaller bubbles in the lungs to get exchanged, mean while giving time for the bigger ones and the slow compartment ones to reduce size and surface tension, will they not be exchanged faster once the diver go a little higher, reducing the possibility of DCS type 2

Don't get confuse about deep stops, I'm only talking for the 12m, 9m and 6m stops, anything below 12m you keep it at your GF recommends.

For example the last portion of the Deco profile tells you the following.
with 50% O2 with a GF of 45/75

12m for 1min
09m for 4min
06m for 17min

what will be the possible or negative effects to redistribute it like this ?

12m for 3min
09m for 6min
06m for 13min

Redistributing the stop times like you suggest does make the deco less efficient, which you can verify in the Subsurface dive planner (or any other) by entering the entire profile except for the 6m stop, and see what it tells you to do at 6m. Although spending longer deeper is less efficient, as everybody has been saying, in reality the difference is marginal. I don't know what bottom time, depth, or gas you were using, so tried to create a profile with similar stop times to yours. Based on 36m for 30min on 21/35 gives something similar with GF 45/75 (1 minute less at 9m).

Subsurface dive plan
Runtime: 54min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min
- 21m 1min 33min EAN50
- 12m 1min 34min
- 9m 3min 37min
- 6m 17min 54min
➚ 0m 1min 55min

Now if we redistribute the stop times so that you spend 3min at 12m, 6min at 9m (entered at 1min to ascend, 5min at stop), you end up requiring one additional minute of total deco.

Subsurface dive plan

Runtime: 55min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min EAN50
➙ 21m 1min 33min
➚ 12m 1min 34min
➙ 12m 3min 37min
➚ 9m 1min 38min
➙ 9m 5min 43min
➚ 6m 0min 43min
- 6m 12min 55min
➚ 0m 1min 56min

Redistributing further, so you spend 7min at each 12m and 9m, you need another minute of total deco....

Subsurface dive plan

Runtime: 56min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min EAN50
➙ 21m 1min 33min
➚ 12m 1min 34min
➙ 12m 7min 41min
➚ 9m 1min 42min
➙ 9m 6min 48min
➚ 6m 0min 48min
- 6m 8min 56min
➚ 0m 1min 57min

But you seem to really like bubble models (they're becoming less popular based on studies in the last few years, but each to their own). VPM-B +4 gives pretty much the same profiles for this example (NB it gets less conservative compared to Buhlmann for longer and deeper dives).

"pure" VPM-B +4 profile

Subsurface dive plan

Runtime: 54min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min
- 21m 1min 33min EAN50
- 12m 2min 35min
- 9m 3min 38min
- 6m 16min 54min
➚ 0m 1min 55min


Redistributed stop times (7min at each 12m and 9m)

Subsurface dive plan

Runtime: 56min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min EAN50
➙ 21m 1min 33min
➚ 12m 1min 34min
➙ 12m 7min 41min
➚ 9m 1min 42min
➙ 9m 6min 48min
➚ 6m 0min 48min
- 6m 8min 56min
➚ 0m 1min 57min


As you can see, redistributing stop times has a small effect, but not terribly significant in these examples. However, if spending longer deeper meant you were delaying a gas switch to oxygen at 6m, you you see a big difference. Go ahead, test it yourself: Subsurface is free

 

[clownfishsydney]

As we observe all current Deco algorithms/profiles and so called strategies, the focus is on long times @ 6m, but since there is NO On-gassing from 12m and up and only Off-gassing is present, why is it that these Algorithms are not re-distributing the time @6m between 12m, 9m and 6m, not necessarily means divided by 3 but the time can be re-distributed.

The way I understand it, bubble size is kept smaller at 12m than at 6m and the surface tension as well is lower, and there only Off-gassing, it appear now days that the trend is as well shorter deco times @ 6m if we compare GF-50/70 to GF 50/80. or VPM+5 to VPM+2 for example.

Is there a particular reason why it is not done this way ? am I missing something or misunderstanding it.?

You appear to be missing a lot of things. Of course there is on gassing from 12 m and up. There is on gassing at 3 m! What I assume you mean is there is no on gassing from 12 m if you are on high PPO. Certainly if you are on O2 from 6 m there is no on gassing.

 

[lermontov]

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

Once your compartments are saturated the bubbles will form in the different compartments at different sizes while the diver is ascending correct ?, while some in the faster compartment will diffuse faster as they reduce in size faster than others,( fast compartments ), will it not constrain the ability to diffuse the bigger ones in the blood stream ? looking it like at a conveyor belt in regards of size, the lungs can exchange so much at a moment in time and the smaller bubbles will be the 1st to do so correct ?

If you give time for the smaller bubbles in the lungs to get exchanged, mean while giving time for the bigger ones and the slow compartment ones to reduce size and surface tension, will they not be exchanged faster once the diver go a little higher, reducing the possibility of DCS type 2

Don't get confuse about deep stops, I'm only talking for the 12m, 9m and 6m stops, anything below 12m you keep it at your GF recommends.

For example the last portion of the Deco profile tells you the following.
with 50% O2 with a GF of 45/75

12m for 1min
09m for 4min
06m for 17min

what will be the possible or negative effects to redistribute it like this ?

12m for 3min
09m for 6min
06m for 13min

Not sure if your talking about the relationship between tissue tension and inert inspired gas pressure as it relates to ambient pressure ?
switching to a higher o2 reduces inspired inert gas pressure thus creating a steeper gradient without exceeding ambient pressure
or
are you talking about the rate of exchange controlled by bubble size ( i.e. your conveyor belt picture) I see it more like a reducing funnel rather than a conveyor belt so off gassing can occur from both slow and fast tissues at the same time rather than "waiting in line"

deco for divers has a good section on page 101

 

[victorzamora]

What I'm trying to understand is the Bubble size, the efficiency of using high O2% and keeping it at 1.6PP I understand.

This is actually an incredibly common misunderstanding that I see/hear all the time, even from people who should absolutely know better. Having a high PO2 isn't good. It's not what helps you off-gas. It hurts you. It's not a good thing. HOWEVER, replacing "inert gas" (mainly N2 and He) with O2 means you're offgassing those gasses better.

The below isn't 100% scientifically accurate, but should be a good-enough way to think about it.
Let's say you do a dive into deco and you're ascending into your stops. Your body is too full of nitrogen (and maybe helium). Your tissues are dumping N2 and He into your venous system (carrying "used" blood back towards your lungs for recharging with O2). All of that blood is saturated (theoretically). When it gets to your lungs, your blood has more N2 and He in it than what's in your lungs (because you're ascending and offgassing) so they equalize. You then exhale all of the N2 and He that was able to transfer from your blood into your lungs (which isn't all of it). The more N2 and He you have in your lungs, though, the less is able to transfer. It's a simple gradient. The more difference there is, the steeper the hill is for the ball to roll down. When your blood and lungs both have the same amount of He and N2, the hill becomes flat and there's no more gas transfer.

What you're trying to do by switching to deco gases is breathe in the lowest amount of the inert gases possible. That's limited by oxygen toxicity.

As for "you can't on-gas no matter what at 12m"....that's just completely wrong. When you're at the surface, your body is equilized at a PN2 of ~0.79. When you're at 12m on air, you're exposing your body to a PN2 of ~1.74. The ball will roll down the hill from 1.74 in your lungs to the 0.79 that's in your body and you'll be on-gassing across all compartments.

 

[Remy B.]

Redistributing the stop times like you suggest does make the deco less efficient, which you can verify in the Subsurface dive planner (or any other) by entering the entire profile except for the 6m stop, and see what it tells you to do at 6m. Although spending longer deeper is less efficient, as everybody has been saying, in reality the difference is marginal. I don't know what bottom time, depth, or gas you were using, so tried to create a profile with similar stop times to yours. Based on 36m for 30min on 21/35 gives something similar with GF 45/75 (1 minute less at 9m).

View attachment 404445
Subsurface dive plan
Runtime: 54min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min
- 21m 1min 33min EAN50
- 12m 1min 34min
- 9m 3min 37min
- 6m 17min 54min
➚ 0m 1min 55min

Now if we redistribute the stop times so that you spend 3min at 12m, 6min at 9m (entered at 1min to ascend, 5min at stop), you end up requiring one additional minute of total deco.

View attachment 404446
Subsurface dive plan

Runtime: 55min

depth duration runtime gas
➘ 36m 4min 4min (21/35)
➙ 36m 26min 30min
➚ 21m 2min 32min EAN50
➙ 21m 1min 33min
➚ 12m 1min 34min
➙ 12m 3min 37min
➚ 9m 1min 38min
➙ 9m 5min 43min
➚ 6m 0min 43min
- 6m 12min 55min
➚ 0m 1min 56min

The heat bar below have a different color intensity ( yellow glare ) between the two profiles, is that the amount of tension or Bubbles ? to what refers that ?, can I assume the more yellow the color then it is worse correct ?

I'm refering to the GF-45/75 and the modified version of redistributing the stops between 12m-9m and 6m (1st and 2nd heat bars ).

I was actually expecting somebody to modulate these two profiles with a Heat Bar, Thanks a lot.

 

[Remy B.]

You appear to be missing a lot of things. Of course there is on gassing from 12 m and up. There is on gassing at 3 m! What I assume you mean is there is no on gassing from 12 m if you are on high PPO. Certainly if you are on O2 from 6 m there is no on gassing.

To my understanding there is On-gassing at the begin of the dive, once your tissues are saturated and you go up to your stops you are OFF-gassing, it go by differential pressure or not ?? if not it will be a never ending story that you will never be able to decompress the way I maybe misunderstood it, not saying I'm right.