Ron Natalie wrote:

a tank that small only lasts a few hours on a charge... They have larger
ones.

The currently most advanced system is EDS-D1 by Mountain High
http://www.mhoxygen.com/

It's very popular among glider pilots because its sophisticated
pulse-by-demand-functionality reduces the oxygen flow considerably. Just
put the cannula on and forget it. Not cheap, but worth the price.

Stefan