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Interesting point.......I'm wondering about how much I could get out of an "opened then closed" valve....? One breath, half a breath?
It depends on your equipment, your depth and how you measure a 'breath'. For me, I can get two breaths at 10' with just what is in my line. Instructor closed the valve at the bottom of the pool so I would know what it feels like to run out of air, just in case the SPG was not working, i.e. giving false readings. Gave him the OOA sign and he opened the valve back up.