This is something that is rarely brought up even in welding courses. Low side- set line pressure highside- tank pressure FACT: Never run the high side pressure in the acetylene tank below the low side pressure of the oxygen tank. For instance, if your oxygen pressure is set to 25psi, your acetylene tank is effectively empty at 26psi on the high side acetylene gauge. 1 psi more than lowside o2 pressure is the cutoff level. The reason for this is that when o2 low side exceeds Acetylene high side a backflow can be created which lets o2 flow backward in the line into the Acetylene tank. Backflash preventers don't recognise this and will not stop it. Free acetylene gas is unstable above 12psi and is explosive at 15 psi. and the introduction of oxygen into it creates a very hazardous situation. Conjecture: While I have no sources or figures to support my hypothesis, I believe that most tank fires that are blamed on backflash are in reality originated in the tank by the very condition described. The company I have spent most of my life with has never had a Oxyacetylene incident so I have no first hand experience with tank fires. I have only seen the result of one and that one burned about a foot off the tank end of the hose which tells me the fire originated on that end.