Well I have now fired two seat mounted air bags under more or less
controlled conditions.  Not sure what Duncanwood was mumbling about but
all it takes is a low current 12 volt source connected to to the
appropriate wire and BANG, off it goes.  Smells remarkably like a very
large percussion cap. The load on the seat appears to be minimal - the
first test we bolted the seat to a pallet and then trapped teh pallet
under a fork lift truck.  The second test we just rested the seat on a
pallet with a couple of studs (without nuts) passing through two
drilled holes.   All very satisfactory.  Now we feel that we know what
we are doing we'll deploy another 8 - 10 under rather more formal
conditions including some high speed filming.