Why do we put oxygen in aluminum cylinders?

[2airishuman]

The AL40 has become the standard deco bottle.

It is made, of course, of aluminum. Aluminum burns far more readily than steel in the presence of hyperbaric oxygen. There have been accidents where technical divers and the people who support them have been severely injured by oxygen fires in aluminum cylinders.

Exploding scuba tank kills one - Florida

Divers Alert Network, Trust But Verify

The hazards of aluminum components in medical oxygen systems are well understood although the overall track record of the medical gas industry appears to be far better than the track record for deco gases:

https://www.usfa.fema.gov/downloads/pdf/publications/tr-107.pdf

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. Fires and explosions resulting from cylinder contamination are rare particularly given the enormous number of cylinders in service. In medical uses, regulators are a more frequent source of fires than cylinders.

New LP50 steel cylinders are available from Faber and readily available from several importers. LP40 and LP45s have been available in the recent past from Worthington and PST. All of these have size, weight, and buoyancy characteristics similar to AL40s.

Now, someone please tell me why technical divers use AL40s for oxygen (and deco blends) instead of a similar sized steel cylinders. I am confused.

 

[tbone1004]

buoyancy characteristics aren't anywhere near the same. Luxfer 40 is 0.8lbs negative when full, LP45's are -4. That is HUGE when you are trying to clip it off to one side, or to your butt. A 40 you barely notice, a 45 you can feel. If the steel tanks were more positive then we would use them for the reasons you mention above, but they aren't...

The medical industry uses almost exclusively aluminum cylinders as well btw, but that's largely due to cost.

 

[LiteHedded]

because they're lighter

 

[Wookie]

I have a wonderful story of a diver clipping off his deco bottles (OMS45s) on the hang line and swimming away. He was diving OC with a bunch of rebreather divers, and the rebreather divers, when finished with the 20 foot stop, would come to the hang line and clip off their bailouts, and finish deco on the loop. So Stinky (everyone got a nickname on the Spree) swims over, and being cool like the rebreather divers, clips off his deep deco, clips off his shallow deco and swims away. We can see what he is doing, so we don't pull the hang line up. Stinky gets to the end of his deco hose (about 40 inches) and comes up a bit short, as he was still on deco. So, he realizes what he has done and sheepishly swims back to the hang line and unclips his bottle to clip it back on his rig. Now, I was always taught to always have hour deco gas clipped off at least one clip. But Stinky, who could have stayed at the hang line all day, decides to fully unclip his shallow gas bottle and drops it. Straight down 200 feet. And it's a borrowed bottle. So he aborts deco, gets up on the boat, yelling, grabs a dive weight, and ties it to his spool, and throws the weight over the side. Except he doesn't have 200 feet of line on his spool, so now he has a reel, a weight, 200 feet of line, a OMS45, and a regulator at 200 feet. The boat is moored on a 500 foot line, so it's swinging in maybe a 300 yard arc. Stinky manages to toss over a net, a gaff, and something else before Melanie gutted him with a rusty filet knife.

The charterer who brought Stinky is now tasked to retrieve this collection of heavy things from the bottom, well off the wreck, at the cost of him missing his dive. We got it all back, but the toll was a 7 hour deco obligation. We wrapped the deck up at 2 AM.

That's why we like deco bottles that float when empty.

 

[2airishuman]

buoyancy characteristics aren't anywhere near the same. Luxfer 40 is 0.8lbs negative when full, LP45's are -4. That is HUGE when you are trying to clip it off to one side, or to your butt. A 40 you barely notice, a 45 you can feel. If the steel tanks were more positive then we would use them for the reasons you mention above, but they aren't...

Well, let's use LP50s as an example, filled to 40 cubic feet. The buoyancy difference is one pound, with the steel being less buoyant:

LP50: 25" long, 5.5" diameter, 19 pounds empty, -2.4 buoyancy full, -1.7 buoyancy if filled to 40 cubic feet. source

Luxfer AL40: 25" long, 5.2" diameter, 15 pounds empty, -0.7 buoyancy full. source

That doesn't seem like a big deal. My AL19 pony is more negative than that and it isn't hard to clip off.

because they're lighter

Well, again, using an LP50 as the example, 4 pounds makes a difference?

 

[Capt Jim Wyatt]

Stinky is a dumbass

 

[ScubaWithTurk]

Stinky is a dumbass

With a terrible nickname that made me laugh each time I read it.

 

[2airishuman]

That's why we like deco bottles that float when empty.

Hmm. Gear solution to a skills problem? Perhaps...

 

[Jack Hammer]

I haven't slung an LP50 so i can't compare it to anything in the water. AL40s sling awesome, you barely know they're there. I've considered trying an LP72 as I've heard a few old timers swear they make awesome deco bottles, but haven't needed that size O2 bottle too often.

 

[tbone1004]

Hmm. Gear solution to a skills problem? Perhaps...

negative, swim around with an LP45 filled with O2 to even only 2200psi for half an hour and let me know how much you like it. Then do it with 2 of them and let me know how you like it. Then realize an LP50 isn't enough and switch them both to 72's. You will roll over and see a lot of what was supposed to be on your right side