You don't need to explain anything. If you go to the grocery store, do you explain what you want the two cases of Pepsi for? Are you in the US? If so, just go to a gas supplier and tell them you want to rent a bottle of Nitrogen. I'm sure they'll ask you what size bottle you want. Do your research first and make sure you get a big enough one for your needs. Obviously, you can't talk to them about connecting a full face diving mask to the tank. That's up to you to figure out how to do that, and there's a lot of info on SaSu for doing just that. You can call the gas place first on the phone inquiring about how much it costs to rent a bottle of nitrogen, what size, etc. If you're renting a bottle of Nitrogen, I think it safe to assume that the company is going to assume that you have a legit use for it and should already understand about what valves will fit it and such. You do your research ahead of time and become familiar with the fittings on the bottles so that you can sound intelligent when speaking about it. After making a phone call, that easily sets the stage that follows that you would show up and get one after that. Don't volunteer any information that you don't need to. You will only sound suspicious. People go in these places every day to get tanks, get their tanks filled, exchange tanks, etc.