Something I never understood about hard hat diving

[mello-yellow]

Some early hardhat divers had really gruesome accidents. Their air was pumped from the surface. If for some reason the pump failed, and had no failsafe to seal the hose, the diver ended up with a pipe on top of his head leading to the surface, and ambient pressure would instantly squeeze him into mush. In some cases the diver's entire body ended up inside the helmet.

What I do not understand is this:

I can easily see how it would happen if the hose to the surface were a rigid pipe. Water tries to go up the pipe, and human body being essentially water... ends up with a very red mess. But the hoses were not rigid. The same pressure which tries to collapse the diving suit around the unfortunate diver, should also instantly collapse the hose and thus close the connection with the surface pressure. Why was that not happening?

My apologies for a grotesque subject, but this question has been bugging me for a long time.

 

[Nwcid]

I am on slow internet so I am not sure how complete this video is, but Mythbusters did it,
Apparently my link didn't work. Google Mythbusters crushed diver

 

[John C. Ratliff]

There are a few things to understand before I get into the physics; this happened in the very early days of helmet diving. In those days, the pump was manned by two men. It was a manual operation. Also, the flexible hoses were not of good quality, and could break. Finally, there was no check valve in the helmet to prevent loss of air. With this in mind, here are some calculations.

Body Surface Area Calculator

Using the body surface area calculator, we can get an idea of the pressures involved. At 3 atmospheres of depth (99 feet), the pressure over surface pressure is about 44 pounds per square inch. The surface area of the diver (minus the head area, which is inside the helmet) is conservatively over 1500 square inches. If you multiply those two factors, the square inch factors cancell, and the resultant is around 66,000 pounds. That's 33 tons of pressure pushing the diver's body up into the helmet, which is incompressible. If the diver is only at 66 feet, that's 44,000 pounds; at 33 feet, 22,000 pounds, or "only" 11 tons pushing the diver's body into the helmet.

Modern helmets have a lot of safety features that prevent this from happening, and modern lines and compressors, check valves, etc.

The term for this is a "squeeze." It is the same effect as if you squeeze a tube of toothpaste. The paste shirts out the opening of the tube. I'll post a drawing of this from The New Science of Skin and Scuba Diving. We skin and scuba divers also can suffer from various squeezes (sinous, ear, mask, and dry suit are most common). But we get away from the deadly helmet squeeze by not using a helmet, and equalizing all body air spaces.

'Hope this helps.

SeaRat

PS, the Mythbusters video above shows graphically what happens. Thanks for posting that episode. They even caved in the helmet, which demonstrates the tons of pressure better than any calculation. By the way, 300 feet / 33 feet/atmosphere = 9 atmospheres x 14.7 pounds per square inch/atmosphere = 134 pounds per square inch x 1500 square inches = 201,000 pounds / 2000 pounds/ton = 100.5 tons of pressure total on than fake human pushing up into the helmet. That's why the helmet itself also caved in.

The drawing is from "the NEW SCIENCE of SKIN and SCUBA DIVING," Conference for National Co-operation in Aquatics, Association Press, 291 Broadway, New York 7, NY, 1962, page 40.

 

[mello-yellow]

It did help.

To be pedantic, I already knew everything you wrote, and your post did not address my real point of confusion -- why didn't water pressure squeeze the hose shut, and prevented the air pressure in the helmet from dropping to 1 atm? But as I tried to visualize the forces involved, it finally made sense. Air rushing up the hose prevents it from squeezing shut because all of that air is initially at the bottom pressure, which at any given point along the hose is higher than ambient pressure.

 

[John C. Ratliff]

mello-yellow,

You are correct. The greater pressure from below keeps tho hose open. But, if you add the check valve, then the hose would collapse. So with more modern helmets, or one's without a compromised non-return valve, the water pressure would close the hose down tight. Air would remain in the helmet and suit, but the diver would eventually suffocate without air coming down. Later helmets had an air reservoir built in, and used higher pressure hoses that would not collapse, giving them a better survival and allowing the use of mixed gases (helium-oxygen).

SeaRat

 

[captain]

To be a bit more grotesque, bodily fluids would be pushed up the hose at a pressure equal to surrounding water pressure, since fluids are incompressable the hose would not collapse.

 

[John C. Ratliff]

Actually, there is not enough "body fluids" to equalize the hose. If the hose is 2" in diameter, and 300+ feet long, that's about 11,000 cubic inches, or about 78 cubic feet of area. The hose would still collapse, just a bit higher.

SeaRat

 

[captain]

Never saw a 2 inch umbilical, or anything near that big. Most are about 3/4' OD with a 3/8" ID.

 

[John C. Ratliff]

Okay, my mistake. Thanks.

SeaRat