Carbon fiber tank weights 50% less than aluminium one but holds twice as much O2.
Go to the bottom to get specs comparison:
http://www.michiganinstruments.com/Thumper%20Mobile%20Oxygen%20Specifications.htm

It is pretty amazing but on expensive side. DD size tank will run you about $450. I handled one last week at the buff show for EMS and I wouldn't mind carrying it at all. Great tool for search and rescue or whenever space is limited and you need more Oxygen.

Matt