Inerting is an excellent way to ensure a safe cut or weld on a tank, BUT -
It requires a device (expensive) to monitor the oxygen level inside the tank.
It also requires bottles of whatever gas you will use (we used CO2) and special regulators, hoses for those bottles (expensive).
You must continue injecting the inert gas during cutting and welding procedures, uses a lot of gas.
This method is great commercially, not so for the backyard guy.
You can fill an old propane tank with water, meaning COMPLETELY fill it to the point water comes out the top valve, make sure it sits level. Make sure there are no gas/air pockets in it. Let it sit a while, drain it and repeat. Then, after you get it cut, build the great mother of all fires in it and burn it out.
I have seen several comments from various members about hazardous things "getting in the pores of the metal" inside used tanks and later causing health problems. NOT SO!
Didn't figure they would believe me, so I called the kid. Son Michael's degree is metallurgy and powdered metal technology. Guess what? Metals (cold rolled steel, etc.) DO NOT HAVE PORES! We do, pigs do, metals do not. Powdered metals (metal powder placed in a mold and subjected to tremendous pressure, then heated in an oven at tremendous temperatures in the presence of various gasses) are porous.
So, build your smoker and enjoy!