No reason, just read alot about hellium CTB as It seems more talked about. I was aware Nitrogen was basically the same effect. Found a 2.2L N2 that comes with a twin gauge regulator for quite cheap. 2.2 litre's enough? Should give about 30 mins. Any reason why Helium has become more popular?
It was quite popular method before when one could use cheap ballon canisters with an exit bag. You still can but a few companies American especially have started mixing air into the helium to stop people from Ctb.
The knowledge on how to ctb with inert gas has risen exponentially these last years and more serious setups and alternatives to the exit bag has risen, like scuba scba or hood. The search for high purity inert gas for the more serious setups naturally found nitrogen to be more available and cheaper than helium.
I would say at least on this site that nitrogen is more popular choice when doing a more serious setup than a cheap ballon tank bag setup. Maybe because the tread connection is more standard and adapters more available for regulators.
2.2 liters at 200 bar? PPH list it as enough with 30 min of inert gas. I prefer atleast 40-50 min for trail rounds but if your sure 30 min will be enough with 15 min of time to spare.