First item, I would suggest is to call the manufacturer of the system and ask him how to fill.
Next step, talk to the local hospital or gas supplier in town about filling bottles.
Thirdly, use an A/C vacuum pump to evacuate the bottle, shut the tank valve off to retain vacuum, place bottle onto an A/C charge scale, hook up supply bottle, and add 10 pounds, or whatever bottle capacity is, and fill.
Empty bottle weight does not come into the picture, just the weight of the contents inside. Keeping everything frozen, keeps gas at a liquid state.
Hope others here submit their 2 cents....