OP
Mekotronix
Registered
We are not wrong.
Just to make sure there is no misunderstanding, I was decidedly (though perhaps not clearly) NOT claiming all of you were wrong. It was intended as a "I don't really understand why this is the case, but y'all have way more experience in your pinky toe than I have in total so I'll take your word for it."
A snorkel will work for a test but a short piece of garden hose will be better. See if you can breath through it at 6 inches of submersion. You will not be able to draw a breath at 6 inches of water column but just barely and certainly not repeatedly as it would be exhausting. If a regulator had a cracking effort of 6 inches it would not sell very well to say the least.
Like many others here I assume, I did the garden hose to the bottom of the lake when I was a kid. (~15 yo) I went down 12-15' though... never tried it just below the surface. I'll have to give it a go.
Sucking water with your mouth is a little bit different than inhaling air into your lungs.
Try this:
Go to a pool and breath at the surface with you body vertical and your head above water. Then try breathing with a snorkel with your head under the surface with your body still vertical. See if you notice any difference in difficulty inhaling. You may need to hang onto a ladder or something to keep from floating to the surface or becoming horizontal.
I will try this (and the exercises suggested by Nemrod) when I get the chance. Nothing like personal experience to drive a lesson home!
The replies I received really got me wondering about the physics involved here. At first glance, a 6" pressure differential to suck water into a straw should feel the same as a 6" pressure differential to crack a 2nd stage reg. (Assuming both volumes << lung working volume. If you reach the end of you lung capacity trying to crack a reg I imagine perceived WOB goes up.) What factor am I missing?
Is it the fact that your lung are submerged and have to counteract the increased pressure in order to fill? But then, isn't that what the pressurized air is for? To increase the pressure in our lungs (and mouth, reg, etc.) as the ambient pressure increases? It's not obvious to me that explains the drastic changes in perception. Seems to me increasing the ambient pressure would more or less be a wash.
Maybe it's that pressure gradients are so much larger in water than they are in air? 10m deep in water puts us at 2 atm. We'd have to stand in a hole ~5.6 km below sea level to experience 2 atm in air. If my calculations are correct, that 6" pressure differential at 10m underwater is roughly equivalent to 840m pressure differential in air at 2 atm.
Huh... I'm not sure what to make of that. Given no amount of vacuum is going to pull water over 64' high (given 2 atm ambient), I'm either way off base with my line of thinking or very surprised anyone could breathe a reg adjusted to 6" at 10m.