What about this safety valve I keep hearing about that sometimes blows
out in a hot car or room? I'm worried about that one. It sounds like a
jet taking off when it happens, apparently.

Nikki

It's a burst disk, rather than a valve. Opens once, then needs to be
replaced. The minimum blowout pressure for a CO2 cylinder is usually
something above 150 bar.

There are two ways to reach that pressure in a CO2 cylinder:
1) Grossly overheat the cylinder.
2) Overfill and somewhat overheat the cylinder.

Leaving a full cylinder in a closed car in the sun where temperatures
are reputed to reach 60 degrees within a few minutes will fulfill #1.
Even at 49 degrees a 100% full cylinder will only be at around 135
bar.

At 32 degrees a cylinder will reach 150 bar if it is 125% over filled.
What are the chances that simultaneously your fill station attendant
screwed up, and you have a day that warm in the U.K.? (Just teasing -
I live on the Wet Coast of Canada.)

In addition to the soda fountains and beer dispensers mentioned by the
previous poster, there are also thousands of CO2 fire extinguishers
surrounding us fitted with the same burst disk arrangement. Burst disk
rupture is a rare event that usually has to be worked towards.

It occurs to me that burst disks are only required on SCUBA cylinders
in North America, not in Europe. It might very well be that you don't
have them on CO2 cylinders either. No worries, eh?