"Tom Shilson" wrote in message
...

IIRC, from ages ago, every 30 feet of water is 1 atmosphere of pressure
(15 lb. / sq. in.) If he is going down, say 6 feet, 15 lb/sq.in. should
be fine. If he tries to go deeper, he will suck air, get none, and come
up to where he can breath.

I can tell that you have never actually tried this.

Run a little experiement. Get a vacuume gauge. Stick the hose in your
mouth and try to inhale. I bet that the gauge will barely budge.

If you close the airway into your lungs and then suck with your mouth you
may be able to suck a few PSI, but that is using the muscles of your jaw,
tongue, cheeks, etc. To get the air into your lungs you need the diaphragm
to be strong enough to pull a vacuume, and it just isn't that strong.

Rod