Physics of diving with a "straw"

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The tube, assuming it is sufficiently rigid to resist collapsing along its length, has created an area of 1ATA through a water column that is higher pressure. The water pressure will attempt to push into that space through the open end, but since you are attached to that end it will be compressing *you* into it first...though you'd have to hit a pretty decent depth for that to happen, I would think.
You mean I can't snorkle the Titanic?

Damn.
 
The tube, assuming it is sufficiently rigid to resist collapsing along its length, has created an area of 1ATA through a water column that is higher pressure. The water pressure will attempt to push into that space through the open end, but since you are attached to that end it will be compressing *you* into it first...though you'd have to hit a pretty decent depth for that to happen, I would think.

For this very reason a commercial diver's helmet/mask has a check valve. If the hose gets severed at a depth nearer the surface than the diver is then yes, the pressure differential would try to squeeze the diver into the hose if the check valve failed. I think some Japanese divers who did not employ check valves on their helmets found this out the hard way in the 1930ies-40ies. The Mythbuster's video posted earlier shows a pretty graphic example of this.
 
The one thing that strikes me in this thread is the lack of knowledge of diving physics by supposedly "certified divers".
 
Kgallowaypa,
I would like to thank you for posting this question. Don't listen to all the "know it all" people that like to make you feel like an idiot by telling you how much they know. You pose interesting food for thought. Is anyone really worried that you are going to grab a 50 foot garden hose and jump in a lake? The mere fact that you ask this question and people read it is proof that SOME people read these posts to learn from someone else's perspective. As a relatively new diver, I am well aware that I don't know everything and to think I should have learned everything in a basic OW class is ridiculous. That's why I read these posts and other literature..... To learn. I guess if you posted this question in the advanced technical diving forum I would expect some ridicule. But seeing how it's in the basic discussion section, I for one enjoyed reading it and reading the responses, especially the video. My theory is you can never stop learning no mater how much you think you know. What some people think is a stupid question will probably spark some answers that anyone can learn from. So thanks again.
 
I don't suppose you could explain how that would happen.:shakehead:

Yes. The air pressure in the tube will remain at 14.7 psi and the water pressure around your body will increase by 14.7 psi for each 10 meters you go down. It won't be very deep before the outside pressure is pushing the soft parts of you up through the tube.
 
1. You always need your supply air to be at some absolute pressure above your ambient pressure. Since the pressure increases by an atmosphere for every 10 meters / 30 feet (approx.), at that depth you have no more supply pressure. At exactly that depth you could in principle still suck air through a wide tube, but see point 2 below.
2. There are straightforward formulae for the pressure LOSS for a fluid or gas with a given viscosity through a tube of a specified diameter and length. The details depend on whether you are in the laminar or turbulent flow regime (Reynolds number). So with a finite tube diameter and the viscosity of air being a known quantity, you can calculate the pressure loss for a given flow rate such as that involved with breathing. You must supply an overpressure to overcome this pressure loss; of course this is why our standard scuba regulators deliver a certain pressure (relative to ambient) from the first stage and then down regulate further with the second stage to a breathable ambient pressure at depth.
3. To illustrate this, suck a milkshake through a narrow straw, then a thick straw. You'll notice a huge difference in flow rate of milkshake into your mouth.
4. This is pretty basic physics, and is included (for example) in the simple physics-for-premed type courses. It tells you about flow rates you can get through various hypodermic needle diameters, etc.
 
This thread kinda got me thinking a bit, so let's take the idea further and add a "magical" vest that allows you to breathe at depth, but doesn't change the ambient pressure on your body.

You mean a submarine?
 
The one thing that strikes me in this thread is the lack of knowledge of diving physics by supposedly "certified divers".
Agreed. Most divers (like normal people) have a very tenuous grasp upon physics. And that is okay. For most people.

Instead they are taught "rules" that they should not break. And they do not understand why.

This is kind of like cars & seat belts. Most people do not understand that if unrestrained, they will fly through the car windshield at amazing low impact speeds. Instead of teaching the physics so they are aware of the speed threshold, they are told to belt up.
 
1. You always need your supply air to be at some absolute pressure above your ambient pressure. Since the pressure increases by an atmosphere for every 10 meters / 30 feet (approx.), at that depth you have no more supply pressure. At exactly that depth you could in principle still suck air through a wide tube, but see point 2 below.
2. There are straightforward formulae for the pressure LOSS for a fluid or gas with a given viscosity through a tube of a specified diameter and length. The details depend on whether you are in the laminar or turbulent flow regime (Reynolds number). So with a finite tube diameter and the viscosity of air being a known quantity, you can calculate the pressure loss for a given flow rate such as that involved with breathing. You must supply an overpressure to overcome this pressure loss; of course this is why our standard scuba regulators deliver a certain pressure (relative to ambient) from the first stage and then down regulate further with the second stage to a breathable ambient pressure at depth.
3. To illustrate this, suck a milkshake through a narrow straw, then a thick straw. You'll notice a huge difference in flow rate of milkshake into your mouth.
4. This is pretty basic physics, and is included (for example) in the simple physics-for-premed type courses. It tells you about flow rates you can get through various hypodermic needle diameters, etc.

You need to try breathing through a 2 foot snorkel and you will get over the idea that you will be able to get some air down to 10 meters. I learned this at about 10 years old by trying it. The outside of your chest is at the same ambient pressure as the inside of your lungs. When your diaphragm relaxes there is a spring action that creates a little negative pressure that allows air in. This tiny negative pressure is easily overcome by 2 feet of water and you can't suck air in. In fact the air will go out.

I forgot to mention that atmospheric pressure is additive to the water pressure so at 10 meters you have twice as much pressure as at the surface, not the same pressure.
 
your hose will collapse probably a few feet from the surface. there is no equal air pressure to counter the water pressure.
 

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