O2 at 80/20 or 100?

[rcohn]

I had the opportunity to ask Dr. Bill Hamilton at a talk last year what he thought of the 80/20 vs. 100% deco. He replied that he believes in doing what works, and 80/20 and 100% both work.

Ralph

 

[Dr Deco]

Dear Readers:

Decompression – The Differences

Once Dr Hamilton remarked (I am told)’ “It is more important that you do some decompression, than what kind of decompression you do.” By this he meant (I believe) that the actual nature of the decompression is less critical than the general, slowed ascent.

Decompression is not like a lock and key, it is not like a downhill slalom race. There are not any critical points, no gates through which you shoot. Certainly, if you had a decompression schedule that was developed especially for you, it might be that critical, but for the real-world schedules, with their built in conservancy, that is simply not realistic. Real decompression is not threading a needle. That is why the various decompression meters and tables all work – there is a lot of “wiggle room” in real tables.

Now, that is not to say that you can simply toss the meter aside and expect it to work. It will go into some type of a default mode. If you wish to do this – and I do not necessarily recommend it – do not dive with a computer. Use a table.

The entire system is meant to be a road map and not a license. Back in 1988, I had a talk entitled “Decompression Tables – Road Map of License?” It had as its message the concept that tables showed you one decompression scheme. It did not purport to be the only scheme or necessarily the best one. What a diver did was under his control and these tables did not assure freedom from the DCS.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[TheDecoStop]

Something else that should be looked at when trying to determine if 80/20 or 100 is better for your particular dive is the rapid CNS runup that you are going to get while on 100%. I know a few great divers that don't use 100% at all just for that reason alone. I also know a few that say it is a bunch of crap... but still something to pay attention to.

TDS

 

[fins wake]

O2 at 20 feet has a higher partial pressure (all that matters) than 80/20 at 30 feet

Really? No, not really. It is exactly the same partial pressure of oxygen, ppO2 1,6. Perhaps all that matters.

The one reason for using pure oxygen at 6 metres (20 feet) is that 80/20 at 10 metres (30 feet, well, actually 9 metres in imperial measurements, please get on board with the rest of the world, folks!) is that if you have any nitrogen in your bottom mix, e.g. air, you'll still be inhaling more of it with 80/20 but not with pure oxygen, i.e. the pressure gradient is more favourable. If it matters ... (and this is where the learned fight, boys and girls!).

but for the real-world schedules, with their built in conservancy, that is simply not realistic. Real decompression is not threading a needle. That is why the various decompression meters and tables all work – there is a lot of “wiggle room” in real tables.

Bingo! Read and re-read! This is what it is all about! Stay on the safe side and ...

Once Dr Hamilton remarked (I am told)’ “It is more important that you do some decompression, than what kind of decompression you do.” By this he meant (I believe) that the actual nature of the decompression is less critical than the general, slowed ascent.

... se how, amazingly, so much general wisdom was actually contained in our first OW class! Go up slowly, take a safety stop if within NDL:s. Don't move about, don't exercise or lift tanks. Relax.

Personally I believe this is more important than which mix you use at 6 or 10 metres for most of us. But hey, it's a personal opinion.

 

[jonnythan]

fins wake once bubbled...
Really? No, not really. It is exactly the same partial pressure of oxygen,

1.53 != 1.61

Also, the pp of N2 at 20 feet on O2 is 0, whereas the pp of N2 at 30 feet on 80/20 is .4

 

[fins wake]

1.53 != 1.61. Also, the pp of N2 at 20 feet on O2 is 0, whereas the pp of N2 at 30 feet on 80/20 is .4

Yeah, this is why I had my *****y - but true - earlier statement about you yanks actually joining the rest of the civilized world ... uh, as in all of it (!) in the SI metric system.

We learned, civilized Europeans (yeah, I'm sarcastic!) always talk in bars and metres, not in PSI:s and feet. That's ... medieval ... y'know, like Bruce Willis with a Samurai sword in some LA cellar? (Hey, I think Bruce is under-rated, so this isn't against him! All righ, Bruce?)

For us, it's so simple. Using the famous 'diamond', 6 metres gives 1,6 x 1 = 1,6 ppO2. 10 metres is 2,0 x 1 = 2,0 bar. (That's using pure oxygen BTW.). 10 metres at 80/20 is 2,0 x 0,8 = 1,6 bar ... voilà! I can't help you're still stuck in the old imperial system which even the British have discarded ... (they have retained the stupid driving on the wrong side of the road-system, though!)

In short (and trying not to be sarcastic or testy), when we start measuring in the same units, you'll get my point. The ppO2 is the same, the N2 gradient is not. Here endeth the lesson. :boom:

 

[fins wake]

... anyone gets really clever, yes, in the true SI system, we don't talk bars, but pascal.

Okay, so we twist the rules. But we're European, and we have better cheese (but worse wine, the Californian wine is IMHO the best in its class in the world! Honest! And don't tell my French friends, okay!)

In any case, at least we twist the rules so that it becomes easier to do the maths.

 

[TheDecoStop]

Something else that should be looked at when trying to determine if 80/20 or 100 is better for your particular dive is the rapid CNS runup that you are going to get while on 100%. I know a few great divers that don't use 100% at all just for that reason alone. I also know a few that say it is a bunch of crap... but still something to pay attention to.

TDS

 

[BIGJC]

O2 can be a double edged sword, that is true. But if you plan the dives conservatively than it should not be an issue. The CNS exposure does increase with the use of O2 but the length of the stop decreases.. in most cases this causes a "wash" as far as CNS exposure... but on really long hangs you may need to rethink your bottom mix.....good point decostop.

O2 has a more favorable partial pressure at 20 feet (6m whatever) than 80/20 does at 30 feet (10m.. Notice I have included the metric conversions!?!?)... that means that you are scrubbing out more inert gas from you tissues on O2 than on 80/20... this is a good thing, it is also the reason why you can throw out your 10ft (3m) stop and calculate all deco to end at 20ft(6m) without fear of injury. I believe the only reason to use 80/20 is if you do not have access to a booster and cannot get high pressure O2 fills... then you may have no choice....

as far as the french go... well thats another story.....

:viking:

 

[Doppler]

I'm only aware of one current decompression software program that takes this into account (v-planner (VPMB)) so several older packages will give you the same run times -- or shorter -- using 80/20 rather than 100%. Doesn't change the "real world" benefits of pure oxygen... in my experience.

We also tend to loose sight of the issues when talking about deco gases... standard mixes deliver many, many wonderful benefits, but all sorts of mixes "work." If you feel more comfortable diving 80/20, go ahead... just make sure all your team is using the same stuff. Horrible wake-up call to see one person swapping gas at 30 feet when everyone else is using oxygen...