By the way, CO2 in an air bottle is safe, but it isn't recommended with composite bottles. As CO2 is in liquid form at high pressure, and vaporises as you use it, the bottle will freeze down (as any CO2 users will know). This isn't ideal with composite bottles as the freeze/warmup cycles are said to put a lot of stress on kevlar/resin weave. So you could be shortening the life of your (expensive) bottle. Personally I'd use a CO2 bottle for CO2- they're only £20 or so after all....