Posted for "Denny" <d.duello@www.mebbs.com>:

Roy S. is a professional welder who has electric arc welded many steel gas
tanks utilizing the shop vac method over more than 40 years.  Of course that
doesn't prove that he was in absolutely no danger, or lucky either.  Until I
become familiar with a more effective method though, I believe I'd trust his
experience.  However, with a respectful nod to the concerns expressed here,
I'd consider doing two things that I haven't seen Roy do.  1.  Dissipate any
possible static charge before taking the vac close to the tank, using the
precautions recommended for use in plane refueling.  2. Lower the
concentration of the first vapors to pass through the vac by introducing
compressed air deep in the tank for a short period prior to taking the vac
tube close to the tank opening. I'm confident that once the vac was running
the concentration of vapors in the heavy flow of air through it would be
very low. I suppose I could just stick a hundred foot garden hose in the
tank and adapt it to the vac sitting way outside the shop too. Too simple.