Optimal Pony Bottle Size for Failure at 100ft?

[OMyMyOHellYes]

FWIW, a quick estimate on a direct surfacing return with an AL40 from the "recreational limit" of 130fsw/39msw:

28 litres/min SCR (roughly same as 1cuft/min stressed volume SAC rate in US Imperial units), with 39meters ( 4.9ATA) depth NDL with one minute stops every 3 meters to surface:

4.9 x 28 x 1 = 137.2
4.6 x 28 x 1 = 128.8
4.3 x 28 x 1 = 120.4
4.0 x 28 x 1 = 112
3.7 x 28 x 1 = 103.6
3.4 x 28 x 1 = 95.2
3.1 x 28 x 1 = 86.8
2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28


Sum Total: 1156.4 litres gas needed for one person to ascend to surface from 39m depth for an emergency contingency with a fairly reasonable & controlled ascent rate.

A 5.5L cylinder (AL40 "pony bottle") filled to 200 bar pressure supplies 1100 litres.

(Note: this is for worst case contingency option only, with no decompression obligation or barely any remaining air left for a "Safety Stop")

Please help me understand 12, one-minute stops en route to surface (every 10ft or 3M).

If it is a dive within NDL limits there would be -0- required stops en route? 30 ft/minute time would be four minutes, twenty seconds to the surface; 60 ft/minute ascent would be two minutes and ten seconds.

 

[lowwall]

What would your recommendation be then, given an AL40 with ascent from 39m/130'?

Is this a rhetorical question?

If not I certainly wouldn't be making regular stops at depth.

Ok, here's a plan keeping in mind that the OP stated that it was a no deco dive (and 100' but we'll go with 130' for now).

- 60'/minute ascent to 15'
- stop for at least 5 minutes, maybe 10 if it's warm enough, but ascend if you get down to 400psi.
- slowest ascent to the surface you can manage, maybe 2 more minutes.

The point is to get shallow enough that you can calmly and easily ascend to the surface even without gas if something else goes wrong. Then stop and let your tissues unload some N2 and generally get yourself together (do you need to send up a DSMB?) before making the final ascent.

 

[SpaceX Aquanaut]

Please help me understand 12, one-minute stops en route to surface (every 10ft or 3M).

If it is a dive within NDL limits there would be -0- required stops en route? 30 ft/minute time would be four minutes, twenty seconds to the surface; 60 ft/minute ascent would be two minutes and ten seconds.

Is this a rhetorical question?

If not I certainly wouldn't be making regular stops at depth.

Ok, here's a plan keeping in mind that the OP stated that it was a no deco dive (and 100' but we'll go with 130' for now).

- 60'/minute ascent to 15'
- stop for at least 5 minutes, maybe 10 if it's warm enough, but ascend if you get down to 400psi.
- slowest ascent to the surface you can manage, maybe 2 more minutes.

The point is to get shallow enough that you can calmly and easily ascend to the surface even without gas if something else goes wrong. Then stop and let your tissues unload some N2 and generally get yourself together (do you need to send up a DSMB?) before making the final ascent.

The rationale for this instance is that given the OP parameters and choosing an AL40 as calculated in Post #1 ,
as a contingency option for whatever reason to go down to rec limit of 130', should he have to "bail out" to his AL40 -->an easy and controllable ascent profile to follow is a 30 second hold at every 10'/3meters, followed by a 30 second time to travel/move up to the next depth level, and hold there for 30 seconds and so on etc. You can do 20 and 20 as an option; or 10 seconds hold and 10 seconds move (which is somewhat approximate to a continuous 1ft/2sec ascent rate [and essentially the same as 30ft/min overall]).

The point meant is, given an AL40 at 130'/39m, the simplest maximum time ascent profile this cylinder can supply is a 30sec hold and 30sec move.

 

[rsingler]

What would your recommendation be then, given an AL40 with ascent from 39m/130'?

Lots of "it depends". You specified 130'. I'm going to assume (unlike @lowwall ) that this was a deco dive, just to work a worst case scenario. But if you had reasonable gas consumption and didn't panic...
If you'd been single tank diving and hadn't accumulated too much deco, but had accumulated SOME deco...

then I'd ascend at 30fpm instead of 60fpm for the deeper portion of the dive. I wouldn't make any pauses, because that'll either add to your deco, or potentially put you in deco. It's been pretty well established now that pausing on the way up only adds to your slow compartment N2 load, and raises your risk of DCS.

So....30 fpm up to a "safety stop depth", which I'd do at 10' instead of 15', conditions permitting. Under the parameters listed above that still leaves you 790 usable psi, or 10.5 extra cu ft.

With that gas, you could extend your 10' stop from 5 minutes to 18 minutes, or perhaps a 2 minute stop at 20' (if your computer asked for it) and 15 minutes at 10'. You'd surface with 160 psi left in the tank, or breathe it down to fumes and get a couple more minutes of deco.

It's hard to see how even diving a 100 cu ft single tank plus pony, that a 40AL couldn't get you out of single tank deco requirements with reasonable gradient factors in an emergency.

My two cents. Nothing personal. I just don't think you'll find many adherents of repeated pauses on the way up from depth.

 

[SpaceX Aquanaut]

<snip>

My two cents. Nothing personal. I just don't think you'll find many adherents of repeated pauses on the way up from depth.

Ok, you could do that profile. But why on Earth would you do?

You claim it's easier to do these ascend and holds rather than a continuous moderate ascent. I'm not sure that is true, especially since there is no need to maintain a precise ascent rate. Anything between 30 and 60 feet per minute is fine.

But much more important is risk mitigation. You want to quickly get your butt out of the deep water with its high Nitrogen uptake and gas usage. Save the gas for a safety stop or rough surface conditions or other unforeseen conditions.

No . . . but IMHO better repeated pauses -under control- than constantly looking at your computer's ascent rate indicator all the way up from 130'/39m, making sure it reads no more than 30fpm (better to just stop and hold for 10 seconds, and then move up taking 10 seconds to ascend 10'/3m -->essentially again, an overall rate of 30fpm/9mpm. . .

 

[lowwall]

The rationale for this instance is that given the OP parameters and choosing an AL40 as calculated in Post #1 ,
as a contingency option for whatever reason to go down to rec limit of 130', should he have to "bail out" to his AL40 -->an easy and controllable ascent profile to follow is a 30 second hold at every 10'/3meters, followed by a 30 second time to travel/move up to the next depth level, and hold there for 30 seconds and so on etc. You can do 20 and 20 as an option; or 10 seconds hold and 10 seconds move (which is somewhat approximate to a continuous 1ft/2sec ascent rate [and essentially the same as 30ft/min overall]).

The point meant is, given an AL40 at 130'/39m, the simplest maximum time ascent profile this cylinder can supply is a 30sec hold and 30sec move.

Ok, you could do that profile. But why on Earth would you do?

You claim it's easier to do these ascend and holds rather than a continuous moderate ascent. I'm not sure that is true, especially since there is no need to maintain a precise ascent rate. Anything between 30 and 60 feet per minute is fine.

But much more important is risk mitigation. You want to quickly get your butt out of the deep water with its narcosis hog, fast Nitrogen uptake, and high gas usage. Save the gas for a safety stop or rough surface conditions or other unforeseen conditions.

 

[rsingler]

IMHO better repeated pauses -under control- than constantly looking at your computer's ascent rate indicator all the way up from 130'/39m,

We'll have to agree to disagree...

 

[lowwall]

No . . . but IMHO better repeated pauses -under control- than constantly looking at your computer's ascent rate indicator all the way up from 130'/39m, making sure it reads no more than 30fpm (better to just stop and hold for 10 seconds, and then move up taking 10 seconds to ascend 10'/3m -->essentially again, an overall rate of 30fpm/9mpm. . .

I have maybe a tenth of your dives, but I don't find holding a rough ascent rate to be that difficult. Again, this is not supposed to be a deco dive that requires any great precision in the ascent profile.

I may be completely mistaken, but I think what we are seeing here is what happens when an engineer creates a model that is too simplified.

It looks like you have defined the problem as, "Given x gas, what is the simplest ascent plan?". I think the more appropriate question is "Given x gas and all the confounding possibilities of real world diving, what is the safest ascent plan?".

IMO, the answer to the latter question starts with: get shallow quickly.