How do you determine how much gas is left in an oxygen bottle? I
don't want to run out in the middle of a soldering job.
I'm assuming you're talking about the refillable type, not the
dispossable ones about the size of dispossable propane tanks.
In the US regulators used on refillable oxy & fuel gass tanks
usually two guages on them. The one on the right (as you're looking
at the guage) shows the pressure in the tank. The one on the left
shows the pressure of the gas as it's leaving the regulator.
While it's not the same as a Full, 3/4, 1/2, 1/4 type guage the
pressure indicated on the right hand guage is a good indication of
the amt of gas in the cylinder. An Oxy cylinder typically starts out
at about 2000 psi when it's full. Noting where the pressure is
between it's full pressure & zero will give you a good indication of
the amount of oxy left.
Typically, the oxy pressure doesn't fall very fast unless you're
using a humongeous torch (not likely for jewelry work) or have a leak
somewhere in the system.