Recertifying nitrous bottles?

What you probably need to consider doing is to set yourself up with a much larger "mother" bottle. There's been a lot of BS but the fact is most ANY standard pressure bottle is easy to transfer

1..Get a mother bottle and depending on the setup, arrange it for liquid transfer (for NO2)
2..Get the appropriate hose and fittings.
3...Cool the empty bottle in a freezer
4..Hook up the hoses, crack the target bottle hose end fitting and "blip" the mother bottle valve to bleed the hose.
5...Slowly open the target, then the mother valves, and transfer contents. To minimize liquid loss, after transfer, and before disconnect you have to get "gas" in the hose. If the mother cannot be easily handled/ inverted, then close the mother valve and leave the target bottle valve open a few minutes. This will allow the liquid in the hose to migrate into the bottle and vaporize into gas, anything that remains in the hose is hopefully now gas.
6...Close the target bottle valve, recheck that the mother valve is closed.
7...While filling you can "rig" the target bottle onto a scale to get some idea of where you are. You will want to first get a "tare" weight on the target bottle and to determine the filled weight. After filling, take a more accurate weight.
8...If necessary, re-chill the bottle in the freezer and top it off

It is not necessary to bleed, empty, or evacuate the target bottle unless there is reason to think it is contaminated. It IS necessary to accurately weigh bottles and to KNOW what the tare and filled weights are going to be, and leave a "fudge" (underfill) for safety

I've transferred a LOT of refrigerants, CO2, Nitrogen, and even some Argon mix gasses using this method, as well as O2 to a smaller "suitcase" Oxy-Acet outfit for portability.

You know, that bank job where we .........................