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In the underwater scene in Mission: Impossible - Rogue Nation Ethan Hunt is told that he can't take any oxygen tanks because there are metal detectors.

So why does it have to be metal? I know this might be a little bit naive of me, but with all their technology they aren't able to use rubber bags/balloons of air maybe?

Just pump some air into a non-metal container and take it down with you!

Maybe I missed some detail, but was there any other constrains for the underwater scene?

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haven't seen the movie but depending on how deep underwater: it would need to be something that can withstand high pressure without distorting its shape. – KutuluMike Jan 28 at 13:05
a helium balloon would have been suffice. – user30438 Jan 28 at 23:59
He free dived, so it's not deep. Even a pvc 40 psi tank would give him a few extra minutes of air. So, yea, plot device. – cde Jan 29 at 0:17
@cde and he breathes from it how? The mouthpiece/fittings would certainly have metal. – Catija Jan 29 at 1:28
@catija a small plastic valve. Low psi, relatively shallow water. We are talking about a few extra breathes. For reference a standard scuba tank lasts a beginner 1 hour at 10 meters. He would have been fine with even one extra deep breathe worth of air. – cde Jan 29 at 2:29
up vote 30 down vote accepted

In order to have sufficient air to breathe in underwater situations, the air is very highly pressurized (3000 psi on average). The first and second stage of the scuba regulator work to reduce the level of the pressure to the ambient level for the person using the air.

Scuba regulator

In order to hold that high-pressure air, it's necessary that the container be made of metal, otherwise it would explode... imagine an over-filled balloon.

At lower pressure levels, it's certainly possible that someone could create a heavy-duty plastic cylinder but many of the fittings on a regulator are also metal, at least in part, so you'd reduce the amount of metal by a huge amount but it still wouldn't be no metal.

Regardless, there's one other problem. Air is extremely buoyant. It does not want to stay underwater... it wants to float. As a test, take a capped, empty three liter soda bottle into a pool and try to stay submerged... it's not easy... and if you do manage to get deep enough, watch it compress under the water pressure.

Part of the benefit of the solid metal - the standard steel tanks generally weigh 30-40 pounds - is that it usually helps counteract this buoyancy. If Ethan had simply taken a plastic container of air down with him (despite likely only getting a couple of breaths), he'd spend a good amount of energy working to stay underwater, against the buoyancy of the air. And the more energy you expend underwater, the more air you need.*

Generally, when you scuba dive, this is counteracted with the weight of your equipment (generally around 50 pounds) and additional lead weights, if necessary. Neither of these are options for Ethan, who can't have any metal on him.

The density of the most dense non-metal is iodine, at 4.933 g/cm3. The density of lead is more than double that, 11.34 g/cm3. To combat the pull of the air, Ethan would have to haul around a ton of iodine... which, again, would take more work and more air. It's simply not feasible.

*In scuba training, they encourage you to move as little as possible and to keep your movements controlled and smooth because you will breathe less and go through your tank more slowly. When I dove in the Caribbean, I would use about 2500 psi at 60 feet over the course of about 50 minutes. Our dive master used half that, only about 1200 psi. Clearly he's a much more efficient diver than I am.

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Great answer! +1 – sanpaco Jan 28 at 18:29

Firefighters use composite pressure tanks that are quite light, so there's no problem with finding non-metallic construction that will withstand SCUBA class pressures.

No, the problem is with the "take it down with you" part. Those firefighter cylinders weight very little in air, but in water they will have negative weight. That is, they float. In fact, a nearly-empty aluminum SCUBA tank will float, just barely. So if Ethan Hunt had a nonmetallic compressed gas tank with him, he'd need to also carry a lot of weight to get and stay underwater. While this could also be non-metallic, he'd need to carry 3-4 times the volume of rock compared to lead weights.

End result? He'd be carrying a small air tank and a couple of bowling balls. And I haven't even gotten to the non-metallic regulator he'd need...

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Are you certain that the firefighting cylinders are non-metal? The info I have found is that they're aluminum with a fiberglass or carbon fiber wrapping... Plus, the fiberglass would be extremely buoyant... Here's a conversation with people asking about them on a diving site. – Catija Jan 28 at 20:27
@Catija What would be the purpose of the aramid composite (Kevlar, Dyneema...) wrapping? SCUBA demonstrates that Al by itself is adequately strong. LMGTFY I see that both full composite and gas tight Al foil lined tanks are available. – mpez0 Jan 28 at 21:15
I'm not a specialist... all I know is what's on that link I posted (which is, admittedly a bit old). It sounds as if it's a very thin aluminum shell that's then strengthened with the composite because it's lighter weight than solid aluminum or solid steel. – Catija Jan 28 at 21:18
Oh, and I think I saw something about issues with Helium leeching... the composites may not be air tight enough to prevent Helium from getting out of your mix, which is bad for technical divers. The small amount of metal prevents that loss. – Catija Jan 28 at 21:33

There are such materials as granite, carbon ceramic and graphite, all of which would be able to sustain high levels of pressure, and in the case of graphite be a true future-tech material that could be used in conjunction with some type of stone to counteract the buoyancy issue.

In short, see above for at least 2 viable solutions to a relatively trivial problem...

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But if they don't exist... they can't be used in the film... and that doesn't address the fact that all the rest of the fittings for the regulator would also need to be made out of these sorts of materials. – Catija Feb 20 at 23:20

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