Portable oxygen cylinders are small tanks that contain a specific amount of compressed oxygen that empties as you use it.
Larger cylinders contain more oxygen and last for longer periods of time
Smaller cylinders must be changed more frequently, but are easier to carry
Since all portable oxygen cylinders require frequent changes, you must plan ahead when traveling to make sure you bring sufficient replacement cylinders for your trip.
Resource: Duration Guidelines for Oxygen Cylinders
You can use portable oxygen cylinders if you have a prescription for oxygen from your doctor. RANA can help you choose a suitable cylinder size.