The duty cycle on acetylene tanks has to do with how fast the
acetylene gas can bubble out of suspension.
Acetylene gas is explosive if compressed to over 15 PSI. The way
they get it into the tanks at “higher” pressure is to dissolve it in
(There’s diatomatious earth in there as a buffer too, but it’s the
acetone that’s doing the work.)
It’s sort of like an open bottle of soda: no matter how open the top
of the bottle is, there’s only so fast the bubbles will come out.
If you pull the gas off the top of the cylinder faster than it can
bubble out of solution, you start sucking acetone up your lines,
which starts rotting out your seals. (this is also why you don’t
want to put an acetylene tank on its side for any length of time:
the acetone goes after the valve stem seals.) The way you spot this
is that (A) you can smell the acetone, and (B) your flame goes
In the short term, it’s not a problem, but it’s not something you
should make a habit of doing. As soon as you notice it, shut down.
The rule that I remember is that you can only pull off about 15% of
the remaining volume per hour. If you’re running less than that,
you’re OK. Alternately, you need to wait a while if you’re pulling
more. A B tank can support a casting torch for a few minutes. So if
you’re doing small(ish) melts, and you’re only going to be doing one
melt at a time, you should be OK. At a guess, you’ve got about 10
minutes at those sorts of draw rates before you suck the tank down.
I cast successfully for years with a B-tank. The trick is to
remember to let it rest, and to get your melts done fast. Don’t
screw around with them. Which is good from the metal’s point of view
I don’t use rosebud tips. They’re too sensitive to overheating. (get
them hot, and they start flashing-back. Deeply annoying) I use a
small cutting head. They’re not nearly as sensitive to heat, and
have a slightly better mix ratio. Just make ABSOLUTELY sure that
you do NOT hit the cutting jet handle.
On one of mine, I flipped it up backwards along the head, and wired
it in place, so it couldn’t be used. I also mix just
ever-so-slightly reducing. Still a very hot flame, but just a bit of
un-burned fuel to use as a shield over the melt. Don’t mix it up Oxy
heavy the way you would for cutting. Just the faintest bit of the
rooster-tail still, and you should be good.