Nitrox theoretical question

[soltari675]

This came to mind in another thread, and I asked it there, but the thread was dying off anyway, so I only got 1 or 2 responses. Probably good to not hijack that thread anyway. So here goes.

Let's say I am diving EAN36 or some mixture close to. I am very close at my max depth for that mix. I have a pony bottle with me that is just plain 21% air. No big deal. (I don't actually get that close to my max depth on nitrox, but this is just a what if scenerio).

So while swimming along, I get caught in a strong down current. I quickly drop to and past my max depth. Would it be safe to switch to my pony bottle, long enough to get out of the current and back to a safe depth? Or would having been breathing EAN36 and having a higher oxygen air running through my system still be a major problem? I know it is possible to swim out of the current and ascend, etc. But I am just curious about the air and if it is safe to do something like that.

 

[Stoo]

Assuming your "maximum depth" calculation is based on the traditional 1.4 ata, I can tell you that exerting yourself in a current with an elevated p02 is potentially a short ride to "dead". The magic p02 where 02 becomes a diver's downfall will vary with the individual, but I am personally aware of one fatality last year where the deceased had (if I recall correctly) a p02 of just under 1.5, but probably while exerting himself somewhat, in cold water.

The MOD of a Nitrox mix isn't something to be taken lightly, ESPECIALLY during a time of exertion. It's called a MAXIMUM Operating Depth for a reason.

So the answer to your question is you would be better to go on to your air... but remember that it will take a little time to reduce your elevated p02 to a safer level. And of course your decompression profile will be altered as well...

 

[t-mac]

I think your question assumes you realize this before you reach your MOD and your are wondering about the O2 still in your system. Is that right? In any event I would think you'd be better off trying to switch to air until you get back above your MOD. As you point out you will still have extra high levels of O2 in your system but your body is consuming it so once you switch it seems to me your risk starts dropping not only because your are diluting the O2 in your system but because you are using it up. Whether this does any good probably depends on how fast you switch (among other things like your susceptibility and whether your pony will last long enough). Hope I understood and that this is helpful.

 

[Rhone Man]

Sensible replies. The other thing to add is that if you go below the MOD it will not cause you to suddenly explode. I have certainly done plenty of dives where I have dipped below the MOD and had a ppO2 of 1.7 ATA or so for short periods - early in the dive when your accumulated oxygen is small that is pretty low risk. Not something you want to do too often, but if you can get back above the MOD reasonably expeditiously, I would focus on doing that rather than fiddling with the pony bottle whilst stuck in a down current. Conversely, if it is towards the end of the dive and you have already been breathing O2 at a high partial pressure for a long time this is a great concern. But to be honest, unless I was a long way past the MOD and/or I felt the onset of any symptoms I'd probably just try to focus on just getting shallower.

The other point to bear in mind is that if you are in a down current where you are not going to be able to get back above your MOD within (say) 5 minutes or, you are probably in some pretty serious do-do. Pony bottles don't last long at depth under serious exertion.

 

[Stoo]

The nasty thing with an 02 tox hit is that there are no warning signs... And that susceptibility varies greatly between individuals, and within one individual depending on any number of factors... exertion, previous 02 exposure (as you mention), dehydration, thermal stress... Hell, probably even what you had for breakfast!

 

[boulderjohn]

Remember that O2 toxicity is caused by a combination of high PPO2 and time. If you were to shoot to 40 feet past your MOD and stay there for a minute or two before getting back up, the odds are excellent that absolutely nothing would happen. If you are at all unsure, switch to the air bottle by all means. On the other hand, you are right that switching mixes is not going to have an instantaneous effect. If you are going to hang around at depth making the switch, you are probably better off just swimming up.

 

[soltari675]

Thanks guys! Yeah this all answers it. I was wondering if it takes a while for the elevated o2 in your system to go away, and if so, would it be fast enough that a switch to air while swimming away from the down current would keep you safe. As I understand from the nitrox class, an O2 tox hit is pretty sudden and often without warning. And also noting that I don't stop to switch to my pony. I know exactly where the reg is and would do it while swimming, wasting no time. Like I said, I don't get that close to my max depth. It was just a theoretical question of "what if?"

 

[Centrals]

There are only couple of things you can do as other had already suggested.
But what would you do if you did not have the pony?

 

[weaserm99]

Id stick with the EAN mixture but im going to explain the problems that i think that could arise by switching over to the air tank.

OK here are the issues that i see with making the switch from the nitrox mixture to the air mixture.

1. If you are using a computer unless you have a computer that can switch gases it will screw up the nitrogen loading if unable to tell the computer that you are now diving air. This can cause issues as what the computer thinks you are at and where you are at are two different spots.

2. If table diving did you calculate the switch into your dive plan.

Now I know this is a hypothetical question but im gonna nit pick a little bit. If when making your dive plan why did you not choose a lower mixture say of 32, 33, 34, 35 percent of oxygen, this way you do not have to worry about coming close to your MOD.

Next question, why if you know your MOD are you venturing as close to it as possible if you are worried about an o2 hit.

These are just the hypotheticals that i throw out there.

 

[TSandM]

It only takes two or three minutes for the blood to equilibrate with the new breathing mixture. It probably takes the tissues a bit longer, but we are almost certainly talking periods in the single digits.

However, I'd only go through the effort of switching if I were plummeting in a serious way, or on a high O2 mix. Dropping twenty feet below MOD on 32%, when I intended and expected to correct that error immediately, wouldn't motivate me to take my attention off correcting my depth long enough to change gas.