BTW, one thing to keep in mind is that during the time of swapping your one working regulator to the other tank, you're going to be without ANY working regulator.
And your hands will be numb and shakey if CO2 is building up in your blood since some sort of horrible episode is on like Donkey Kong.
..........Breathing out of a tank is fine in the pool. It's harder to feather that value with numb hands and the shakes. I use cheap Thermo valves and they are NOT easy to feather.
I believe the standard thinking is pistons are better for swapping versus diaphragms, but I unfortunately know that diaphragms do work.
A crushed second stage or badly cracked reg body on a stage bottle is not what I call catastrophic since it's one of those thing can can happen without you noticing it until it's time to use it. Turn it on, see the needle firm up, but all you get is water. If the reg has a big enough hole or crack in it to prevent negative pressure from activating the value it's trouble and you'd have to have the mind to push the purge and still you get lots of water. If the rim, purge or body is cracked the diaphragm that pushes the second stages valve may be prolapsed or out of place so it might not work either.
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