On Mon, 11 Jul 2011 20:05:40 -0400, "Carl Ijames" wrote:

CO2 is the cheapest purge gas. You could mostly fill the tank with water
and then purge the remaining headspace with CO2 gas or get some dry ice and
toss it into the tank and let most of it sublime then start heating. To
remove 95% of the air you need three tank volumes of gas, assuming the
exhaust line is long and skinny to keep air from back-diffusing into the
tank. Without water, 22 gal is 82.5 L so 3x is 247 L. If you have a
flowmeter on your MIG CO2 tank you can crank the flow up and calculate how
long to wait. You will get roughly 1000-fold expansion from the dry ice so
247 L/1000= 0.25 L of dry ice. Winging the density that would be about 500
g or 1.1 lbs, so get two or three pounds and wait until 2/3 or 3/4 has
sublimed then fire up the torch. Again, you want the exhaust line to be
long and skinny, not just the fill neck :-).


3X purge is not required with CO2 because CO2 is so much heavier than
air. Put it in at the bottom of the tank and it will displace all air
and vapour as it fills. 20% more than tank capacity is all that is
really required - but I always play it safe and add a bit more part
way through the job - particularly when brazing oil pans.

And a long and skinny exhaust is NOT required if the filler kneck is
located at the top when filling/welding.
-----
Regards,
Carl Ijames
"Ignoramus24437" wrote in message
m...

I have a 22 gallon (or so) fuel tank that originally had gasoline in
it.

I want to use it for diesel.

I would like to drill it and install a through-hull fitting, which
would be for the fuel return line. Ideally, I would like to braze the
fitting in place also.

My question is how do I drill it and braze, so that it would not
explode.

The tank has not had gasoline in it for a couple of weeks.

Today, I recently set it up with the fuel cap open, turned it over so
that the fuel fill hole pionts down, and set it out so that it would
becmoe quite hot under the sun.

Would it be correct to assume that after a few days I could purge it
with compressed air, and then drill and braze it, without exploding?

Would purging with argon be a good idea?

i