In the manual supplied by the gas company here (NZ)they
advise to turn the oxygen valve on 1 1/2 turns, they say the
oxygen is the more dangerous as this adds to a burning fuel and
should be turned off as quickly as possible.
The reason for opening the oxy tank valve fully in the US (don"t
know how the NZ oxy valves are made) is the design of the valve.
In valves, the packing around the valve stem & the packing nut
are the only things that keep the fluid/gas from leaking into the
atmosphere. In low pressure systems, under 500psi, (acetylene is
usually about 375psi for a full tank) this is usually adequate.
For higher pressure systems (oxy is usually about 2000psi in a
full tank) the valves are constructed with 2 seats, 1 in the full
closed position & 1 in the full open position. By adding the 2nd
seat, full open, and opening the valve fully, the only possible
way for the gas/fluid to exit the syst=em is out the valve
opening. If the valve is only partially opened the high pressure
gas (oxy) can also force its way out the along the valve stem.
While adding a little extra oxy to the atmosphere may help, it
sure doesn't do anything for making the oxy supply last for
The reason the acetylene tank should only be opened a little
(1/4 to 1 1/2 turns) is to allow it to be turned off faster in
the event there is a problem. Fires need 2 things, fuel & oxy.
It's difficult to remove the oxy (it's in the air) from a gas
fire, sooo the best thing to do is be able to remove the fuel
(gas) in the shortest possible time.
However you treat your welding/soldering system, do it with care
& respect. It'll serve you faithfully & trouble free.