Once you receive your liquid nitrogen rental you will need a source to supply you with the liquid nitrogen. Airgas is the easiest answer because they have thousands of locations, but they can be expensive and cumbersome to work with. The cheaper and less hassle alternative is probably your local welding supply store. Technically, you don’t need to know how to fill these because they will do it; however, it’s nice to know what to expect and make sure they do it properly. If you get a worker that doesn’t care and/or know what he/she is doing, then they might fill it as fast as possible, put the neck cork or withdrawal system back on it and say your good. This is wrong, the reason is that you need to let the liquid nitrogen dewar cool down or you will get unnecessary boil off.
We teach our employees to cool it down with 2-3 liters, depending on dewar size, dump it out and then fill it up. The reason is the first couple liters of liquid nitrogen dispensed will immediately convert to a gas since the inner tank is hot. Since you are cooling the tank so fast, you will also get “snow/”frost” in the neck and if you keep filling, the snow/frost will fall into the liquid nitrogen and stay. The snow/frost is just the moisture in the air being cooled off and converted to a solid. If you dump the liquid nitrogen out after the initial cool down of the 2-3 liters, the inner tank will not create the snow/frost. If it is filled to quickly, you will also see boiling. It will continue to boil and will lose liquid as long as it is boiling. A typical 5 or 10 liter dewar can cool down in a couple minutes and a 20-30 liter might take more like 5-10 minutes depending on how good the vacuum is.
Ask the filler to top it off for you once it stops boiling if you are needing every drop. Replace the neck cork and make sure it never enters the vehicle with you.
If you have any questions, feel free to email us and we will be happy to help!