Current best practice for deco gas: 75% vs 100%

[nadwidny]

You lot are useless really. I ask a question and all you can do is pedantically attack my question.

So, hands up who uses 100% in
1) flat calm conditions inland
2) the sea at all
3) in the sea with some swell.
4) in the sea without an 'anchor line'

As in, what conditions do you actually dive in while needing a strong deco gas?

100% O2 all the time in all of your conditions.

 

[TrimixToo]

No, this is not right. What matters (if you believe Buhlmann, Haldane etc) is the difference between the ppInert in the blood and in the lungs. The ppInert in the tissues will be something like 3 at the last stop at 6m. With o2 the lungs will be at ppInert of 0 so the delta is 3, with 50% it will be 0.8 so the delta is 2.2. So to begin with the advantage is only about 25%, as time goes on that advantage gets bigger, but eventually yo surface before it gets to 100% (ie twice as fast) due to double the delta. (I am probably wrong about the 3, but it still applies even at say 2, when the delta is 2 vs 1.2, so faster but not twice as fast)

I'm not quite there yet, which probably means I am misunderstanding something. The blood/lung gradient speeds the offgassing from the blood. The lower blood PPinert speeds the offgassing from the tissues. But the latter rates are not linear because the tissues themselves have different degrees of perfusion, yes? What am I missing?

 

[RainPilot]

Something I am curious about. On 100%, the inspired gas will be zero inert, but the gas in your lungs will not be, as the tissues offgas into your lungs and you are not exchanging the entire volume. As you breathe you will flush a fair amount of that inert gas but not all.

So, in theory, the gas in your lungs is, say, 80 or 90 or even 95% O2, does that not then mean that there is a benefit to doing the deco shallower even on 100%? Would the 10' stop not offer more gas gradient than the 20' stop?

I am genuinely curious whether this would be lost in the "noise" of our deco algorithms or whether there is a noticeable effect. Maybe @Dr Simon Mitchell or @David Doolette could help me understand this better?

 

[KenGordon]

I'm not quite there yet, which probably means I am misunderstanding something. The blood/lung gradient speeds the offgassing from the blood. The lower blood PPinert speeds the offgassing from the tissues. But the latter rates are not linear because the tissues themselves have different degrees of perfusion, yes? What am I missing?

It isn’t non linear (wrt mix) even if you only consider one tissue compartment. You don’t surface at a ppInert of zero, but at some much higher than ambient ppInert. The gradient between that higher ppInert and inspired ppInert is what matters to rate of off gassing, changing that between 2 and 2.8 (50% vs O2 at 6m given a ppInert of 2.8) only improves the rate by 40%.

The advantage may be greater for slower tissues as they have lower limits on ppInert and so the delta is smaller and so the advantage of exogenous closed to the 50 vs 100 ratio.

I fiddled about in Multideco (50/80) with a bunch of dives between 30m and 60m with bottom times of 35 to 40 mutes and final stops up to about 45 minutes and the biggest advantage I found for o2 was 4 minutes (with a 2 hour plus runtime and CNS limits exceeded) and sometimes the o2 plan was a minute slower. Changing the last stop depth between 3 and 6m for an 80% final stop also made little difference.

You also need a reasonable lost gas plan, If you lose the deep deco gas and can only get off back gas at 6m you spend quite a while on a terrible gas at 9m and the deco extends considerably.

It could be argued that the non leading tissues would have lower ppInert following o2 deco since the difference between 80 and 100 is most marked in the tissues with the smallest delta. I’d need to run plans and print them out to check. I suspect it would be very marginal.

The only people I see using 100% as a deco or bailout gas are GUE divers. The practicalities of 100% are poor, it always needs boosting, takes longer to fill and likely you will end up with a shorter fill. Perhaps if you are doing land based diving n a place with vast banks rather than half a dozen Js in a cascade it will be easier. That is not how it is here.

 

[rjack321]

This is awesome. As I am yet another year older this week, this thread makes me feel like its 2000 on techdiver.net

 

[JonG1]

80 is very popular in UK amongst rebreather divers for bailout as is 50 for the intermediate oc deco bailout.

Partly as your cns could be comnensurately higher after a fixed ppo2 dive but also with bailout often being the limiting factor in ccr dives and most impact on reserves impacts deeper gases so 80 pushes the switch deeper to improve the balance of margins.

 

[huwporter]

Something I am curious about. On 100%, the inspired gas will be zero inert, but the gas in your lungs will not be, as the tissues offgas into your lungs and you are not exchanging the entire volume. As you breathe you will flush a fair amount of that inert gas but not all.

So, in theory, the gas in your lungs is, say, 80 or 90 or even 95% O2, does that not then mean that there is a benefit to doing the deco shallower even on 100%? Would the 10' stop not offer more gas gradient than the 20' stop?

I am genuinely curious whether this would be lost in the "noise" of our deco algorithms or whether there is a noticeable effect. Maybe @Dr Simon Mitchell or @David Doolette could help me understand this better?

The actual quantity of inert gas off-gassed in to your lungs will be totally negligible. You can calculate that the total volume of inert gas off-gassed during during even a quite significant tech dive decompression is in the order of a litre or two, total, over an entire deco of a couple of hours - that is compared to roughly half a litre tidal volume every breath, say 15 litres every minute. Compared to all the other wild variables involved in our messy biology, your lungs would be so well flushed that any effect from this would be completely discountable.

Edit: Reference Nitrogen Uptake During Air Diving. Can go through the full calculation if required.

 

[leadduck]

@Germie courses should definitely be done with 2 gases in anticipation of full trimix, and while I would certainly prefer to have someone throw an O2 bottle on the hang line.

60m for 20min on 50/80 GF's.
Backgas, 106min deco
100%, 51min deco
50%, 37 min deco
50% and 100%, 31min deco.
While I'd rather have a safety bottle of 100%, or maybe 80% depending on the surface conditions, I'm certainly not carrying a second bottle to save 6 minutes of deco.

@ajduplessis Please explain how 50% is a less ideal choice than 100% for the dive profile above? Note the GFlo being 50 on that, it gets far more useful if you dive a lower GFlo like many still do

The impact of 50% vs 100% (one deco stage) on the needed amount of back gas is also considerable. In your example with EAN50 you need 2709l back gas on the bottom and 487l during ascent to 21m plus 1300l EAN50. You also want to be able to bring an OOG buddy to 21m and hence have at least 2709+2*487 = 3683l back gas. With 100% you need 2709l on the bottom too but 1570l back gas for deco and the ascent to 6m, resulting in 2709+2*1570=5849l of back gas.

That's 60% more bottom gas to take with you if your single deco stage has 100% instead of 50%.

 

[leadduck]

(...)

Personally, my reasons for preferring o2 over 80% fall under the following:

1. Best treatment for DCS, or other diving injuries except CNS oxtox, is 100%. I like to have 100% on-hand on deco dives for this case anyway, means there's one less bottle to fill.

(...)

I wouldn't be so sure that 100% is actually better than 80% for first aid. While elevated O2 supply is certainly highly recommended for DCS, hyperoxia also brings disadvantages such as brain vasoconstriction and reduced cardial output, that limits its positive effects. In life support after CPR for example lower pPO2 seems to yield better results than 100%. So I'm quite sure that 50% is better than air after DCS, and would always provide the highest O2 percentage available, but also wouldn't bet much on whether 100% vs 80% actually makes any difference and which is better.

 

[Matan]

Does anyone really give a **** if they’re at 19 or 21ft during the oxygen stop? Come on guys. Get it together.

gotta love the metric system, sound much nicer to say we don't care if 5.7m or 6.3m.
+1 for using Oxygen in ocean (Mediterranean sea), and you can start counting any gue tech2 diver out there.

Matan.