Hey DB,
I just wrote an article about packing for technical diving for ADM's E-zine. (If you haven't signed up for it yet, here's the link
Log In and it's free.)
Frankly, getting a rebreather onto an airplane in carry-on might be tough although I have honestly never never heard any stories about that one way or the other.
Assuming you could get it through security, there is then the problem of it actually fitting in the overhead bin. Each airline has a links page that details its size limits, that would be worth checking out because it might make the decision for you.
Also, airlines have begun publishing weight limits for carry on bags. In my experience for the most part they are not actually enforcing them --yet. I do know at some airports there are now (ominous) scales at the security area. That might be a factor.
In the ADM E-zine article I wrote are some links to resources pages that might provide some helpful information. Perhaps the most important information is which airports you will be flying to, through and from. The track record of each airline at specific airports is important.
For example, if you are flying through Philadelphia on USAir, there is a very good chance you will have a bag delay.
One thing I try to do is avoid airports with lousy bag delay records.
I know the feeling about checking gear. As a photographer, I don't have any choice because the camera gear has to be with me. Everything else is checked --almost always overweight and often with an extra bag.
Hopefully this has been helpful.
Jeff
