The pressure at the bottom of a column of water is about 1/2 psi per foot of height, so I would imagine that you would need about 5 or 6 feet of water head to achieve a pressure of 2.5 or 3 psi.

Mixing Compressed Air With Water Makes Awesome Explosions. Don't try this at home. By Avery Thompson Published: Sep 09, 2016 4:47 PM EDT. Save Article. The Backyard Scientist // YouTube.