[Old Professer]

It opened the valve and held it open against water pressure.

I think I misstated the volume of the room; it is, of course 800 cubic feet.

[supercat]

It opened the valve and held it open against water pressure.

That requires a 120CUFT N2 bottle? Why not use a pilot-valve arrangement like that used in electropneumatic pipe organs (in a pipe organ, a small valve allows pressurized air into a bellows which then operates the main valve; for water, I'd sugget having a small valve let water into a piston to do likewise).