Nitrox to an OOA Diver?

[Jax]

only 19cf? You're gonna die!

How much gas do you need to make a direct ascent to the surface for a non-decompression dive?

It was a joke, CD, and no, I don't want to play.

 

[Puffer Fish]

Oh, and my personal choices: pony carried on all dives, bailout-only (i.e. not part of dive plan), 19cf, ascent-only but with stops, backmounted, left-side, on cam band, with bracket, valve-up, valve on, mini SPG, OPV, air-only, necklaced reg, for me alone. Any other choices are totally crazy and indefensible.

That is so wrong, in so many ways. Everyone knows you don't put the bottle on the same side as your snorkle

 

[Cave Diver]

It was a joke, CD, and no, I don't want to play.

Does that mean you don't know the answer?

 

[Deefstes]

I can't believe you'd even consider using only one pony bottle and not have two configured with a doubles manifold. What? Did I just hear you say you prefer independent doubles ponies? You're gonna die.

 

[Cave Diver]

i think it depends on how many licks it takes to get to the center of a tootsie pop.

283

 

[MXGratefulDiver]

I think it depends on how many licks it takes to get to the center of a tootsie pop.

As a more serious answer it depends only on two things ... your depth and your consumption rate. Consider that for a no-decompression dive, making stops is purely optional ... recommended, but not mandatory.

So from a depth of 120 feet, you should take at least four minutes to surface. If you have a surface consumption rate of, let's say, 0.5 cubic feet per minute, then you can calculate it easily enough ... over a four-minute span, coming up at a constant rate of 30 feet per minute, you will have an average depth over that four minute time span of 60 feet. Convert that to pressure at that depth ... 2.82 ATA. Heck, let's use a little bit of scuba math and round up to 3 ATA.

The amount of gas you'll need to surface from 120 feet will be 0.5 CFM x 3 ATA x 4 minutes = 6 cubic feet.

That's the theory ... in reality it'll probably be much higher, because you're likely to be stressed out and breathing hard. Then consider that for most recreational divers, they won't really be used to making an ascent directly to the surface without some kind of line to hang onto or at least look at ... so there'll probably be a bit of up and down going on. So in reality it'll probably be closer to 10 or 12 cubic feet of gas ... possibly even more than that.

So if you're sucking a 19 cubic foot bottle, you're probably OK. Of course, I used a pretty low surface consumption rate. Calculate yours ... use scuba math (round up) ... and see if that pony you've got is really big enough for the depth you're going to. For most folks a 19 cf will be fine at recreational depths ... as long as you ain't planning on sharing any of it with a buddy ...

... Bob (Grateful Diver)

 

[Jax]

I went for the rule of thirds method - as long as you are carrying a pony with the same as one third of your backgas, you'll have enough to recover safely no matter when in your dive plan you experience trouble.

 

[Jax]

Well, now that we've all had our fun with this thread, here is the rest of the story that made me think of the question . . .

DISCLAIMER: I know nothing more than what I recount here, so please don't ask me! Feel free to make and state assumptions.

Some tec divers were out at a popular dive spot where it's possible to go to 160 ft. Also there was your standard dive boats.

The guys were coming up from a deep dive and had EAN 80 as their deco gas. They were somewhere around 45' on a deco stop but not yet on their deco gas. Along comes Mr. OOA from behind. He grabs a diver's EAN80.

"Donator" tried to give the OOA diver his primary, but OOA was having nothing of it (apparently violently objecting). The tec divers [apparently] believed the OOA would OxTox if he stayed on it. They reluctantly began ascending, but drew the line at ??? 20? 30? fsw.

By then, the DM from the other boat found them, immediately realized the situation, (and since OOA was calmer), snatched the deco gas out of the divers' mouth and replaced it (shoved it) with his octo. The DM took OOA to the safety stop to the immeasurable gratitude of the tec divers.

Later story was that Mr OOA said, "Nitrox? They "gave" me Nitrox? I could have been poisoned! I'm going to sue! Yada, yada." Further rumor was that the DM and Capt informed him in no uncertain terms that they were going to kick him off the boat and make him swim back if he kept up the ****. (He must've been a fan of NCSI.) The boat ride back was 60 min on a fast boat. It's also in Mexico so boat capitan rules.

So . . . What would you have done were you the tec diver in deco? What if the DM hadn't come along?

 

[HowardE]

Let the guy breathe the 80% and tell the guy, "I could have just let you die"

 

[Jax]

Uhhh . . . Well, if the mod of 80 at 1.6 ata is 33 feet (if I didn't screw up the calc), if you let him keep the 80%, he could OxTox at greater than 33 feet. Do you not see any issue with
leaving him on the 80%? Especially since it is your deco gas?