Cave 1 single tank restriction scares me...

[PfcAJ]

The H valve isn't the real problem. Its the lack of gas behind the valve. I distinctly remember being at or around the peanut restriction on a single al80, and thinking how far up a creek sans paddle I would be if something broke, or if I ended up off the line, or if my buddy had a real problem at some point during the exit.

The guys over at ADM did some testing of just how much gas was lost relative to time for various failures. A low pressure hose failure drained 14.45cuft in 15 seconds. That's a lot of gas in a short time span. 1/3 of a single 80 is about 25cuft. See what I mean about not having much gas to deal with problems? When that gauge is dropping and the pucker factor goes up, time is not your friend. And time is gas. But lets look past a dramatic failure. Think about 'time to solve a problem'. Lost buddy? Lost guideline? Siltout slowing your exit (you're intro, remember? I bet you're not a boss at zero vis exits just yet)? All these things take TIME, and time is exactly what you don't have when you're diving with a small reserve.

The reality of things is that an intro diver is probably not going to be a Zen master if an LP hose goes, or if they're lost off the line. Heck, I wouldn't be a Zen master, either! But being in a cave with a small reserve volume when you're brand new is just ASKING for it. Luckily, the cave diving community has kind of moved away from single tank intro diving, for the most part, and I think that's a good thing. I also consider the above situations not resulting in a fatality, but in a 'close call' and scaring someone away from cave diving. When you've got a bunch of gas, a moderate problem becomes an inconvenience, not an experience that ruins your diving career.

Learn to cave dive with a large volume on your back. Stack the odds in your favor, play it save, build some experience, get more training, then take it to the next level.

 

[TSandM]

You know, I'm on the side of the angels -- oops, doubles -- here. But I own two sets of LP72s, and 4 HP 130s. The single tanks and the doubles have almost the same amount of gas (and weigh darned near the same amount, too!). The difference between an HP130 with an H valve and a set of 72s is the isolator (and the fact that a single tank neck o-ring failure can't run you out of gas).

I don't know about anybody else, but I've done over a thousand dives without a tank neck o-ring problem.

Bozanic has an article about isolators -- although I haven't read it in a while, I believe he was unable to come up with any anecdote reporting a situation where an isolator solved a problem.

I think a single Al80 is not enough gas for anything but a Mexican cavern dive, but a BIG tank with an H-valve may not be that horrible a thing.

 

[PfcAJ]

What's greater, 5/6 of the 72s or 2/3 of the 130? Remember intro rules are 1/3 of a single or 1/6th of doubles.

~120cuft for the 72s at 2060psi
~90cuft for the 130 at 2295psi

Its late, and I might have really goofed on the numbers (someone double check, seriously, and the math changes a bit if you overfill the 72s. Someone else can do that, I'm sleepy), but I think that totals up to about a 30cuft advantage for the 72s. The real bugger of it all is that the 130 gets you farther into the cave (~43cuft penetration gas vs ~24cuft for the 72s) with less reserve gas. I think if you cave fill the 72s you get like 35cuft of penetration gas.

I know which one I'd pick, given the choice.

 

[ucfdiver]

Bozanic has an article about isolators -- although I haven't read it in a while, I believe he was unable to come up with any anecdote reporting a situation where an isolator solved a problem.

I know of at least one recent death caused by an isolator being shut that shouldn't have been, but I can't think of any reason that one would save someone's life.

I still dive with one, in the interest of standardization-- and as long as you check it there's no danger, but I do agree that its benefits are marginal at best.

 

[kotik]

you're not going to convince me that a H valve is safe in a cave.

I'm not trying to convince you or anyone else if a single tank with an H-valve is safe. I welcome you to do your own thinking and make your own decision about it. The discussion with AJ and James was merely to point out that the viewpoints on this matter are personal opinion only, not universally accepted and not supported by hard facts, so probably not really appropriate for unrestrained bashing. Hopefully by now there's agreement that this is a matter of personal decision and that the perceived risks are up for interpretation. If not, it's not that big of a deal to me, it just means tuning out.

Apparently you don't have a clue about what I asked or why.

Unless I'm mistaken, your original post asked that if your certification to dive doubles expired, could you dive two tanks anyway.

WHERE YOU GOT ALL THIS CRAP ABOUT HOW I FEEL ABOUT INSTRUCTORS OR TRAINING IS BEYOND ME

Again, if I'm not mistaken, I did point out that what you're asking to do definitely sounds like it would violate the limitation, and I suggested twice that rather than do that, it might be better to consult an instructor for a way to solve the problem. On the second time, you said you'd be sure to tell your mommy what you're doing. That's your words, not mine...

I do agree that the NAUI or TDI route sounds like a better fit for you.

 

[nimoh]

3. I have had a first stage fail to "closed"

4. I have had a first stage freeflow and lose all the air in 1-1/2 minutes.

5. I have had a dip tube clog shut.

6. I wouldn't be too surprised if a neck o ring failed.

In 200 dives you have had all of this? Perhaps it would be good to look at getting better equipment and/or better maintenance habits. All of these issues are very rare for most divers.

 

[LiteHedded]

you need a new first stage

 

[TSandM]

AJ, your point is made. I wasn't thinking about diving 6ths on the 72s, versus diving thirds on the 130. My limits at Intro and C1 were always 6ths (basically).

 

[kotik]

What's greater, 5/6 of the 72s or 2/3 of the 130? Remember intro rules are 1/3 of a single or 1/6th of doubles.

~120cuft for the 72s at 2060psi
~90cuft for the 130 at 2295psi

Its late, and I might have really goofed on the numbers (someone double check, seriously, and the math changes a bit if you overfill the 72s. Someone else can do that, I'm sleepy), but I think that totals up to about a 30cuft advantage for the 72s. The real bugger of it all is that the 130 gets you farther into the cave (~43cuft penetration gas vs ~24cuft for the 72s) with less reserve gas. I think if you cave fill the 72s you get like 35cuft of penetration gas.

I know which one I'd pick, given the choice.

Ok, let's take a look at something:

Situation 1:
Kotik and his bride are diving a no-flow system. She is wearing a single HP 120. He is wearing two HP100's. All tanks are filled to their rated capacity. Both divers have exactly the same air consumption. They are diving to 1/3rd's
As they hit turn pressure and turn around, it triggers a freeflow in her regulator, which evacuates the entire remaining tank contents in 1.5 minutes. Each diver has used 40ft^3 at this point, so Kotik has 160 ft^3 to divide up between them for the exit. So each diver now has 80ft^3 of gas to make the exit, twice as much as what they needed to travel that distance going in.

Situation 2:
James and AJ are diving identical circumstances, except both divers are equipped with double 120's. This time it's the isolator valve that lets go at turnaround. Each diver has used 80ft^3 at this point, so one diver has 160 ft^3 to divide up between them for the exit. So each diver now has 80ft^3 of gas to make the exit, exactly the amount required to get back, hope nobody's air consumption goes up.

I'm convinced. You guys should be diving 1/6ths.

Any number of scenarios can be dreamed up to prove a point you want to make. IMHO, the relative probability of the scenario and the vehemence put into making the case for it should be somewhat proportional, and sweeping loaded statements about what the world should do should be used pretty sparingly in general. That's all I'm sayin'

 

[PfcAJ]

You've just figured out why thirds in a no flow cave is a somewhat aggressive gas plan.