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Why You Need Preflow When Welding



When you're about to start MIG welding or TIG welding, it's easy to forget to bleed the gas. Why is bleeding the gas through the cable important?

Kevin gets out his AHP AlphaMIG 250, which has a nice long MIG cable on it. The machine is cold - it has not been turned on yet. He is going to put on his welding safety gear and show you what happens when you don't use preflow or purge the line.

Kevin has the AHP AlphaMIG 250 set on 23.5 volts and 377 inches a minute. He has some 1/8" mild steel plate that he's ground down the edges on and clamped down to the workbench. He asks viewers to pay attention as he welds.

After he welds the seam he asks, "Did you hear the difference?" Kevin could hear it plainly. He shows the beginning of the weld where there is some porosity. That happened because the gas hadn't yet gotten from the gas bottle to the all the way to the end of the cable. When the gas is at the gun and you pull the trigger - Bam! - that gas is ready for you.

AHP also actually programmed in a little preflow for this MIG welder to try to stop that problem. It works pretty darn well. If he had used one of his older, big machines that does not have preflow you'd see a big old blob of porosity, and then all of a sudden the weld would look good.

Preflow is really imperative with TIG to help flush the line and get gas to the weld so it can do its job.

So what if you don't have a preset capability like this machine does? Just bump the trigger for a second. That'll help get the gas going. If you're TIG welding, just stomp the foot pedal for a second get the gas flowing. Or set up your preflow on your welder. I like mine set at about a 1/2 to 3/4 of a second.

So that's just a friendly little reminder for you guys who are used to welding and don't even think about it and Kevin hopes it's helpful for new welders, too.

Kevin is ready to go back to work, but you might want to stick around for a moment to see Kevin explain the importance of passing gas .....

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