PDA

View Full Version : Breathing mixed gasses



DarinMartell
03-04-2008, 22:31
I pretty sure I know what the answer to this is but I was wondering if it is an issue for a person who was not trained in mixed gasses to breathe from a mixed tank? My thoughts are about an Out of Air situation when one buddy has mixed gas and the other does not. I know any air is better then no air, my question was more about if any different precautions are needed after breathing it.

Scubastud16
03-05-2008, 01:41
Well if your buddy is on EANx, the only thing you want to worry about is being below MOD (maximum operating depth), but if he's past it, you're both probably going to take an ox-tox hit.

Trimix is a whole different story, but tech divers rely much more on themselves than others. (total generalization as to not get into the specifics :D )

CompuDude
03-06-2008, 01:56
Well if your buddy is on EANx, the only thing you want to worry about is being below MOD (maximum operating depth), but if he's past it, you're both probably going to take an ox-tox hit.

Trimix is a whole different story, but tech divers rely much more on themselves than others. (total generalization as to not get into the specifics :D )

A little over-simplified, but a decent summary nonetheless. :smiley20:

cummings66
03-06-2008, 07:05
As a Nitrox diver I don't worry about donating the mix to my buddy, after all I won't be below my mod and so he won't either.

The important knowledge a Nitrox class teaches is how to safely dive with Nitrox, you can assume your buddy using it is safe and thus you are if you share it. It's a trust me situation, but really, what other option do you have?

UCFKnightDiver
03-06-2008, 10:50
If you take the class you can figure our or at least be familiar with the mods of a certain mix that way if your buddies diving it you can still know :) 32% a pretty standard mix and the mod(maximum operating depth) is around ~111 (thats 1.4 ppo2)

I would suggest taking the class as it is imo a very good class to help you learn about nitrox but to also help you better understand some of the theories behind diving.

texdiveguy
03-06-2008, 11:39
A basic nitrox class is fun and fairly informative....not alot of $ or time.

Miked
03-06-2008, 11:55
If you take the class you can figure our or at least be familiar with the mods of a certain mix that way if your buddies diving it you can still know :) 32% a pretty standard mix and the mod(maximum operating depth) is around ~111 (thats 1.4 ppo2) the equation is ATM+1*PPO2%=MOD

I would suggest taking the class as it is imo a very good class to help you learn about nitrox but to also help you better understand some of the theories behind diving.


Rox, no offense intended, but while your MOD for a 32% mix is correct, I think the formula you showed is for the one for calculating the partial pressure, rather than the MOD.
IIRC, to get the mod, we would do a 2 stage calculation:

1) [(max "safe" ppo2) /(% mix as a decimal)]= ata (absolute pressure, in atmospheres)

2) (ata -1) * 33= MOD

to illustrate: if you wanted to use the 1.4 as your max safe ppo, for a 32% mix, the calculations would go something like:
1) (1.4/.32)= 4.375= ata
2) 4.375-1= 3.375; 3.375 *33= 111.375'= MOD

IIRC, from nitrox class, they created a single formula that combined these 2 steps:

{[33* "safe" ppo]/[mix ]} -33 = MOD

but the instructor said that the 2 -step process is simpler to keep track of.

To Darinmartell, sorry for the detour in your thread for all this Math stuff. :)

UCFKnightDiver
03-06-2008, 11:58
oops I think your right my bad also the equation for pp02 is still wrong lmao I deleted it anyways but it should have read (atm +1)*mix=PPO2 but dont mind my equation lol can I blame it on the 3 hrs of sleep last night:)??? stupid roommate

It is a good idea though to know your buddys mod of the mix he is using.

MSilvia
03-06-2008, 12:54
It's generally a bad practice to buddy with someone using a different mix than you are, as it will almost certainly mean you're diving different profiles. If it's time for you to surface, and your buddy still has additional bottom time alloted (or vice versa), you're put in the position of either being a bad buddy and separating, or wasting the money and advantage inherent in diving mixed gas.

Rather than wonder if it's safe to use the gas your buddy is using, get a buddy diving the same gas you are, so there isn't any misunderstanding or confusion. At the very least, make it part of your pre-dive buddy check. That said, if the only gas available is from a diver using a different gas, what choice do you have but to breathe what they give you? So long as you let the donor donate, and don't just grab the first regulator you see, you should be okay. If you do get grabby with a mixed gas diver, you might indeed get a mix you shouldn't be using, and central nervous system toxicity convulsions may result. That, as they say, "will learn ya".

If you want to understand the dangers, benefits, and other implications of what you breath, txdiveguy's right... take a nitrox class and find out. They're easy to find and usually short money.

Rockhound76
03-06-2008, 15:44
It's generally a bad practice to buddy with someone using a different mix than you are, as it will almost certainly mean you're diving different profiles......

What he said...Never dive in "mixed pairs".

( just deleted the rest of this, as it is a repeat of most of what MSilvia said. I should have read his post more closely.)

JahJahwarrior
03-06-2008, 15:53
Grabbing a reg off another (unknown) diver in a panic might have really bad consequences. A stage or deco bottle at depth or trimix or an unused "deep mix" with a low PPO at shallow depths would be an ugly accident waiting to happen.

This is why many divers advocate never taking a bottle past its MOD. Nothing you can do about hypoxic trimix about it's minimum operating depth, except put something over the mouthpiece.

Of course, I would argue that letting anyone who doesn't know that they can't use air from certain tanks at certain depths, dive with someone who carries stages, deco mixes or uses hypoxic trimix, is a bad idea. Only dive with people who aren't likely to kill themselves.



Personally, if you are diving air and your buddy is diving 32% or vice versa, I don't see the problem in an OOA situation. You surface immediately, neither of you will be in any problem. The only place it gets dicey is if your 32% buddy was near NDL's and moves to air, but that would mean that you, breathing air, ought to have a deco obligation already. Have both of you breathe your gas for the length of your stop and surface. Both of you should have plenty of air to allow for this if you follow something like the rule of thirds.

My primary dive buddy breathes 32% while I breathe air.

Simplest way to avoid any issues is to never put yourself in an OOA situation. I mean really, we talk about it all the time, but what would really make you be OOA? As long as you follow rules like the rule of thirds, you should always have enough air to support yourself, correct?

MSilvia
03-06-2008, 16:11
My primary dive buddy breathes 32% while I breathe air.
So long as you both dive the more conservative of the allowable profiles, that's fine. I assume your buddy is using EAN to be conservative, and not to extend his bottom times?

texdiveguy
03-06-2008, 16:31
I am posting this link for those not totally familiar with or those wishing a brief review of the basics of CNS 'OXTOX',,I might suggest this is a good start of your reading, though other sources of good information about this important topic are available this is just one that reads easy.

DAN Divers Alert Network : OXTOX: If You Dive Nitrox You Should Know About OXTOX (http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=35)

doczerothree
03-06-2008, 16:54
I'd be cautious sharing gas in situation like the one you have discribed. I can't think of why I would hand an OOA diver my one of my deco bottle below thier MOD.....:smiley11:.....

cummings66
03-06-2008, 18:37
I think a lot of people read things into the question that I don't believe were there. I assumed that this was a normal rec dive and that the diver using the mix was essentially using it as his backgas and not deco so that meant that if it was safe for him it was safe for his buddy. We also were told in the OP that it was the OOA air diver who needed to breathe Nitrox.

Certainly you can find situations where you'd have a mix that would cause tox but you wouldn't be using it then either. Now if we tossed in a deco dive and such it's not a rec dive anymore and it's not likely this would happen.

IE both of you doing a deco dive would have the training for it. See the key part of the question is, the buddy was not trained for it. A deco diver is not going to buddy up with a non deco diver. Therefore the non trained diver is not going to get a mix that will make them tox.

In other words, if I'm breathing a Nitrox mix and am buddied with an air breather which I often do, you can assume that I'm watching out for my life and as part of my predive brief we'll talk about tox, we'll talk about mod. We will not be going below the mod so that means it's safe for me and by extension safe for the air diver. If it's not safe for him it's not safe for me either.

JahJahwarrior
03-07-2008, 00:01
If the EAN diver isn't close to a tox, then the air diver who switches to EAN should be nowhere nearly as close to a tox....he won't have as much oxygen saturation....right?

So no worries of MOD, no worries of tox, no worries at all. Except the fact that your buddy let himself go OOA...

Scubastud16
03-07-2008, 00:08
I thought it was all about the ppo2 you were breathing the gas at?

DarinMartell
03-07-2008, 00:47
Thanks for all of the replies! The reason I was wondering is because I know a Nitrox class is on my list. However, my buddy that I got certified with has not cought the bug like I have and we will probably only dive together a few times a year. I am very safety minded and am expecting my ST 30cf pony any day (sold my Spare Air after reading things here) So I am confident that I won't run out but you sometimes don't know about your buddy. I know this will all be answered when I take the class but I appreciate reading things from different perspectives.

cummings66
03-07-2008, 07:04
I thought it was all about the ppo2 you were breathing the gas at?

It is, but you won't be below your mod no matter what so your buddy won't below the mod if you share.

cummings66
03-07-2008, 07:07
I am very safety minded and am expecting my ST 30cf pony any day (sold my Spare Air after reading things here) So I am confident that I won't run out but you sometimes don't know about your buddy.

I'm sure we discussed it when we talked about the secret handshake, but I've forgot for sure what was said in that regards and now just have my opinion.

Regarding your pony, keep air in it so that you never have to worry about it's depth and whether or not you can use it.

Rockhound76
03-07-2008, 07:51
I'd be cautious sharing gas in situation like the one you have discribed. I can't think of why I would hand an OOA diver my one of my deco bottle below thier MOD.....:smiley11:.....


It ain't the "handing them the mix" that's the problem. It's all the "grabbing" of whatever they can see--especially if it's someone else's buddy you never saw until they grabbed you, grabbed the reg, mask and anthing else they could get (hint: been there, done that. I never saw the guy until he was on my back).

However, I agree: you should avoid carrying any mix deeper than MOD as a matter of "normal" procedure. That would avoid the problem, altogether.

Rockhound76
03-07-2008, 07:53
If the EAN diver isn't close to a tox, then the air diver who switches to EAN should be nowhere nearly as close to a tox....he won't have as much oxygen saturation....right?

So no worries of MOD, no worries of tox, no worries at all. Except the fact that your buddy let himself go OOA...
That should have been my first answer. Short and to the point.