Pony bottle ascent schedule?

[dstrout]

I'd be interested to hear peoples thoughts on a hypothetical situation I was considering on the way to work this morning.

Imagine I'm cruising along at the end of my 130' dive on 32%. I'm within my NDL and dive plan as I reach the ascent line. Just then, Neptune appears, decides he hates me, and puts his trident through my back gas. Luckily, I have my handy 30 cu ft pony full of air, so I start my ascent using that.

The question is, what is my ascent schedule? I was planning on using nitrox to ascend, but now I'm on air at 130 ft.. My computer doesn't know what I'm breathing (nor even if I'm breathing). So I'm breathing more nitrogen and less oxygen than I had planned on as I ascend.

My instinct, based on no information or knowledge, would be something like this:

130 -> 70 @ 30 ft/min
70 - 1 min stop
70 -> 50 @ 30 ft / min
50 - 1 min stop
40 - 1 min stop
30 - 1 min stop
20 - 5 min stop or breathe it down to about 300 psi, whichever comes first.
Done diving for the day

So, do I have a lovely case of DCS, or shall I crack open a beer and have a cigar?

thanks,
dave.

 

[elan]

His trident is pretty big so if he manages to put it through the back gas it would most likely go through the pony as well. Check your LDS they should have anti-trident stickers

BTW: they might ague but 130' on 32 is a bit of a stretch

 

[udtfire]

depending on your sac rate the gas you have is enough for the return and then some as for the dcs who knows its all theory but extend your safety stop as long as possiable the go have a drink because you out of the water for 24 hr but if your pony matched your back gas well no worries maybe thats your answer in a ndl profile let me know what you think

 

[Blackwood]

Using air instead of 32% wouldn't concern me in the slightest. That's more than enough deco.

 

[bfisher]

I know you're just thinking hypothetically, and I've never pushed this, but if you'e at 130' on 32% EAN you're already dead. Or am I wrong on this, too.

I know this has nothing to do with ascent rate, but hey.........

 

[SkimFisher]

I know you're just thinking hypothetically, and I've never pushed this, but if you'e at 130' on 32% EAN you're already dead. Or am I wrong on this, too.

I know this has nothing to do with ascent rate, but hey.........

You're not dead. But you're pretty close. I get a PPO2 of 1.54 at 130'. Well beyond the MOD.

...and one more reason to invest in a computer that handles gas-switching. Or work out the schedules prior to diving. Or dive the same mix in your pony.

 

[GrumpyOldGuy]

I think you have 2 logical choices for ascent schedules. Both based on KISS.

1) You have plenty of gas and can monitor the SPG, so you use your standard ascent schedule. You are used to it, don't change anything. You already fff'ed up, no need to complicate things and go for seconds.

2) You are not sure you have enough gas, you do a basic PADI style ascent at 30FPM to your s/s and then hang out as long as possible. It is an adequate (not ideal) protocol and reduces the chance of running OOG a second time and drowning.

In both cases, your diving is done for the day, because you screwed the pooch, not Neptune.

 

[Blackwood]

You're not dead. But you're pretty close. I get a PPO2 of 1.54 at 130'. Well beyond the MOD.

Depends on what you consider MOD.

Personally, I won't plan to dive 32% @ 130 feet, but I'm not going to suggest that you die at 1.6ATM (which is slightly less than the PO₂ I'm exposing myself to when I switch to the bottle on the bottom in my current avatar).

In any case, that's not really the point of this thread as I understand it. I believe the OP is asking if one needs to account for using a different gas (with a lower FO₂) on and sub-NDL emergency ascent.

My take is: no. A slightly lower FO₂ given this profile is unlikely to cause any problems, but even if it did, not drowning is the paramount concern.

..and one more reason to invest in a computer that handles gas-switching.

Wait a minute, so I run out of backgas and now I'm going to fumble with switching gases on my computer at depth while sucking down a pony bottle? No thanks, I'm going up.

 

[Peter Guy]

Just for kicks, I ran this profile through iDeco, a Buhlman profile, and came up with what I expected. You are within your NDL when you lose your back-gas so it is merely a normal ascent -- 30 fpm to 40 ft and then 1 minute stops to the surface.

With 32%, your NDL at 130 fsw is 15 minutes (+/-) so you only have fast tissues which have come close to their M values which means, of course, that they off gas just as fast as they on gas (again, more or less).

The switch from 32% to 21% isn't all that important since you have now started your ascent and won't spend very much time at all at depth. In 2 minutes you'll be at 70 feet, assuming the 30 fpm ascent -- just not much time to on gas.

End of the day? Well, if you have more tanks and/or new regs, why stop diving?

This is really one of those that show how "fragile" deco theory may be.

YMMV

 

[Blackwood]

This is really one of those that show how [-]fragile[/-] misunderstood deco theory may be.

fixed