If you could change one thing about dive training...

[Edward3c]

I was taught to use my BCD as a last resort if I had an OOA situation.

When the negative aspects of breathing off a BC/ABLJ became known BSAC dropped the skill from the syllabus. Its history came from the days before SPGs and when a dive was terminated when divers went onto their reserve.

 

[wetb4igetinthewater]

After all how many recreational divers bring a pony bottle with them?

I have no idea about the percentages of divers who use pony bottles or are in twinsets/sidemount. It makes a lot more sense to have redundant air (something I advocate in all my courses as buddy separation does happen, even with highly trained DIR folks).

To me, redundant air is a kind of seatbelt.

 

[TMHeimer]

Wow! Run out of air at 100’ and have to breathe off your BC air to make it to the surface.
I would think the prudent thing to do would be to train divers extensively (drilled into their heads) to check their air supply and keep their stuff serviced and in good shape. Buddy or no buddy, no excuses!
I wonder if pilots are trained to look at their fuel gauge and keep their planes maintained, or do they just carry a parachute and have to jump every once in a while?

Well of course Eric...We all know how divers should be trained and that they shouldn't do stupid things like ignore their gas supply. What I'm talking about here is some unforseeable catastrophic failure that would put someone in that position. Maybe an actual problem with the reg and it is that one in a million times it doesn't free flow. Maybe some sort of entanglement--I can't think of other possibilities. But it's like a CESA from 30 feet-- you SHOULD never have to do that, but it's nice to know.
If someone is stupid enough (or has narcosis) at 100' to not constantly be checking remaining gas they are probably not swift enough to consider breathing from the BC to ascend anyway. They may be diving solo. Problem solved. Darwin.

Now, re never breathing from the BC...
This is something, as I said, I would never practice. CESA OTOH, is something I may do from 20-30' on every other dive, simply because I usually ascend at some point to check my exact location, so why the heck not. AND obviously, I know how to do it safely.
But, I have to figure (and may well be wrong) that if you're down 100+ feet and OOA yes, you can do a CESA. And the air in your lungs will increase in volume. But, I think it may be that you can't inhale any fresh O2 for your tissues. That is bad, depending on how slowly you ascend. The urge to breathe caused by C02 may also cause you to think "hey, what about the BC air?"
All irrelevant stuff if you are prudent and check your gas supply--and your gauge is not stuck on a number, like mine was once stuck on 1000 PSI. It was good I was doing my usual shallow diving that day.

 

[boulderjohn]

The urge to breathe caused by C02 may also cause you to think "hey, what about the BC air?"

Hard to inhale when air is pouring out of your mouth, which is what happens on deep CESAs.

Practice submarine escapes have been done from 300 feet. Here is the practice tank from 100 feet.

 

[kmarks]

But, I have to figure (and may well be wrong) that if you're down 100+ feet and OOA yes, you can do a CESA. And the air in your lungs will increase in volume. But, I think it may be that you can't inhale any fresh O2 for your tissues. That is bad, depending on how slowly you ascend.

When I did open water, we were taught that you shouldn't ascend faster than 60 ft/s. That has since been revised to 30 ft/s, but if the sh!t hits the fan when you're at 100 feet, ascending at 60 ft/s gets you to the surface in a minute and forty seconds. Your body can easily handle less than two minutes without fresh oxygen. I can hold my breath right now for two minutes.

And I think John mentioned earlier that your tank isn't actually empty, it just doesn't have enough air left to overcome the ambient pressure, so at about 50 feet, you might find that you can get a half breath, and then again at 25 feet. That's part of the reason you keep the reg in your mouth while ascending.

 

[boulderjohn]

When I did open water, we were taught that you shouldn't ascend faster than 60 ft/s. That has since been revised to 30 ft/s, but if the sh!t hits the fan when you're at 100 feet, ascending at 60 ft/s gets you to the surface in a minute and forty seconds.

When I did my instructor exam, it was emphasized that the standard for the horizontal CESA was 30 feet, with no time specified. At 60 FOM, that would take 30 seconds. We were told with emphasis that an OOA diver would certainly go faster the 60 FPM, and we were that was OK, and not to fail them on the exercise unless they were sprinting.

 

[TMHeimer]

When I did open water, we were taught that you shouldn't ascend faster than 60 ft/s. That has since been revised to 30 ft/s, but if the sh!t hits the fan when you're at 100 feet, ascending at 60 ft/s gets you to the surface in a minute and forty seconds. Your body can easily handle less than two minutes without fresh oxygen. I can hold my breath right now for two minutes.

And I think John mentioned earlier that your tank isn't actually empty, it just doesn't have enough air left to overcome the ambient pressure, so at about 50 feet, you might find that you can get a half breath, and then again at 25 feet. That's part of the reason you keep the reg in your mouth while ascending.

I was not aware that air would be pouring out of your mouth ascending from very deep, as John points out. I would think you would not be trying to inhale anyway until you have ascended somewhat.

Having said that, I do understand the other points-- you may get tank air as you ascend and that you body can handle 2 minutes without fresh O2. I guess even more than that before the 4 minute minimum before possible brain damage.
OTOH, I am not good holding my breath more than a minute. Probably can, but I'm sure that varies per individual. You can hold it for 2 minutes-- deep free divers practice in pools holding it for like 8 minutes.
If someone feels they MUST breathe (whether the really must or not), going to the BCD would be an option. Of course you would try the tank air first.
Yes, I also know that getting some tank air is one of the reasons you keep the reg in your mouth during a CESA.

My only point is that it doesn't hurt to know you have air in your BC. If you are prudent, your equipment is serviced, and nothing weird happens, you would never breathe from it, or do a CESA for no reason. It's also nice to know that if (somehow......) you get your first stage kicked out of your mouth (still not sure what weird things would occur for this to happen), you can breathe from your own octo, which is almost never mentioned in OW courses. You do everything right as a pilot and you never need that parachute. But nice to know it's there.

 

[boulderjohn]

OTOH, I am not good holding my breath more than a minute.

You should not be holding your breath during a CESA, and the sensation is different from holding your breath. For one thing, all that gas pouring out contains CO2, too.

Now, think about the mechanism if inhalation. You cannot inhale once your lungs are full and air is coming out because of the pressure. Think about a normal ascent. We tell you not to hold your breath when ascending, yet we tell you to breathe normally, which includes inhaling. That is because if you are able to inhale, then your lungs are not full and in danger of overexpansion.

 

[TMHeimer]

You should not be holding your breath during a CESA, and the sensation is different from holding your breath. For one thing, all that gas pouring out contains CO2, too.

Now, think about the mechanism if inhalation. You cannot inhale once your lungs are full and air is coming out because of the pressure. Think about a normal ascent. We tell you not to hold your breath when ascending, yet we tell you to breathe normally, which includes inhaling. That is because if you are able to inhale, then your lungs are not full and in danger of overexpansion.

kmarks was talking about your body can handle 2 minutes without fresh oxygen. Then he said he can hold his breath for two minutes. I think he meant he could go without inhaling fresh 02 for 2 minutes.
You don't think I (or he?) really meant we are holding our breath during a CESA, do you?

Yes, if your lungs are not completely full during a CESA, or at anytime, you can inhale. If they're full with air coming out as you say the ole ah...........during a CESA you could inhale some if now there is tank air available. If your lungs are completely full at some point you can't inhale, no? If your lungs are completely full but air is coming out because of the pressure change as you ascend, then your lungs are not completely full at that point.
What am I missing here, and what does this have to do with whether you should or shouldn't ever breathe BCD air?

 

[kmarks]

[Y]ou can breathe from your own octo, which is almost never mentioned in OW courses.

That's on your instructor. I teach this in Try Scuba sessions as the third option for regulator recovery: arm sweep, reach behind, alternate.

You don't think I (or he?) really meant we are holding our breath during a CESA, do you?
...
What am I missing here, and what does this have to do with whether you should or shouldn't ever breathe BCD air?

No, I'm definitely not saying you'd be holding your breath. That would be fatal. As you ascend, the air in your lungs expands, and you can continue to exhale as this happens. The act of exhaling will mostly relieve the urge to exhale from built up CO2.

But breathing BCD bladder air is bad for many reasons. Even if you keep it clean with antimicrobials, have you ever turned your BC upside down and opened a dump valve after a dive? There's usually a ton of water that comes out, so you're going to be aspirating that if you try to breathe off the bladder. And reserving enough air in your BC that you'd be able to take a breath off of it risks having that air expand as you ascend and turn your CESA into an Out-of-control ESA.