Mixes >40% O2?

[bleeb]

Trying to understand this newfangled Oxygen thingy, mostly out of interest rather than any immediate plans to dive with it.

Other than 100% and maybe 80%, what other mixes do people routinely use with O2 > 40%? Or do you not use specific mixes?

[gconner]

For deco I use 50% & 100%. I know some people use 80% as well. I've never seen anyone use anything else.

-- Gary

[lowviz]

Hey, bleeb!

Start here. I particularly recommend anything by Shreeves. :wink:

Bibliography: Nitrox (http://home.earthlink.net/~divegeeked/biblios/nitrox.htm)

[rongoodman]

50% and 100% are standard GUE mixes here and are used by many other tech divers too. I often see references to 32% and 80% on the UK sites.

[Shcubasteve]

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.

[PfcAJ]

You don't really need a "full" bottle of o2 for most dives. Even an al40 with ~2k in it is enough for 30-40mins at 20ft with a reasonable reserve.

[String]

Trying to understand this newfangled Oxygen thingy, mostly out of interest rather than any immediate plans to dive with it.

Other than 100% and maybe 80%, what other mixes do people routinely use with O2 > 40%? Or do you not use specific mixes?

Some agencies specify standard gases so you have to use those, others customise for each dive.

If you exclude trimix deco gases then id say the most common nitrox ones by a long way are 50% and 100%. 80% is also common.

If you're tailoring a dive profile specifically you can end up with 75/70 or even 60% mixes and its entirely personal preference if you want to go all that way or not.

I like 50/100 but i have used 70, 90 and other mixes before now - in some cases you dive what they can blend which isnt always the same as what you ask for or would like. O2 isnt in that abundant supply in some areas, booster pumps aren't and so on.

32% is a fairly common non-deco nitrox mix used as a bottom gas. You can/do use 32% as a deco gas but with the MOD of 40m its normally associated with a deeper trimix dive (ive used it but with He in the mix too)

[Scott L]

I am guessing the OP is referring to NDL dives. While on the question, are there folks who utilize >40% for shallow reefs, etc.?

[Peter Guy]

While on the question, are there folks who utilize >40% for shallow reefs, etc.? 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!

[rigdiver]

I've used >40% specifically for coral spawns. Pretty neat to get 90 minutes at depth without any deco.

[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 :no:

[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 :no:

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 :D

[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?

[Blackwood]

I'm not familiar with DecoPlanner (only ever used it in classes), so I don't recall an "optimum deco gas" computation.

How does that work? Does it run through a bunch of gases and pick the shortest runtime?

[ianr33]

I'm not familiar with DecoPlanner (only ever used it in classes), so I don't recall an "optimum deco gas" computation.

How does that work? Does it run through a bunch of gases and pick the shortest runtime?

Guessing you see where your first stop is, then mix a gas that has a pO2 of 1.6 at that depth.

[rjack321]

I'm not familiar with DecoPlanner (only ever used it in classes), so I don't recall an "optimum deco gas" computation.

How does that work? Does it run through a bunch of gases and pick the shortest runtime?

I got the impression Rhone Man's "optimum" aka "best mix" included cf of oxygen required in the mix so there was some sort of $$ vs. "benefit" going on.

[Rhone Man]

I'm not familiar with DecoPlanner (only ever used it in classes), so I don't recall an "optimum deco gas" computation.

How does that work? Does it run through a bunch of gases and pick the shortest runtime?

Basically yes, except it doesn't do it automatically - you have to keep running numbers until you come up with the smallest amount of O2 required - the various trade offs (richer mix means less deco time; leaner mix means longer deco time, and higher SAC rate due to greater depth).


I got the impression Rhone Man's "optimum" aka "best mix" included cf of oxygen required in the mix so there was some sort of $$ vs. "benefit" going on.

Yes. You actually get an S-curve - the absolutely cheapest way to do it is just to deco on backgas (obviously), but if you accept the use of one deco gas, you can work out the way to get yourself out of the water reasonably quickly in the most cost effective way. If you have time to keep plugging numbers through.

Just for the record, I don't actually use Deco Planner (TM). I use HLPlanner, because it is free and I am that cheap.

[Rhone Man]

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?

No buddy on that dive; 30 cu ft; 22 minutes at 180 feet (planned) on 24%; 34 minutes (planned).

It didn't form part of my deco profile or my gas planning, but I also carried 30 cu ft of 30% as my 'oh &$^#!' gas (because up to 30% was free on the boat).

If we want to turn it into a critique, a good place to start would be planning a bottom mix with 1.55 ATA.

[rjack321]

No buddy on that dive; 30 cu ft; 22 minutes at 180 feet (planned) on 24%; 34 minutes (planned).

It didn't form part of my deco profile or my gas planning, but I also carried 30 cu ft of 30% as my 'oh &$^#!' gas (because up to 30% was free on the boat).

If we want to turn it into a critique, a good place to start would be planning a bottom mix with 1.55 ATA.

Thanks just wondering what you were doing.
Weird that up to 30% was free but more than that was $2cf for O2.

[Rhone Man]

Thanks just wondering what you were doing.
Weird that up to 30% was free but more than that was $2cf for O2.

I believe that they had a membrane system which could produce up to 30%, but blending anything richer meant partial pressure blending, and importing the O2 was hideously expensive (this was in Micronesia).

[rjack321]

I believe that they had a membrane system which could produce up to 30%, but blending anything richer meant partial pressure blending, and importing the O2 was hideously expensive (this was in Micronesia).

Odd that their membrane wasn't capable of at least 32% Most can produce 36% even.

[Rhone Man]

Odd that their membrane wasn't capable of at least 32% Most can produce 36% even.
I think it could, but the way it was explained to me was that it was such a hump to do so, that they kept it set at 30%.

[kierentec]

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!

IMHO this is a terrible idea. In a situation where you would have to share air with an out of air diver, and somebody was injured, explaining why you allowed a student to breath a gas they were not certified to dive might be tricky... Just my 2 cents....

[PfcAJ]

IMHO this is a terrible idea. In a situation where you would have to share air with an out of air diver, and somebody was injured, explaining why you allowed a student to breath a gas they were not certified to dive might be tricky... Just my 2 cents....

Beats having to explain why there is a dead diver...

[kierentec]

I dont get it... Who would die if the instructor were diving air and used that to donate??

[Peter Guy]

Who would die if the instructor were diving air and used that to donate??
The instructor would die from DCS after doing all too many CESA attempts!;)

Kierentec -- Your point is, in fact, an interesting one that has several different variations.

a. I don't dive Air -- I dive 32% all the time. Given your scenario, would you be concerned that an OOA student, now suddenly breathing 32%, would have a cause of action for the injury? (BTW, putting aside the notion that breathing high PPO2 mixes could cause an injury, excluding O2 toxing issues.) What possible injury could be caused by virtue of breathing a mix as opposed to Air?

b. Diving trimix when doing deep dives with students who are not trimix certified. I, for one, am quite concerned about diving Air/EAN below 100' -- especially if I am responsible for overseeing a student at a depth down to 130' during a Deep Specialty. I don't think it is safe NOT to be on 25/25 (for example) -- but would I be doing someone "dangerous" by giving a narced OOA student 25/25 instead of Air when I was also narced?

c. I still think the benefits to me of a high O2 mix, the instructor who may be creating a personal dangerous situation by virtue of many ascents/descents, outweigh the potential theoretical liability issues from a student breathing the same mix. And I say theoretical because I can't think of any real liability issues from having a student breathing a mix that is well within the MOD.

[kierentec]

You're right, you would be well within acceptable po2 levels, and would most likely not cause an issue. 32% is much different than 50%, however. PADI allows a nitrox instuctor to let ow students dive nitrox on dive 4 anyways. 50% is beyond recreational limits, and if you are not a tech instructor yourself, then you should not be giving it to anybody. The issue would be during litigation when every detail of the situation was being picked apart... Just one more thing to defend yourself against. Also, if you are doing that many rapid ascents consistantly during training sessions, I would think you already have enough to worry about...

[fdog]

...I think that Peter is better equipped than you or I to assess potential exposure to litigation...

Peter, like you, I believe in taking the best tools for the job, and gas selection falls into that category. OW class dives w/ multiple ascents? 40%. And for the deep dive in AOW, 25/25.


All the best, James

[rjack321]

I'm always concerned by courses where the instructor is unwilling to dive the same gas as the student. If there's a "better choice" then the student should be on that mix too. If the agency doesn't support that choice then find an agency who's standards are aligned with the way it "should" be done.

[kierentec]

No, absolutely not an attorney, which may be why I'm a little more cautious about getting sued. Never been in court, don't really plan on it, either. rjack makes a good point. If it's not safe for you to dive air, then why would you allow your students to do so?

[fdog]

I'm always concerned by courses where the instructor is unwilling to dive the same gas as the student. If there's a "better choice" then the student should be on that mix too. If the agency doesn't support that choice then find an agency who's standards are aligned with the way it "should" be done.

It's not that there is a better gas choice for the student; it's better for me.

The students don't have to yo-yo up and down 8 times or more on the dive doing CESA training and Air-Share ascents and the like. I do. I got tired after dives like this, even with slow ascent rates; after I switched to a richer mix, I have felt better ever since.

I could breathe air, just like them, but why beat up my body like that?

As far as 25/25 goes, there is a learning lesson in taking students on air to 100'-130' for their deep AOW dives. Getting narced in a controlled situation makes for better dive planning in the future. I could breathe air or nitrox, just like them, but I see no benefit in being as drunk as them, and actually an advantage in being sharp.


All the best, James

[rjack321]

I know what you're saying and to a certain extent agree on the richer nitrox mix since you are not doing the same dive(s) as 1 student. You're bouncing up and down with a larger class. Of course you could have smaller classes and bounce up and down less that way too.

As far as 25/25 goes, I use it and other trimixes all the time. But I'm not really a fan of "let the students get narced so they can see what its like". AOW students diving to 100ft naturally push a little deeper and the wise ones figure out their personal limits in a measured way. If they aren't very prudent students one dive to 110-120 isn't going to change that, especially if the come up concluding they "weren't narced". Besides its not that hard for instructors to induce preceptual narrowing and other narcotic symptoms at 90ft with a little task loading and CO2 buildup. Although some agencies (again) may not support these kinds of lessons in classes.

[String]

Odd that their membrane wasn't capable of at least 32% Most can produce 36% even.

Might be a time factor. Membranes take longer and longer the higher the % mix required. Its not linear.

34% for example can take a LOT longer than 36% to blend and so forth.

So if they've got lots of tanks, a short time to fill then it might not be possible to blend rich.

[String]

IMHO this is a terrible idea. In a situation where you would have to share air with an out of air diver, and somebody was injured, explaining why you allowed a student to breath a gas they were not certified to dive might be tricky... Just my 2 cents....

Its a complete non issue.

Question:- is the gas safe to dive at that depth? Answer is yes
Was it an emergency situation? Answer is yes.

You're not going to get grilled over using a safe gas in an emergency situation!

Quite a lot of instructors teach on nitrox (if they get it for free!). The students aren't nitrox qualified. Despite that i don't hear of court cases with unqualified students using nitrox 32 sharing air with an instructor on the AAS ascent and so on.


I'm always concerned by courses where the instructor is unwilling to dive the same gas as the student.

The student isn't doing 3-5 dives a day with numerous ascents/descents chasing students and repeating that sort of day for 3-4 weeks in a row with no break though!
In the same way a student maybe happy in a 5mm wetsuit while the instructor is in a drysuit - he isnt in all day, every time for an extended time period.