Last Deco Stop Depth, Oxygen Window, and Deco Efficacy

[tomfcrist]

How does a higher ppO2 accelerate off-gassing if ppN2 (or ppHe) is already at 0? So which of points 10, 11, 15, and 17 above are you saying is incorrect?

Without going line by line...most of it.

If your posted theory is correct, then the hell with the 20 or 10' stops...just go deco on the boat with o2 and see how that works out for you.

And after all of your "points" you asked is 10' a safer stop than 20' pertaining to the increased risk of breathing o2 at higher partial pressures...answer is no. Breathing O2 at 1.6 is not any more inherently dangerous than breathing it at 1.3....my reasons why were explained.

 

[Michael Guerrero]

Lol, so you're argument is that I'm just wrong? And "my" theory that O2 accelerates decompression is wrong?

And like raftingtigger said in post #2, which I'm sure you saw in your careful reading, you still have to worry about bubbles.

Anyone have anything of substance to add here, because I am genuinely interested in (scientific) responses. I'm am completely open to the idea that the person I heard was correct, I just don't have any information to prove it, and the information I do have would seem to refute it or at least say that it's a neutral consideration (neither good nor bad).

Mike

 

[fire_diver]

You lost me at number 15.

15. Greater off-gassing does not occur between 20ft and 10ft on pure oxygen because the pp of inspired inert gas is 0, and thus the gradient cannot be increased once on pure oxygen.

How is it possible that greater off-gassing does not occur at 10ft compared to 20ft when using 100% O2? Simply by reducing the ambient of moving from 20 to 10 is going to increase the inert tension in the tissues which will lead to a faster off-gassing. The fact of both being at 0% inert PP simply removes that portion of the equation, but it's not the only portion in the consideration.

I'm not saying you are wrong and I am right, but if I'm wrong can you explain where I went astray?

 

[tomfcrist]

Lol, so you're argument is that I'm just wrong? And "my" theory that O2 accelerates decompression is wrong?

And like raftingtigger said in post #2, which I'm sure you saw in your careful reading, you still have to worry about bubbles.

Anyone have anything of substance to add here, because I am genuinely interested in (scientific) responses. I'm am completely open to the idea that the person I heard was correct, I just don't have any information to prove it, and the information I do have would seem to refute it or at least say that it's a neutral consideration (neither good nor bad).

Mike

I am well aware that you still have to worry about bubbles....which is another aspect of staying at 20' that is relevant. Have you ever run the numbers of the same profile with a 20' last stop and a 10' last stop. I will bet that if you add the 10' and 20' stops together on the latter profile, you will have within 1 minute the same run time as just doing the 20' stop. So why risk increase in bubble size at the 10' stop if it's not getting you out of the water faster?

But hey, if you have it all figured out...why ask the question? Probably so you can be a prick when someone answers in a way that doesn't agree with your position.

 

[Michael Guerrero]

I think (and it's really just what I think because I'm trying to make sense of the theory) that you cannot increase the rate of off-gassing any more once you have zero inert gas in the inspired mix. However, you still have to off-gas sufficiently to lower tissue tension so you can decrease ambient pressure (ascend) without breaching the m-values and creating bubbles. The tissue half-times dictate how quickly you're off-gassing, but you've maximized their potential off-gassing because you've eliminated inert gas and maximized the possible gradient (difference between tissue tension and inspired inert gas).

No idea if that's correct though.

---------- Post added September 2nd, 2014 at 07:29 PM ----------

I am well aware that you still have to worry about bubbles....which is another aspect of staying at 20' that is relevant. Have you ever run the numbers of the same profile with a 20' last stop and a 10' last stop. I will bet that if you add the 10' and 20' stops together on the latter profile, you will have within 1 minute the same run time as just doing the 20' stop. So why risk increase in bubble size at the 10' stop if it's not getting you out of the water faster?

But hey, if you have it all figured out...why ask the question? Probably so you can be a prick when someone answers in a way that doesn't agree with your position.

Tom,

I'm not sure why, but you seem to have not ready my posts. I'm not arguing that you'll get out faster by going to 10ft. If you can find where I said that please point it out. And if you read my post you'd see that my point was that a 1.6ppO2 is more dangerous than a 1.3ppO2, and that if you're off-gassing the same amount, it would make sense to me to reduce your oxtox risk by going to 10ft.

I asked you to explain your position about 20ft and you mentioned 1.6ppO2, but didn't tell me why 1.6ppO2 was better than 1.3ppO2 if you're on pure oxygen.

So finally, you're saying that you have increased bubble size at 10ft and so it's better to stay at 20ft. This is the first time you've said it, and it's a reasonable position. With respect to that, my ZHL-16C algorithm ensures I stay at my set gradient factor, keeping me away from critical supersaturation and the likelihood of bubble formation, so as long as I follow that, I shouldn't be at severe risk of DCS-level bubbles. At that point, I would ask myself if I'm at a greater risk of oxtox. It would seem like I am at 20ft vs. 10ft.

So, if there's no additional advantage other than potential bubble formation, which my algorithm should accommodate for based on my conservatism settings, it would seem 10ft is the safer choice (excluding any environmental conditions) due to a reduced risk of oxtox.

And I certainly don't have it all figured out, which is why I am asking for clarification on the points I listed above. But all you did is tell me that it's 20ft without telling me why. How does that help me?

---------- Post added September 2nd, 2014 at 07:33 PM ----------

In relation to the previous point, light finning and moving wouldn't be enough exertion to worry about during o2 deco, but fighting current, or swimming around excessively could cause increased Carbon Dioxide levels....there are many, many, did I say many? Cases of oxtox during increased exercise while breathing high ppo2's. The only explanation I have seen for this is carbon dioxide.

I agree completely, and mild finning is what I'm thinking of when I say mild exertion. In currents I think your recommendation of a jon line makes a lot of sense.

---------- Post added September 2nd, 2014 at 07:40 PM ----------

Breathing O2 at 1.6 is not any more inherently dangerous than breathing it at 1.3

I was thinking about this too tonight so I'm glad you mentioned it. Thinking about this in relation to rebreather divers, I'm under the impression that a set point of 1.3 is common among RB divers. I'm also unaware of any air breaks for RB divers. So, if you're on 1.3 for most of the dive (excluding surface and initial descent, and then catchup on ascent) and aren't taking air breaks, you're effectively at 100% O2 at 10ft right?

So is an air break required if you're on a lower ppO2 like this, or do you guys actually get off the loop for air breaks or lower the ppO2 temporarily? Self-thread hijack!!!

 

[barkleyw]

Is the assumption that we want to minimize deco time also? From an oxtox standpoint 10' seems safer, but what about pushing the boundaries on the M-Values?

It seems as though it would be safer at 10' however, would we not need to better understand how the ambient pressure relates to the specific off-gassing rate along with the diffusion properties related to the off-gassing rate based on our physiological composition? I believe the assumption is the the rate off off-gassing is not dependent on ambient pressure if no inert gas is present - I am not sure if this assumption is correct. If it is then it seems that the rate of off-gassing is more related to our individual physiological composition - we do not have formulas that can specifically tell us this rate.

I think the answer lies in the most safe based on on minimizing deco, reducing the amount we push the ceiling of the M-Value and the pp oxtox factor...if we use the standard M-Value tissue compartments with an agreed upon conservative factor and weight the 3 factors mentioned we can maybe answer the question...just oxtox alone - it seems safer at 10'

Lastly - the exertion stuff is also physiological based - if we are going to add that in we better get some more scratch paper - good post - got me thinking a bit.

 

[rjack321]

If you want to reduce the "risk" of a ppO2 of 1.6 take a backgas break periodically, this is exactly what they do in a chamber - for several good reasons.

Going shallower is doing two things, letting bubbles grow (and you absolutely are bubbling) and giving those bubbles a chance to block a key blood vessel so that tissue will be poorly perfused and potentially stay bent upon exiting. Lastly the gas tension on the bubble (forcing it back into solution) is greater at 20ft than 10ft, although not by much.

Keeping bubbles small and mobile is in your best interest. But you aren't listening anyway since you are too scared of toxing at 1.6 to use it to your best advantage...

I definitely wonder who the hell taught you since you seem too paranoid to actually dive, or maybe its just the armchair logbook.

 

[dumpsterDiver]

If you want to reduce the "risk" of a ppO2 of 1.6 take a backgas break periodically, this is exactly what they do in a chamber - for several good reasons.

Going shallower is doing two things, letting bubbles grow (and you absolutely are bubbling) and giving those bubbles a chance to block a key blood vessel so that tissue will be poorly perfused and potentially stay bent upon exiting. Lastly the gas tension on the bubble (forcing it back into solution) is greater at 20ft than 10ft, although not by much.

Keeping bubbles small and mobile is in your best interest. But you aren't listening anyway since you are too scared of toxing at 1.6 to use it to your best advantage...

I definitely wonder who the hell taught you since you seem too paranoid to actually dive, or maybe its just the armchair logbook.

I like this simple answer.. keeps the bubbles smaller and moving.

Can I ask a question about using oxygen (or 80%) on deco? in this thread? Sometimes, (for probably not good reasons) I have as an objective, to minimize the consumption rate of my deco mix. I figure that when breathing pure oxygen at 20 or so feet, you are getting way more oxygen than you can really use. It seems to me that physiologically the limitation on deco gas consumption is primarily CO2 elimination.

I have found that if i take 3 or 4 deep breaths from my air tank (exhaling fully on each cycle) - which is essentially a very mild hyperventilation- I can then switch to my oxygen tank and have a VERY low gas consumption rate for a couple of minutes. I mean I only feel the need to breath maybe 3 or 4 times per minute. After maybe 5 minutes, my breathing rate seems to go up, presumably as Co2 rises and I "feel" like i can blow it back down with another cycle of deep breaths from my air tank.

Is this common knowledge or does this have some significant hazard I am not aware of? I am familiar with hyperventilation and the dangers it presents with breathhold diving.

 

[KenGordon]

I like this simple answer.. keeps the bubbles smaller and moving.

Can I ask a question about using oxygen (or 80%) on deco? in this thread? Sometimes, (for probably not good reasons) I have as an objective, to minimize the consumption rate of my deco mix. I figure that when breathing pure oxygen at 20 or so feet, you are getting way more oxygen than you can really use. It seems to me that physiologically the limitation on deco gas consumption is primarily CO2 elimination.

I have found that if i take 3 or 4 deep breaths from my air tank (exhaling fully on each cycle) - which is essentially a very mild hyperventilation- I can then switch to my oxygen tank and have a VERY low gas consumption rate for a couple of minutes. I mean I only feel the need to breath maybe 3 or 4 times per minute. After maybe 5 minutes, my breathing rate seems to go up, presumably as Co2 rises and I "feel" like i can blow it back down with another cycle of deep breaths from my air tank.

Is this common knowledge or does this have some significant hazard I am not aware of? I am familiar with hyperventilation and the dangers it presents with breathhold diving.

Hazards:

No idea of actual inspired N2 and O2. Not all the gas in your lungs gets switched each breath even with deep one, so it takes a while for your lungs to catch up with what you are breathing. A normal gas switch lasts a while so that will be less significant, but lots of back to back ones? Puff goes your deco model.

Keeping switching regulator, more risk than not. Are you doing the full process of showing the new gas to your buddy? Suppose instead of alternately breathing bottom mix then 80% you were swapping between bottom mix and 50% (or worse another bottom mix reg) that would hurt.

I was taught that breathing on deco is very important and to try not to breath too slowly, to actually think about breathing.

 

[Kevrumbo]

(27Feb 2004 The Deco Stop)

Poster: Deepstops2003
Re: last stop

Quote:

Originally Posted by Chickdiver “The Oxygen Window refers to undersaturated tissues and blood in relation to ambient pressure. The summed partial pressures of O2, N2, water vapor and CO2 are always less than ambient pressure. Tissue and venous blood are 8-13% unsaturated in relation to ambient pressure. Tissue and blood O2 tensions are metabolically reduced during each transition, and as a result CO2 increases at the tissue phase. CO2 is 21- 25 times more soluble than the consumed O2 and in accordance with Henry’s Law exerts a smaller partial pressure than the O2 it replaces. Oxygen uptake exists because tissue oxygen is always lower than the partial pressure of alveolar oxygen. A CO2 elimination gradient exists because tissue tensions are always higher then the partial pressure of alveolar CO2. Research suggests that the degree of understauration increases linearly with pressure, but decreases linearly with increases in the inert gas fraction of the breathing mixture. The predicted and actual oxygen window differs depending on oxygen extraction in critical tissues. Inert gas partial pressure in a bubble is greater than the tissue because of the oxygen window. The driving force of inert gas elimination from a bubble in tissue is based on the magnitude of the oxygen window.” From NAUI Trimix Instructor Guide- by Weinke, Neal, and O’Leary

Reply Deepstops2003:

Basically, stated with a lot of handwaving; the venous circulation always has less oxygen PP in it than the arterial, and the arterial has less than the alveolar. CO2 elimination is driven by the opposite, of course; the lowest PP of CO2 is in alevolar sacs, then the arterial (gas exchange is not 100% efficient), then of course in the venous side. The consumption of oxygen leaves an inherent unsaturation in the blood on the venous side, as is noted.

But when you are doing accelerated deco on 100% O2, the PP deficit in the alveloar sacs is as high as you can drive it; indeed the inspired gas has ZERO inert gas in it - on purpose. Therefore, the diffusion gradient across the alveloar bed is as high as is possible to achieve, and as a consequence the maximum defecit occurs in the arterial circulation. Your ability to offgas is now limited only by your individual physiology (e.g. how well-perfused are you, in general, etc.) Since inert gas is not metabolized, this deficit can be filled, to a degree, with oxygen - but there is a limit to this, as nearly all oxygen transport in the body is by the hemoglobin and not by direct dissolved gas exchange into and out of the plasma.

This effect, however, is not subject to the PO2 of the inspired gas - only to the PP of the inert gas(es), at least within the human tolerance limits for 100% O2. Thus, you will offgas on 100% O2 equally well at 10' as you will at 20'.

Where people "get it wrong" is the GI3 pronouncements about high PO2 "spikes" being somehow good for decompression. That's not really the case - what's going on is that you're reducing the inert gas inspired PP when you make the switch, and coming off a low PO2 your lungs are not loaded up due to pulmonary toxicity. Therefore, your gas exchange is at an optimum - at least for a few minutes. This is not due to the "oxygen window" per-se; the total PP of all gasses inspired and present in the plasma at a given depth, assuming saturation (e.g. you're in equilibrium) is a constant Heather - it has to be - and your metabolic consumption, and thus the moles of O2 consumed by the body is constant across depth change, varying only with workload. The only way to increase the offgassing gradient in terms of O2 unsaturation is to increase your metabolic rate (e.g. exercise, thus increasing the percentage of the O2 available that is consumed), but we know that's bad during decompression for other reasons, and besides, since most O2 is transported by hemogloben the "win" would not be linear - so that's not an option you want to entertain because if you manage to nucleate then you are greatly increasing the risk of a hit.

The misunderstanding of how this works leads people to say things like:
Quote:

Originally Posted by Chickdiver
It has nothing to do with holding a 10' stop. Deco is done at 20' because the gas gradient ("oxygen window") is most open, and offgassing most efficient.

---

Nope. There is no difference between a 20' stop and 10' stop, assuming both are made on 100%, in regards to the speed of offgassing. Zero. There is no inert gas in the breathing mix, so the gradient is the same for both. A fraction with a zero as the numerator always produces a zero result, irrespective of the denominator.

Think about it Heather. You've got inert gas in your tissues; the only thing that matters in terms of decompression is the gradient between the inert gas tensions in your tissues and that in your lungs, since the gas must diffuse out of your system and into the lungs to be expelled. Oxygen, being metabolically active, plays no part in this; other than the toxicity issues you can ignore it.

The converse of this is why Nitrox gives you more no-stop time than air does. The total pressure absolute is not what controls diffusion - it is the PP of the inert gas and its speed of diffusion (e.g. Nitrogen .vs. Helium) that does that. If the total pressure absolute controlled diffusion then breathing Nitrox would offer no benefit in no-stop times, and enriched gas would offer no decompression benefit.

If you're on 100% whether you make your last stop at 20', 10', 2' or 0' (you breathe the O2 on the surface for a while) makes no difference in terms of how quickly you will offgas, since the inspired gas contains no inert gas of any kind. Therefore, the gradient is the same, for all intents and purposes, at all three depths, as equilibrium is a zero PP of Nitrogen (and Helium, if you were diving Trimix.) (Critical tensions, however, may be exceeded at shallower depth if you haven't spent enough time doing the rest of the deco! [i.e. doing the profile times at deep stops and intermediate deco mixes like 50% at 21m/70' for example]).

Indeed, if you were to breathe pure O2 at 20' for a very long period of time (assuming that you didn't tox or suffer pulmonary effects) you would effectively "de-nitogenate" your body. You'd do the same thing at 10', 5', 2' and 0', and at roughly the same rate, as the PP differential across the lungs is what matters (ignoring potential diffusion through the skin, which while probably measurable, is not large. Indeed, its a good thing its not, or making your final stop at 20' would actually hose you, and using argon as a suit gas would REALLY screw you up.)

The downside to doing the last stop at 20' is that you have to watch CNS loading carefully, as others have mentioned.

Physics and mathmatics win every time.

_____
Indeed, doing the entire stop at 20' has some undesireable qualities, which the following will illustrate.

Go run Vplanner, set up a dive to 250 ' on 16/50 for 20 minutes, with 50% and 100% deco mixes. Set the last stop to 20'. At +2 conservatism this produces the following profile:

V-Planner 3.40 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM-B
DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2
Dec to 200ft (4) on Trimix 16.0/50.0, 50ft/min descent.
Dec to 250ft (4) on Trimix 16.0/50.0, 60ft/min descent.
Level 250ft 15:10 (20) on Trimix 16.0/50.0, 1.37 ppO2, 89ft ead, 108ft end
Asc to 180ft (22) on Trimix 16.0/50.0, -30ft/min ascent.
Stop at 180ft 0:40 (23) on Trimix 16.0/50.0, 1.03 ppO2, 59ft ead, 73ft end
Stop at 160ft 1:00 (24) on Trimix 16.0/50.0, 0.93 ppO2, 50ft ead, 63ft end
Stop at 140ft 3:00 (27) on Trimix 16.0/50.0, 0.84 ppO2, 41ft ead, 53ft end
Stop at 120ft 2:00 (29) on Trimix 16.0/50.0, 0.74 ppO2, 33ft ead, 43ft end
Stop at 110ft 2:00 (31) on Trimix 16.0/50.0, 0.69 ppO2, 29ft ead, 38ft end
Stop at 100ft 2:00 (33) on Trimix 16.0/50.0, 0.64 ppO2, 24ft ead, 33ft end
Stop at 90ft 3:00 (36) on Trimix 16.0/50.0, 0.60 ppO2, 20ft ead, 28ft end
Stop at 80ft 4:00 (40) on Trimix 16.0/50.0, 0.55 ppO2, 16ft ead, 23ft end
Stop at 70ft 2:00 (42) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 60ft 3:00 (45) on Nitrox 50.0, 1.41 ppO2, 26ft ead
Stop at 50ft 4:00 (49) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (54) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (61) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 26:00 (87) on Oxygen, 1.60 ppO2, 0ft ead
Asc to sfc. (87) on Oxygen, -30ft/min ascent.
Off gassing starts at 191.8ft
OTU's this dive: 116
CNS Total: 83.0%

Now this ain't too cool, because those CNS numbers are a bit high. I've also (intentionally) turned off backgas breaks to make a point; you'd want them on, of course, for a real dive with this profile.

Now let's tell the software that we want our last stop at 10'.

V-Planner 3.40 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM-B
DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2
Dec to 200ft (4) on Trimix 16.0/50.0, 50ft/min descent.
Dec to 250ft (4) on Trimix 16.0/50.0, 60ft/min descent.
Level 250ft 15:10 (20) on Trimix 16.0/50.0, 1.37 ppO2, 89ft ead, 108ft end
Asc to 180ft (22) on Trimix 16.0/50.0, -30ft/min ascent.
Stop at 180ft 0:40 (23) on Trimix 16.0/50.0, 1.03 ppO2, 59ft ead, 73ft end
Stop at 160ft 1:00 (24) on Trimix 16.0/50.0, 0.93 ppO2, 50ft ead, 63ft end
Stop at 140ft 3:00 (27) on Trimix 16.0/50.0, 0.84 ppO2, 41ft ead, 53ft end
Stop at 120ft 2:00 (29) on Trimix 16.0/50.0, 0.74 ppO2, 33ft ead, 43ft end
Stop at 110ft 2:00 (31) on Trimix 16.0/50.0, 0.69 ppO2, 29ft ead, 38ft end
Stop at 100ft 2:00 (33) on Trimix 16.0/50.0, 0.64 ppO2, 24ft ead, 33ft end
Stop at 90ft 3:00 (36) on Trimix 16.0/50.0, 0.60 ppO2, 20ft ead, 28ft end
Stop at 80ft 4:00 (40) on Trimix 16.0/50.0, 0.55 ppO2, 16ft ead, 23ft end
Stop at 70ft 2:00 (42) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 60ft 3:00 (45) on Nitrox 50.0, 1.41 ppO2, 26ft ead
Stop at 50ft 4:00 (49) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (54) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (61) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 10:00 (71) on Oxygen, 1.60 ppO2, 0ft ead
Stop at 10ft 16:00 (87) on Oxygen, 1.30 ppO2, 0ft ead
Asc to sfc. (87) on Oxygen, -30ft/min ascent.
Off gassing starts at 191.8ft
OTU's this dive: 108
CNS Total: 56.3%

Notice anything interesting?

First, our CNS is now well within reasonable limits. The insult to your body has been decreased. This is a good thing, all-up.

Note that the 10' and 20' stops add to the original 26 minutes.

The offgassing gradient on your 100% has not changed with the reconfiguration of the software to use a 10' stop!

That is because you have changed nothing in terms of the gradient of dissolved inert gas to inspired gas, because you can't make the inspired inert gas percentage less than zero!

Also note that the CNS exposure is cut by one third by making the last stop at 10'. This is because you are severely penalized for high PO2s, and that 26 minutes on 100% at a 1.6 PO2 is "expensive" in terms of CNS loading.

The latter profile is actually preferrable for this reason IF you can hold the 10' stop. Where its dangerous is if you can't - and find yourself at 5' due to surge. During some part of that 10' time an incursion above 10' could result in taking a DCS hit.

But the latter profile gives you an interesting option that many people miss in this debate about "which way do I run these profiles, with a 20' or 10' final stop?"

Do you see what it is?

You should. Go back and look at it again before you read on....

.............................................

Let's say you plan the dive on the latter table, but you encounter 4-6' seas when you get to the dive site. Holding a 10' stop is going to be difficult! So what? Make the final stop at 15' instead of 10'. It changes NOTHING. Indeed, once the first 10 minutes are gone at 20', you can be anywhere between 10-20' for the remainder of the deco time and you have changed nothing in terms of the total obligation you must serve, as the gradient, once on 100% O2, does not change with depth.

This is because the PP of the inert gas you are inspiring is zero. As such, the gradient does not change with depth. Therefore, the controls on your required position in the water column are the PPO2 toxicity limit (20') and the leading tissue's overpressure. Anywhere between those two points produces the same amount of offgassing over a given amount of time, and is perfectly fine.

This does not work, technically-speaking, with any breathing gas that has an inert PP component, as you will change the gradient if there is any inert gas in the mix; it works ONLY for a gas that has NO inert PP component. Therefore, its only useful for your 100% stop(s), but there it is very useful indeed, as you can make "on the fly" adjustments between the 20 and 10' depths as conditions demand, once you've done the original 10 minutes so your leading compartment pressure is ok for an ascent to 10'.

(For practical purposes it "almost" works on 80/20 too, at least for short exposures, but only because the PPN2 on 80/20 is very low. Indeed, if you do that profile and change to 80/20 instead of 100%, you will see a minute or two change in the total runtime. That's because you have some inspired inert gas, which means the gradient IS affected by the PP change with depth.)