The high pressure valves ( oxygen, Argon, Helium, Nitrogen etc) have a 2 seat valve. One seats when closed and one when all the way open. You want to open them all the way in theory although I don't. Acetylene and alternate fuels you only open 1 turn. This is so in case of an emergency you can shut down quickly. So says my training at a distributor and many years at Victor. Acetylene safety plug is on the top on newer tanks. Acetylene is safe at 15psi and in reality doesn't become volatile until about 30psi and this is why a creeping gauge can blow a big hole in the ground. I'm shocked it doesn't happen several times a day.