Why is it impossible while 5m or more below water to breathe through a hose connected to the surface air, but easy to breathe through a scuba?
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The air you pull down through the hose has a pressure of 1 atmosphere. The air coming out of a scuba tank is higher pressure, so you lungs don’t have to fight the water pressure at greater depths.
There’s a few sides to this.
You can still inhale through a tube at 5m deep, so technically you can breathe. The issue is when you breathe out, the ‘used’ air stays in the tube. Oxygen will run out.
If you breathe out outside the tube, air doesn’t come in easily enough (physics) to last.
There’s also water pressure. This gets worse the deeper you go. It’s like a weight on your chest, making it harder to expand your lungs. I’m not sure at what depth this becomes a big issue. Could be 5m, could be 2, could be 20.
A point of note here which isn’t mentioned yet that humans can only create about 0.1 bars of suction and pressure. If the pressure differential being asked is bigger than this, you either end up pushing all the air out or pulling it all in, being unable to command a breath. This is one of the reasons why people suffocate in crowd rush situations, as people push them in harder than their ribs can exert force to expand.
The person at depth of 5 meters is already too deep to be able to command a breath due to water pushing their chest in at force of about 0.5 bar. Technically, I think having your lungs at depth of about 1 meter should already suffice, and a pipe to surface just shouldn’t allow you to breathe anymore.
When you breathe in, you’re not actively sucking in air. You are letting air be pushed into your lungs by the surrounding air pressure by relaxing your diaphragm. Incidentally this is also why you can blow air harder than you can suck air in.
When you are underwater, the surrounding water pushes on your chest to the point you cannot take in air. Scuba gear is designed to pressurize the air, forcing it into your chest against the water pressure.
How interesting.
Would increasing the diameter of the tube make it any easier?
In my scuba class we learned the co2 you breathe out would sit in the tube. It’s more dense than normal air, so your tube wouldn’t actually get surface air
That’s where the regulator plays in. The deeper you go the regulator opens up more and raises the pressure of the air to equalize to the increased water pressure trying to crush your body. The effect is transparent and just feels like breathing normally to some effect .
The regulator on the scuba tank, puts out air at the exact pressure that a diver is under. That is why you never hold your breath while ascending or you blow up your lungs.
While scuba diving, everything is pressurised. The air is getting blasted into your lungs. You don’t have to fight for it.
In your hypothetical example of snorkeling tho, you have to suck air all the way down the tube, working against the water pressure. Your diaphram just isn’t that strong. It’d be like sucking on a closed off pipe.
To add to what’s been mentioned about the pressure;
If you were somehow able to stick in the air, j the volume of air in the tube would be too small to be able to exhale AND refresh the air in the tube. Essentially you’d be beating your exhaled air until there’s no more oxygen in it and you suffocate.
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