I went to my family Dr. apt. Friday I took 2 portables with me there continuous feed up to 6 liters I need 4 liters at rest & 6 on exertion my first bottle showed I had enough air but while I was waiting I started noticing my air was shutting down so my wife ran to car & got the other bottle. by the way the needle on gauge stuck that's why I ran out. well I used half of the second bottle finally I had to ask nurse if I could use theirs so I would have enough to get home.. when my visit started I thanked my Dr. for the oxygen. his reply was glad it was left here by previous Dr. it wasn't his . I was kind of surprised . so my question is isn't it mandatory or is it an option for the Dr. so this doesn't happen to anyone.