Depends...upon what it is opening into. The gas will expand from it's tank pressure to ambient, BUT (big BUT) not if there is another "container" for it to expand into." Put a lip lock on the valve the way you do a regulator and you can get the full tank pressure. Use the cupped hand method that Dr. Sam described below, and it will be ambient.Q1) What pressure is the gas at once it leaves the tank directly by way of the valve?
About two inches over ambient is the greatest it will be inside the regulator, because of the exhaust valve acting as an overpressure relief valve (in the event of a malfunction and free flow).Q2) What pressure is the gas at once it leaves the tank by way of a first and then second stage regulator?
Dr. Sam Miller described the Scripps Institution of Oceanography using the tank breathing technique as an exercise. But what a lot of people don't realize is that university courses in scuba routinely were either on a quarter or semester basis, and were a 3-hour course (3 hours per week for the entire quarter or semester). This course therefore was hugely longer than current scuba courses, which are at the extreme of minimum to become competent in the water. I helped teach Midge Cramer's Oregon State University course in the mid-1970s, and he took the whole class to Hawaii during Spring Break.
SeaRat