I've welded a few gas tanks. I always filled them with water all the way up to the area to be welded. I just left a small pocket of air where the weld will be (assuming it's a reasonbly small weld area). If you do it this way you'll be safe. The key is to avoid having the empty airspace inside the tank filled with air that contains oxygen. All it takes is mere fumes of fuel or even possibly the gaseous byproducts of the welding process. If there's enough air (oxygen) in their, BOOM. Boom and you're dead, and dead of horrible wounds. This happens all the time, simply google tank + welding + explosion and you'll see. Don't give up the ghost just to save a few bucks repairing an old clunker (or a new one for that matter) - it ain't worth it. And Dale, you mention using compressed air as a way to "dilute" the fumes, thereby keeping you safe during welding. Think again! As you weld you're heating that tank up and driving off fumes inside the tank. Add to that the nice fresh supply of air (i.e. OXYGEN) that's constantly being replenished inside the tank, and you have all the makings of a big bomb. If I were you I wouldn't do that anymore AT ALL. You may have gotten away with it a few times, but your number could come up next time you try it. And goathill piping exhaust in from a motor is a bit too unpredictable. Remember, this is quite literally a life and death situation, you want a failsafe way of doing this. You say the carbon monoxide "dilutes the fumes". It's not that it dilutes the fumes, it removes the free oxygen, which is what's needed for combustion. But there are variables, like how long do you run the motor, how big is the tank, how much ambient air is getting mixed in with the exhaust as it's ported into the tank, and how much oxygen really is left in the tank as you weld? That's a lot of guessing and assumption there. Who wants to guess and assume in a situation like that? Think about what your risking your life for - think about what it must be like to be blown up by a fiery explosion...and for what? To fix an old tractor? C'mon guys, no way. Fill it with water all the way up (with only a tiny airspace left for the weld area), or constantly purge it with CO2 or nitrogen gas in a professional manner (there are even pitfalls with the purging process if you don't do it right though...that's wat I mean by professional manner). Take care, and enjoy those old tractors in good health.
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