Bob Engelhardt wrote:

Shaun Van Poecke wrote:
... The cyllinder ruptured in
an unusual way, splitting right down the side. ... the safety release valve
did not operate, ...

... the Co2 filled up the copper pot ... the welder climbed into the pot
... once he was
inside the pot he was overcome pretty quickly by dizziness and was unable to
get out. ...

Shaun - thanks for the real-world input. Too many of our posts are
speculation, sometimes not based on much knowledge.

While accepting that CO2 is more dangerous than I believed, I'm still
going to store my cylinder in the cellar. It's a probability thing
(like most things in life). The probability that there'll be a fire AND
the safety will fail is small enough for me. Likewise the probability
that my cylinder will leak fast enough to fill up my drafty basement is
small enough. It's a small cylinder.

Bob

It's pretty easy to figure out how much breathable air a leaking
cylinder could displace at atmospheric pressure, since that's typically
the cylinder spec for most gasses (CO2 an exception).

An 80cf Argon cylinder for TIG could at most displace 80cf of breathable
air, or less than a foot off the floor even in a small ~10'x10' shop.
Basically, a single home sized cylinder of inert gas completely emptying
in a typical home size shop couldn't pose a threat to a human, though it
could be a problem for your shop cat.

In a house fire it is very likely that you would be out of the house
long before a cylinder with a defective safety or no safety got hot
enough to potentially fail catastrophically.

Basically under normal home shop conditions you only have to worry about
flammable gasses and oxygen, anything inert just can't realistically
pose a safety hazard.

Pete C.