Can you reach your tank knob to turn it on?

[espenskogen]

How about this one: You're at 80' and you get an uncontrollable free flow.

So you react by turning off your only air source. Then what?

The whole reason why it's important to reach your valves in twins, is because you have a redundant air supply. So you shut down, to make sure you don't loose the air which you can access through the other post. On singles - Well, I find it useful whilst demonstrating air depletion to be able to shut down my own cylinder. And sure, it'd be helpful should you be silly enough to leap in to the water with your air turned off. But I would not consider it an essential skill on singles - Because if you shut down, it's effectively the same thing as if you let it run out. You still can't get air. (Although it may have a purpose in icediving, when you might need to shut down to give the reg time to thaw. But then you should be diving in a redundant config anyway)

In twins on the other hand, it's absolutely essential.

 

[dherbman]

So you react by turning off your only air source. Then what?

Wait until you have to take another breath and turn it back on. If the 1st has thawed, you can make a normal safe ascent. If it is still frozen, take a breath or two, turn it off and wait some more.

 

[TheRedHead]

wow, scuba, that's a very detailed post. You covered just about everything. From reading some of the negatives you point out, every configuration has negatives,

Scuba's post pointed out a potential dangeous situation in donating a primary. With an Air2, you will have to donate your primary so why not a longer hose primary?

 

[pilot fish]

Scuba's post pointed out a potential dangeous situation in donating a primary. With an Air2, you will have to donate your primary so why not a longer hose primary?

Yeah, that was my point, longer primary hose to keep donee out of your face and grabbing stuff. Seems that might be a good idea, huh?

 

[Kim]

Because if you shut down, it's effectively the same thing as if you let it run out. You still can't get air.

You can't get air while it's shut down no - but you preserve the air that you have and are now in a position to 'ration' it back to yourself by feathering the valve on and off as you need the next breath. It's not the same as if it runs out - then there is nothing to be had from anywhere if you have no backup or can't reach your buddy for whatever reason.

 

[espenskogen]

You can't get air while it's shut down no - but you preserve the air that you have and are now in a position to 'ration' it back to yourself by feathering the valve on and off as you need the next breath. It's not the same as if it runs out - then there is nothing to be had from anywhere if you have no backup or can't reach your buddy for whatever reason.

That is true. However, I just can't see the need - On a no-deco dive, you can either secure your buddy's octo, or just freeflow breathe the reg through the safety stop (or as long as the gas lasts) and then surface.
On a overhead dive, whether that's deco or physical overhead, you should be using redundant setup anyhow.

Rather than introducing a bucketload of taskloading trying to siphon air out of the cylinder on your back whilst doing the ascent with all the risks associated, surely it's easier to follow ones training and deal with the problem. I don't think any of the recreational or tech agencies teach the shut-down and sip method of breathing from a single backmounted cylinder. I've seen this kind of thing done with sideslung cylinders, but it is far easier to do this, than to do it from a backmounted one.

It sounds like a bad solution to a simple problem, if you ask me.

 

[dherbman]

It sounds like a bad solution to a simple problem, if you ask me.

So, you would ditch your buddy, blow off a safety stop and likely ignore a safe ascent rate? Simple problems have a way of compounding and killing people when they accumulate.

 

[Kim]

I don't think any of the recreational or tech agencies teach the shut-down and sip method of breathing from a single backmounted cylinder.

It's what I was taught by IANTD during an Eanx Overhead Environment course. This restricts any penetration to the daylight zone and was taught using a single cylinder with no redundancy. Maybe people will say that it shouldn't be taught like that - it's actually similar to Cavern I believe - but in 1999 it was (don't know if it still is). Valve drill was an essential element of the course.

edit: I just checked the IANTD standards on their site for Wreck and Cavern diver. (Page 40 of the manual)

A dual-outlet (H or Y) valve is recommended.

This is definitely for a single cylinder still....and note that even a dual outlet valve is only 'recommended' - not required. There is no requirement for another redundant cylinder such as a pony or whatever.
I don't know about other agencies at this entry level stage.

 

[espenskogen]

So, you would ditch your buddy, blow off a safety stop and likely ignore a safe ascent rate? Simple problems have a way of compounding and killing people when they accumulate.

Absolutely not - If you dive with proper buddy contact you get your buddy's octo, and ascend normally.

If not, then you freeflow breathe the regulator through a controlled ascent, keep your stop until you're out of gas, and then ascend. Since you would have planned your dive, and agreed what to do in case of buddy separation, your buddy would then search for a minute and then ascend, upon which you wack him one for being so far away that you could have been forced to do a CESA.

You would also know, of course, that doing a cesa, you should not exceed a safe ascent rate - You just have to blow your safety stop which, on most dives, isn't a big problem - I.e if you can't blow your stop, then you should have dived with redundant gas supply.

I have a very strong feeling that doing a shutdown-open for each breath will compound problems far faster than simply doing a controlled ascent - Either on your buddy's AAS or if he's left you to die, in a cesa.

It's not exactly rocket science you know..

 

[Kim]

I would say though that the main reason an ordinary diver might want to be able to reach his valve is for when he jumps in with it turned of. It can really ruin your day if you can't reach it.
I never understand the problem some seem to have with this very simple skill. You just make sure that your tank is set up so you can reach it and then IF you ever need to, you can. Where's the problem with that?

Like you said......'It's not exactly rocket science you know!'