helios marathon. Hello, My Anthem insurance covers one H850 liquid oxygen unit. As a backpack, C1 is an OK setting for just sitting around but I need C4 or C5 for walking around or going up stairs. As a result, I often run out of oxygen at dinner, a movie, opera, theater, shopping etc. I think I need to purchase an extra portable. The Helios Plus H300 seems attractive. Or maybe I should just buy an extra H850 unit. Any advice or suggestions? I really must have more than what I have now. HARLAN KESSEL in Oakland CA