Pressurized diving suits - how?

Last week the Mythbusters tested a myth about an old-fashioned deep sea diving suit losing pressure. The results were not pretty (youtube).

What I don’t understand is how the suits working in the first place. It seems to me that if the suit was pressurized, then either the air alone would squeeze the guy, or the water would just push on the air and that would squeeze the guy. So how does it work?

I think you’re right - the diver’s body would be under the same pressure regardless, either from the atmosphere within the suit, or from the surrounding water - but that’s OK - being in a pressurised environment is OK, if it’s done properly.

The problem is that loss of pressure in the suit causes the flexible parts to mechanically squeeze into the rigid parts - squeezing a tube of toothpaste does little to the toothpaste, until you remove the cap.

If the pressure is acting uniformly - as for example in an unpressurized suit on a SCUBA diver - then the diver doesn’t feel a thing. Case in point, you’re sitting at your desk and you are currently under about 14.7 psi of pressure all over your body. No problemo.

The old-fashioned suits were not SCUBA (Self-Contained Underwater Breathing Apparatus), they were connected to the surface via a hose. at a depth of 100 feet, the water pressure is about 50 psi, so the air pump on the surface had to supply that much pressure to the hose that fed the suit in order to drive fresh air down.

If the pump craps out, then 50 psi of water squeezes in on the suit, and the air inside the suit gets pushed out that hose back up to the surface. With a dead pump it’s not possible to build up any kind of pressure inside the suit, so the water keeps squeezing everthing inside the suit toward the spot where that low-pressure hose is connected - namely, into the hard brass helmet.

Is there supposed to be some sort of relief valve that would close off, thus keeping the diver safe for a bit anyways , with internal pressure built up before the pump crapped out.


I’m at work, so I can’t watch the movie, but I can guess what it shows.

Think of a vacuum cleaner. It doesn’t really “suck” - it reduces the pressure near to what you want to vacuum, and the surrounding air pressure pushes stuff into it.

Now think of a box with a low pressure in it, even a vacuum. Attach that box to a hose, stick the hose on your arm, open the valve. The low pressure volume “sucks” on your arm, but really it’s the air pressure around you trying to push you up the hose!

If a old diving suit loses pressure in the hose, say due to pumping failure, then you’re effectively attached to a low pressure area (the surface) by a hose, and the water pressure around you is going to try and push you up the hose. Which isn’t good if we’re talking several atmospheres of pressure. It would be like a super vacuum cleaner trying to suck you into it.

A simple non-return valve on the helmet would help out a lot. Or even a floppy hose that collapses on itself when it loses pressure.

Also, you need to understand a few thing about gases, pressure and volume. If you don’t mind me going over to metrics (bar and atmospheres are so much more comfortable to work with than psi. Just remember that we are at one atmosphere at sea level, and pressure increases with one atmosphere per 10m.

When you drop down to say 30m, you’re under four atmospheres of pressure. This means that the volume of a bag of gas (such as your lungs, or a pressure suit) is a quarter of what it would be at the surface. So what you do is increase the pressure of the air as well, either by a surface pump or through pressurized air tanks (where the correct amount of gas is released by a regulator), which allows your to expand your lungs to their full volume and move around in your suit.

The air pressure doesn’t affect you as much as the water would, simply due to the difference in density between water and air. You do feel air as noticeably thicker at depth though. But as long as you’re moving around in your tiny bubble of air, you’re fine. Puncture that bubble though (but not supplying pressure from the hose), and you create a pressure differential in your suit, where the water is squeezing the air out of the hose. Since the inhabitant of the suit isn’t made of very staunch stuff, this differential will do some nasty things to him.

I’m not an expert in the field, but good safety design fundamentals would dictate a check valve (not a relief valve) to prevent backflow up the fresh-air hose in the event of pump failure.

Although if you’re 100+ feet down and the fresh air suddenly stops flowing, you’re pretty much screwed anyway as there’s not a lot of oxygen in the tiny volume inside that brass helmet. Maybe they can hoist you to the surface before you run out, maybe not; if they can, you’ll be dealing with the bends when you get there.

Yes there is and if it’s in working order the compression doesn’t happen. The Mythbusters went with the assmption that this valve may not have been maintained very well on some suits so disabled it to show what could have happened.

This was removed or defeated in the Mythbusters episode.

Just watched the episode on my DVR last night. At the first stages of myth investigation, they talked to an expert on the old-style diving suits, and he showed them that there was indeed a valve on the helmet that was designed to close in event of compressor failure and save the diver’s life. The expert mentioned, though, that the valves were often poorly maintained by the divers themselves, and thus didn’t operate properly.

I’m a little troubled that the younger Mythbusters had such a joyous reaction to the ghastly results; I kinda hoped at least one person would remark, “Jesus, what a horrible way to die.” Apparently not.

Many of the myths they investigate involve horrific death/disfigurement; the basic tenor of the series involves greeting/exploring these myths with a degree of levity, so I don’t see why this one would be any different.

I think the joy involves seeing “success,” i.e. a myth proven true in a very dramatic manner, since so many of the myths they investigate tend to be proven false with an undramatic fizzle.

Actually, hypercapnea (carbon dioxide toxicity) will kill you long before asphyxia.

A variation on this.

If you were in a suit like this working at one depth, and fell to a significantly deeper depth, you’d could easily be crushed enough to die beforea marginal pump could reequalize the pressure.

Say you fell off of something and ended up 30 feet deeper. Thats 15 pounds PER square inch. It might not squeeze you up into your helmet. But it would probably break all your ribs and collapse your lungs pretty fast.

I am kinda of amazed that divers werent keen on keeping that valve in good working order. I suspect most of them must have bought into that macho/danger mentallity a bit too much.

If the opening at the bottom of the helmet has a 10-inch diameter, that’s an area of about 79 square inches; 15 psi would give a total force of about 1180 pounds trying to extrude you into the helmet. I’m thinking that would be fatal.

The manifold that contains the check valve also provides a connection to a diver carried high pressure tank ( called a bail-out bottle ) that the diver can activate in case of a loss of pressure in the umbilical. Doesn’t help much if the check valve fails though.

Except that no one actually died. It was a dummy and fake blood. How is it any different from watching the Coyote get blown up in another failed attempt to catch the Road Runner?

If I remember correctly, the Mythbusters never said anything about finding an actual documented case of this actually happening in real life; they were testing the myth that it could happen.

Frankly, I thought it was incredibly cool to see. Like the Mythbusters, I would have bet that the squishing would happen evenly, and there would be no way that the fake body would get stuffed into the helmet. Seeing that actually happen was just amazing. And I’m far from young.

Actually according to the text info on the YouTube page, it was a real pig carcass. Actual flesh and blood, just not human.

I just remembered the valve from reading Eric Flints 1634, King Christians skunk works came up with it , and a condemmed prisoner was used, the results were pretty much what mythbusters came up with, sans valve and the thought process for a new form of excution was pretty much the characters way of putting the best light on failure.


This can happen (and has) to saturation divers as well. If I remember correctly, the interviewee (one of the technicians on the oil rig) described the results as “strawberry jam”.

Oh, I agree that it was visually impressive, especially seeing the helmet itself crush like a tin can under the pressure. I guess I’m just especially sensitive to the concept of being crushed to death; of all possible ways to die, being crushed in, say, a building collapse, is in my mind the worst way to go. So, yeah, I know that they do other horrible death myths, but somehow imagining, for example, a farmer’s pants exploding due to a build-up of chemicals and heat is not nearly as ghastly to me as being squeezed until you pop. I just expected or hoped that they’d have some pause imagining that real people had possibly died in the same fashion.