Mixes >40% O2?

[ajduplessis]

An argurment for the use of 80% is that, without a booster, the max pressure you'll ever get is 2250+- out of a FULL o2 cylinder. At 3000 PSI, you have to add 2220psi of o2 then top up with air to get 80%. You could spend tons of $$$ on an oxygen booster, or dive 80% with a standard compressor.

Some WKKP members would refer to "stroke mix" in this case

 

[fdog]

I'm often tempted to use 50% when I'm teaching an OW class since I tend to have to go up/down so often!

I'm not tempted; I actually do use 40% on the dives with lots of up-and-down.

I feel better. Seriously.


All the best, James


PS: I, too, decline using 80%. I'll stick with 100%, 50% and 35/25.

 

[String]

Some WKKP members would refer to "stroke mix" in this case

Whereas the others will go diving with it and emerge safety. In some places you dive with what you can get. If O2 supplies and boosters are lacking then its 80% or more diving.
Most sane people would opt for 80% in that situation.

 

[fdog]

Gotta agree with that - if 80% (or 75%) is all I can get, I'll take it. It's the price for diving interesting, remote places when oxygen comes from a membrane system.


All the best, James

 

[HowardE]

Generally I use 50% or 100% because they are readily available in my area. 50% is pre-mixed even.

Anything greater than the % of O2 you are using on your bottom mix will accelerate your decompression somewhat. It really depends on the "cost benefit analysis" of carrying other gasses for me.

Really, you have to run the numbers to see if 80% is really even better to use than 50%, since you can get on the 50% sooner than 80% you may have less deco all together just using 50%. Also - you need to figure out if 2 gasses will benefit you enough to go through the hassle of carrying two or more gasses. If you only save 4 minutes of deco by slinging 2 bottles... that doesn't seem worth it to me.

 

[rjack321]

Whereas the others will go diving with it and emerge safety. In some places you dive with what you can get. If O2 supplies and boosters are lacking then its 80% or more diving.
Most sane people would opt for 80% in that situation.

Actually the reason 80% is almost never needed are way more complex than this. Especially in the ocean.

GUE/DIR people for the deeper dives will use 21/35 bottom mix (at least) and 50% as the first deco gas. It reduces rock bottom and gets you onto the deco gas sooner as well. And for short exposures with mostly fast tissue accumulation, along with the helium in the mix, the 50% makes perfect sense to eliminate the He fraction deeper than 20-30ft.

Only for the longer exposures would a second deco gas (100%) get added and this times on each gas will be roughly equal. So there is little or no need for massive quantities of shallow-ish deco gas per a Buhlmann schedule. Going from EAN50 at 70ft onto EAN80 at 30ft is just bringing along a second bottle for marginal benefit and by the time a 21/35 diver has ascended to 30ft on EAN50 the only gas left to eliminate is nitrogen anyway, most of the helium has already been eliminated.

If you can make 80% by PP you can obviously make 50%. So 40-80cf of 50% and 25cf (an AL40 filled to 2000psi) are sufficient for up to an hour of deco. Beyond that, its not too bad to carry an al80 of O2 filled to 2000psi (50cf). This pretty much covers the range of normoxic-ish trimix ocean diving. (100-240ft).

If you are doing real hypoxic trimmix diving (12/65 etc), you pretty much need a booster to handle the helium fraction anyway. So IMHO "most sane people" get 50% first, then lessor pressures of 100% aren't a big deal.

Those still diving "deeper" air or lighter trimixes frequently use EAN32 and EAN80 deco mixes but they are running completely different shaped profiles than those of use diving 21/35, 18/45 etc trimixes.

 

[robertarak]

I dive 40% for teaching OW.

 

[ajduplessis]

Whereas the others will go diving with it and emerge safety. In some places you dive with what you can get. If O2 supplies and boosters are lacking then its 80% or more diving.
Most sane people would opt for 80% in that situation.

I would rather take 100% at a lower tank pressure than 80% at a higher. My 2 cents

PS: Nobody said 80% was unsafe. It was a "joke" referring to Mr Irvines comments

 

[Rhone Man]

It really depends on the "cost benefit analysis" of carrying other gasses for me.

Really, you have to run the numbers to see if 80% is really even better to use than 50%, since you can get on the 50% sooner than 80% you may have less deco all together just using 50%.

I was recently on a liveaboard where they mixed rich mixes at US$2 per cu ft for O2. I used my deco planner for the optimum mix for a single deco gas, and then asked them to blend me some 64%.

Got some funny looks, but it was all good.

 

[rjack321]

I was recently on a liveaboard where they mixed rich mixes at US$2 per cu ft for O2. I used my deco planner for the optimum mix for a single deco gas, and then asked them to blend me some 64%.

Got some funny looks, but it was all good.

You convince your buddy to dive the same mix and switch at 50ft? How much 64% did you bring? What was your profile and bottom mix? How much deco did you have?