What pressure would you fill a pair of AL80s to?

[diverdarren]

What Jay said is right. Faber stated that 10,000 fill cycles to 4000psi was their guarantee. However 3900psi fills in an Al 80, if not stupid, is at least not too bright, plus the extra amount of gas is negligible compared to doing this with a low pressure steel cylinder.

[Nitrogenius]

So, you're right....there IS no "no failure" threshold. The phenomenon by which this occurs is plastic deformation. Under a certain strain threshold, the deformation is "purely" elastic and therefore has no permanent crystalline structure changes. By "purely" elastic, I mean 99.999%+ elastic. Plastic deformation is a permanent crystalline structure shift. This is what "hardens" the tank. I'm not sure what the pressure is (tank mfgs won't release sufficient specs for my math, and I'm NOT cutting a tank apart for this forum )......but there's a point where plastic deformation actually becomes "relevant." The higher the pressure, the more lifespan reduction. From what I understand (and based off of math based off of a bunch of guesses).....our "normal" fill pressures will give our steel tanks long enough lifespans that rust and physical damage will take the tanks LONG before elastic deformation causes an issue. This is all under the assumption that we fill our tanks to "normal" pressures at "normal" speeds and treat them like "normal" people.

Thanks Victor. That's what I figured and was trying to say.. It has been a while since that class but that's what I took away from it..

[victorzamora]

What Jay said is right. Faber stated that 10,000 fill cycles to 4000psi was their guarantee. However 3900psi fills in an Al 80, if not stupid, is at least not too bright, plus the extra amount of gas is negligible compared to doing this with a low pressure steel cylinder.

I absolutely agree about not over filling an al80. The deformation of aluminum is such that the plastic threshold is MUCH lower and much less forgiving than most steels. Super high carbon steels are the exception, but they'd have terrible performance as a dive cylinder.

[GregO]

Can someone explain this to me? It was my understanding that most (if not all) firefighter's gas/air tanks were made out of some material OTHER THAN aluminun. Fiberglass, maybe? Anyway, something with a limited cycle life. So I am not clear what they have to do with the question at hand.... Neither steel nor aluminum tanks "expire" after 15 years, I understood that the fiberglass (?) tanks do... These are fundamentally different types of tanks, no?

While I wouldn't say SCBA cylinders are fundamentally different, the lack of the U in SCBA drives a different set of materials to reduce the weight while handling the higher pressure. In other words, we don't have buoyancy in water to help the weight issue!

Current SCBA cylinders are mostly 4500 PSI though there are now 5500 PSI available. The construction is based on a thinner wall of aluminum then in SCUBA and these cylinders are then wrapped in aerospace carbon fiber and then coated with a glass and epoxy sacrificial layer. Current carbon fibers have a 5 year hydro and I am not aware of a maximum number of hydro's for these. There life is 15 years.

Keep in mind, due to the nature of their use, there are design, cost and lifecycle policies that are in place to protect the safety of FF's and wells as manage the liability of manufacturers.

[Gobey]

Every dive in Mexico (cca 3480 psi)



I suppose they fill it daily to that pressure

John

[jlillest]

Super high carbon steels are the exception, but they'd have terrible performance as a dive cylinder.

Can you elaborate? This would be great for the sake of discussion.

I've done a lot of really cool dives in carbon cylinders. We used these cylinders because they made more sense for the mission (and the guy buying them had deep pockets). Of course they require *a lot* of lead, but most of the lead makes a one-way trip to the dive site. Carbon tanks are way better for carrying above ground than any other cylinder out there.

-Jon

[FW]

Can you elaborate? This would be great for the sake of discussion.

I've done a lot of really cool dives in carbon cylinders. We used these cylinders because they made more sense for the mission (and the guy buying them had deep pockets). Of course they require *a lot* of lead, but most of the lead makes a one-way trip to the dive site. Carbon tanks are way better for carrying above ground than any other cylinder out there.

-Jon

I think he is talking about high carbon steel alloy, not carbon fiber wrapped.

[victorzamora]

I think he is talking about high carbon steel alloy, not carbon fiber wrapped.

That's exactly what I meant.

Jon: I'm sure you know that steel comes in a bunch of different alloys with a wide array of properties. Steels can be very soft and malleable, other MUCH stronger but brittle. One method of making steel stronger is by introducing Carbon into the alloy. High carbon steel is used a lot in tools, especially in things like endmills. The issue with high carbon steel alloys is actually their strength. Alloys that are that strong are very brittle. A high-carbon steel tank would fail all at once, with no warning, after very few cycles, and they would actually do so VERY inconsistently due to differing fill rates and treatment methods. I mean, if we treated a full high-carbon steel alloy tank like we do our current tanks.....we'd have a bomb in our hands.

[TracyN]

This conversation is way over my head, lol! Sounds absolutely Greek to me. Never took a physics class in high school or college. Go engineers, lol!

[caveflyer]

This conversation is way over my head, lol! Sounds absolutely Greek to me. Never took a physics class in high school or college. Go engineers, lol!

Great observation - engineers rock

To give some perspective imagine a scuba tank made out of the type of steel a [bendy] nail is made from or the type of steel a manhole cover [not bendy] is made of.........