2 cents from a dive shop.
Sitting here in my repair area turned around, grabbed a second stage tuned to 140 psi.
Fed it with 50 psi, and it worked fine.
I Cannot speak for all regulators, but I have breathed a whole bunch of scuba tanks
empty, and with a good high performance regulator
, well tuned, resistance to breathing begins to be felt at tank pressure of about 3 bar OR 45 psi. I would hazard a guess that a lot of that resistance comes from the tank valve and first stage as they the smallest openings in the entire system.
tank gets tough to breath from as it approaches ambient pressure.
Nothing scientific here, just observations from fixing dozens of regs and diving
most every day for for the last ten years.
I think I will go and get one of my easily adjustable first stages and crank the pressure way down and see what happens. Okay just took a Mares abyss 1st stage and dropped the I.P. down to 50 PSI and it breathed fine on the surface.
Dropped it down to 30 psi and there was some breathing resistance.
I think this will work
fine at shallow depths, no spring swapping required.