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What is the pre-flow setting you use?
I am asking because I upgraded my torch and the lead is longer (25’) than the supplied torch. Currently my PP205 is set at about 2 sec using the supplied torch which has a 12’ lead, I think. I want to make sure when I start welding that argon is shielding the weld and not air.
Argon is slightly heavier than air. Once your hose is purged, it tends to stay full of argon and only a brief pre-flow is required, no matter your hose length. One big issue is not starting your arc when the initial blast of argon comes out. Pre-flow lets that settle down. A lot has to to with the size, length, and composition of the hose before the solenoid valve. If the hose is long, large bore, or soft, it acts like a reservoir of argon and you will get a big blast, when the solenoid opens. Every setup is different, but one sure way to get near perfect starts every time is to tap the pedal so you are running in the longer post flow, then before that stops, you start your arc. With that technique you can set your pre-flow to zero or close to it. Then you know exactly when your arc will start, too. There will be no delay from when you hit the switch. This can be helpful in being as precise as possible. You will get a consistent start and there will be no arc deflection from that initial argon blast, and no waiting, unsure of just when the arc will start.
Thanks for the info, that makes sense.
My Airco's built-in pre-flow is about 1 second, seems to work out great.
Realistically, you'll want to set the pre-flow to the level where the sudden 'whoosh' of gas out of the nozzle has subsided, and stablized. If you set the pre-flow too short, the arc will try to initiate when the gas flow out of the nozzle is too high, and cause problems with hard starting (and leaving some arc striking contamination-like evidence as well.) If you set the pre-flow too long, you are wasting your time waiting for the excessive pre-flow to conclude every time you start the arc, and wasting gas as well.
The factors that go into how long of a pre-flow you need don't generally relate to how long of a torch lead you use. What's more important is how long that "whoosh" of gas out of your torch lasts, which is determined by factors such as how long of a hose there is on the back of your machine (and what diameter it is) between the flow regulating valve and the solenoid, and even which torch components (collet body and collet, etc) you are using.
Edit - just read rambozo's reply more carefully, and he is pretty much beat me to the punch. :)