My understanding is that sucking water up a straw works by pressure differential. The diaphragm lowers the pressure inside the mouth, so the water is pushed up the straw.
If you could generate a vacuum, you could suck water up about thirty feet. But, of course, you can’t generate a perfect vacuum.
So, under ordinary circumstances, what’s the limit to how much of a pressure differential a human can make? How far can a human suck water up a straw?
It may not be the best cite in the world, but the answer is consistent with what I remember. 35 feet
Nevermind, I see you are asking a different question.
You can create more of a vacuum with your tongue blocking your throat, just using your mouth. The vacuum you create with your diaphragm amounts to trying to inhale water, which you’re not going to do.
IIRC the last time I tried it I could create about -4 psig, or 1/4 atmosphere, this way, so I could draw water up 8 feet. However, this would depend on the diameter of the straw, even neglecting capillary effects. The problem is that this is a batch operation, and the tongue can’t keep grabbing a new position to draw more flow.
I think trying to create pressure this way was harder but I could do about +3 psig, and by diaphragm it was down around 2 psi either way. Practically anybody can exceed around -0.5 or -1 psig, which you actually have to do if you are floating upright in water and breathing. Your lungs are about that deep.
I saw this done on Mr. Wizard when I was a kid, and if I recall correctly, the little girl performing the experiment was able to lift the water column a few feet. They repeated the experiment with a vacuum pump and were able to lift the column a few tens of feet.