My only concern with taking down spare air would that it would lead to you pushing yourself past your limits. It's all very well to say I wouldn't and that it is only there for an emergency, but personally if I had the canister with me and had a good dive I'd be tempted to take on an extra few metres sub conciously thinking 'no worries if something goes wrong I've always got the spare air'. Maybe that's just me, but can anyone truely say they wouldn't be tempted?? ![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)