I did similar tests! At my work we have huge plastic bags. Im also the boss so i have a nitrogen tank in my office. When i get super depressed i would fill the bag with nitrogen and inhale as needed with hopes of getting myself to come close to cbt. Its a great feeling to see my vitals drop.
Sometimes id have more of an urdge at home to ctb so i took a few bags filled with nitrogen home with me. I inhaled them and could not get my levels of oxygen to drop below 90. After lots of testing and frustration i realize putting nitrogen in a bag, the nitrogen gets weaker and weaker due to excaping the bag. The bag remained full appearance wise but the concentration drops. I have an oxygen meter and a fresh bag of nitrogen makes the numbers drop fast. But if you leave the bag over 24 hours the concentration of nitrogen is so low that it prevented my oxygen level from dropping below 90.
So just wanted to mention this incase anyone has the same problems i had.