I'm going to tell you my experience:
I bought a 5 liter cylinder of dry nitrogen and a pressure reducer. I also used an old airtight CPAP (sleep apnea) sleeping mask that my mother had with duct tape directly at the outlet of the pressure reducer.
Pressure reducer, do you mean a regulator?
Cpap mask often struggles to achieve a good airseal seal, only the high end mask manage this and it's not uncommon that when sleeping/unconscious the seal fails. Maintenance is also important to keep the seal, when you mention it's your mothers old mask it doesn't strike me with confidence.
An easy test would have been sealing the tube with your hand and attempt to breathe from the mask. I suspect you would have been able to breathe, maybe struggle but breathe nonetheless. I for example could breathe with quite easily with a full face mask when I had a beard, even if the mask was secured tightly to the face, the hair was enough too keep air coming because of the vacuum.
Did you duct tape the Cpap hose directly to the regulator outlet? I don't know the dimensions of your model but I can definitely see a problem keeping that connection airtight, even with duct tape.
apparently the pressure reducer did not have a suitable plug at the outlet of the nitrogen cylinder and when it rose more than 10 bars it leaked. Maybe if I had put the hose in a bag and tied it around my neck I would have achieved my goal, but for "comfort" I chose the CPAP mask...
I don't understand this sentence in bold
10 bars, do I assume right that you didn't use a flowmeter?
You're correct you'd had probably died using a bag. Even if it leaked from the regulator it wouldn't have let air in the system since nitrogen is continuously pushing it out.
Mask setups are incredibly sensitive to leaking since every time you breathe a vacuum is formed sucking air inside. The mask has to prevent this to be successful, this isn't as crucial in the design of the Cpap mask. The fact that you woke up after the nitrogen ran out tells me that there is a leak. The nitrogen flow was enough to render you unconscious, but the small leak from the mask provided enough oxygen to keep the cells alive. When the nitrogen ran out, the oxygen level rose allowing you to wake up.
I also took 2 mg of anxiolytic and 20 mg of hypnotic to be relaxed. The strange thing is that I did it in my car, despite the gas escaping, I think it should have affected me. Maybe 5 liters were insufficient.
Gas escaping in the car wouldn't have done anything in such low quantities, inert gas isn't poisonous. The mechanics is replacing the normal air with inert gas to starve the body of oxygen, the car is just too big to continuously push out the oxygen.
I'm sorry this happened to you, it must have been a terrible experience waking up and facing the fact that the attempt failed. I would been severely depressed after also so I understand.
Not to be deliberate rude, but did you do any research before trying this? Because there are quite a few points many would have warned you before trying this setup.
The cpap mask idea reminds me of an image the circles around other websites, which encourages this exact approach you did but with helium instead. Did you find inspiration from somewhere or was it your idea to use the mask?
I appreciate you writing this as a warning, inert gas especially with inferior mask are a huge risk of failure. I'm sure people will take your story to heart and think twice.
Again I'm sorry this happened to you and the situation it put you in now.