Pony bottle size

[Vtach]

Folks, the best thing you can do to figure out what you need for a redundant gas source is calculate your SAC (surface air consumption) rate.  Once you figure out your SAC rate you can accurately calculate what your needs will be for a redundant gas source at depth by multiplying your SAC by the ATM of pressure and time at that depth.  Don't forget to multiply that result by 2 to account for an additional OOA diver (worst case scenario).

No offense to anyone but even a 19 cu ft bottle won't be of much use to 1 diver much less 2 divers OOA at depths of 100 or more.  SAC rates will be much higher in an emergency than at rest.