I have expierienced (How do you spell that vonash???) problems with perosity when using higher than 20 CFH. A oldtimer told me, and now I tell those slightly younger than me by 20 years, that using a higher flow rate, will make the argon (whatever gas you are using) rush by the puddle, and suck the air in the atmosphere into the puddle. I have always used this advice, and never had any problems unless running out of gas, not clean enough, or way too windy conditions. Even at home.
Have a great day
Mike