Why do we put oxygen in aluminum cylinders?

[Bob DBF]

The medical gas environment is an inherently cleaner and more highly regulated one than diving. Oxygen fills are regulated by the FDA and OSHA. No blending takes place and fill whips are dedicated to 100% oxygen

Aside from what you stated, from what I could find the portable bottles used by the public are limited to 2216# (some at 2015#) service and the largest one is 24 cuft. In addition valves are specific to the gas, and size of container, so you cannot physically connect an air device to an O2 bottle, or a HP tank to an LP tank. It is harder to make a mistake if you are prevented by physical interlocks.

Now if these procedures and interlocks were instituted in tech diving, it would probably kill the sport. On the other hand, the accidents would stop.


Bob

 

[PfcAJ]

Completely off topic and I'll start another thread, but I'm curious. If you are cave diving, obviously you need enough gas to get out with thirds. If you are rebreather cave diving, you need enough gas to get out from the place where your rebreather fails. But on an extremely deep push, you want a rebreather because you need the extended dive time a rebreather allows. So, if you are back 3 or 4 hours in a cave (say 3 for scrubber life), can you get out on 2 95's? Do you plan team bailout? I have nothing against team bailout, and I'm not trying to start anything, I'm just wondering what the protocol is for carrying bailout on long pushes.

It's worthwhile to add safeties early and often. Wondering if you have enough gas isn't a good feeling to have.

 

[kensuf]

Completely off topic and I'll start another thread, but I'm curious. If you are cave diving, obviously you need enough gas to get out with thirds. If you are rebreather cave diving, you need enough gas to get out from the place where your rebreather fails. But on an extremely deep push, you want a rebreather because you need the extended dive time a rebreather allows. So, if you are back 3 or 4 hours in a cave (say 3 for scrubber life), can you get out on 2 95's? Do you plan team bailout? I have nothing against team bailout, and I'm not trying to start anything, I'm just wondering what the protocol is for carrying bailout on long pushes.

Frank,

When going for linear penetrations, yeah you'll need adequate bailout to get you home. The "party line" is for the team to carry 1.5x the amount you'll need from furthest penetration, but I'm a weenie and shoot for each diver carrying 2x the amount necessary. So to go 8000' back in a cave, you may need substantial bailout.

However, cave CCR isn't just about linear penetration. You can also use it to explore multiple different tunnels on the same dive.

Take Devil's Ear Cave, for instance (map below). The entrance to the cave is in the lower left corner ("Devil's Eye Entrance"). An open-circuit dive may only allow you to explore one section per dive, for example you may go visit the "double lines" section (top of the map), but once you hit your gas reserves you have to go home. On a CCR, as long as you have adequate bailout, light battery, and deco gas, it would be trivial to first explore one tunnel, then another, and another. So a CCR cave diver could explore the "double lines" then "Hiller Tunnel" to the "Dome Rooms" (upper right of the map), then explore the big room, bone room, and white room on one dive. Never once going more than 1700' from the exit, but easily seeing 4000' of cave passage.

 

[kensuf]

at 100ft, 0.7cfm SAC, 50fpm kick speed, and 1.5x safety factor, cave filled 95's can theoretically get you out from 4500ft. This is a rather conservative number since in an emergency you'll probably be moving a bit faster than 50fpm, and the average depth of most of the tourist caves isn't 100ft, but closer to 80ish, but that is the number I would personally use.
4500ft is a LONG way back, and well into the range that you should be using a DPV because of the decompression obligations. That's ~180 minutes of bottom time which is going to give you probably 90 minutes of deco. That's quite a long dive, so the need for anything more than that necessitates scooters and proper dive planning

Most people I know actually go slower when the **** hits the fan rather than faster. It's not that they are intentionally going slower, it's the snowball effect.

 

[2airishuman]

Aside from what you stated, from what I could find the portable bottles used by the public are limited to 2216# (some at 2015#) service and the largest one is 24 cuft. In addition valves are specific to the gas, and size of container, so you cannot physically connect an air device to an O2 bottle, or a HP tank to an LP tank. It is harder to make a mistake if you are prevented by physical interlocks.

That's mostly true, I think. Typical medical oxygen bottles are either 3AL2216 or 3AA2015 with a +, these last always being spun rather than deep-drawn, and therefore quite heavy.

The size "E" portable bottle is about 24 cubic feet with the steel ones having somewhat larger actual capacity than the aluminum ones, since the outside dimensions are the same but the wall thickness differs. Larger cylinders are used for medical oxygen for home use and in various institutional settings, but are not considered portable.

The valves being specific to the gas is true for both medical and industrial gases. Portable medical cylinders use a pin-register valve from which the original SCUBA yoke valves were originally derived. Various combinations of pins correspond to particular medical gases but the valves are otherwise compatible. Gas misidentification still occurs, and there are fatalities every year from it. The pin register system can be inadvertently defeated by using two sealing washers (the Bodock seals used instead of a face o-ring). Color codes aren't standardized with some institutions color coding cylinders by facility area or ownership. And since in medical use hoses are typically connected to regulators using a barbed fitting, it is possible to connect a hose to a cylinder and regulator that match each other but which as a pair are not the gas needed for the situation.

Medical also has a unique problem with mixups involving very large refrigerated liquified gas containers. Because of the low pressusre and the design of the containers it is physically possible to swap discharge fittings on them while they are full, and there have been some tragic accidents where liquid argon has been mistakenly delivered in place of liquid oxygen, and then connected to a hospital oxygen piping system by a maintenance person swapping fittings to "fix" the incompatibility.

Now if these procedures and interlocks were instituted in tech diving, it would probably kill the sport. On the other hand, the accidents would stop.

I'm not sure it would help. It's been tried in Europe, where Nitrox is supposed to require a larger version of the DIN connector. Gas blending mistakes happen in the medical gas world also, interlocks or not.

 

[Bob DBF]

Gas misidentification still occurs, and there are fatalities every year from it.

I was more addressing the fire and explosion hazard which is reduced by lower pressure, smaller tanks, and regulators and other gear that cannot be used on other, non O2, tanks and fill whips to transfer contamination.

In SCUBA it would be quite a change, including a special valve fitting for use with O2, both valve to reg and tank threads for O2 only cylinders, along with lower service pressure. And the eternal question of where to draw the line on Nitrox, perhaps all pp blending in O2 tanks and continuous blending with a stick and N2 separation up to 40%? for air tanks.

Any system can be defeated by applied stupidity, but it sounds like they have the fire / explosion issue under control. This is impressive since the SCUBA use of O2 is nothing compared to medical use.


Bob

 

[steady1570]

In SCUBA it would be quite a change, including a special valve fitting for use with O2, both valve to reg and tank threads for O2 only cylinders

Here in Europe we have the M26 valve / thread size for use with rich O2 blends. Relatively straightforward to get fills etc at most places in the U.K. at least with these sizes, but for travelling outside CE-land they are a PITA as most rental cylinders I've seen for example have standard DIN fittings and so require an adaptor... meaning quite a lot of people swap out the M26 even at home and the system is defeated since it's also no real trouble getting a fill on an (O2 clean) DIN.

-Mark

 

[The Chairman]

When going for linear penetrations, yeah you'll need adequate bailout to get you home.

Very well put. Lucid even. Don't let it go to your head!

 

[2airishuman]

I was more addressing the fire and explosion hazard which is reduced by lower pressure, smaller tanks,

Lower pressure, yes.

To the original point of this thread, there's no reason a neutral, lower pressure, steel cylinder couldn't be manufactured. The lower the design pressure, the thinner the walls, the more floaty. I have an LP53, 3AA1800 cylinder that is something like +6 when empty. No need to go that far, of course, it's a sliding scale. 3AA2015 cylinders are the standard for medical and industrial oxygen. Similarly rated cylinders, but deep drawn rather than spun, and sized appropriately, would probably be ideal for deco. I think. What do I know.

... and regulators and other gear that cannot be used on other, non O2, tanks and fill whips to transfer contamination.

I think it's an interesting question why the incidence of oxygen fires is so much lower in medical than scuba. The number of O2 fills for medical use has to be 1000x greater or more than the number of O2 fills for scuba. I'm not sure transfer of contamination has much to do with it.

 

[The Chairman]

there's no reason a neutral, lower pressure, steel cylinder couldn't be manufactured.

This might help put it into perspective:

• Reasons to manufacture a neutral, lower pressure, steel cylinder
  • We want it
• Reasons to not manufacture a neutral, lower pressure, steel cylinder
  • Economics (no possibility of a profit)