Michael Doelle wrote:

 I would assume that since their units are actually in use, the scrubber
regularly gets changed by someone who knows how to do that. This is not the
case in an emergency unit that 'just sits there' for any amount of time.
And on a dive boat moisture would very likely be an issue.

You could be right. I'd be surprised if that was the only reasons, though.
With no more knowledge than I have on the subject, it would still seem
possible, and even logical, to seal the scrubber in something air tight
which could then be opened when needed.  I still find the idea of something
so compact that could provide O2 for an extended period.

As you may recall, my desire to have an emergency O2 supply does not relate
to diving.  It's is specifically because Jayna got CO poisoning in the open
cockpit of a flybridge boat.  We returned to shore and put her in an
ambulance on O2 as soon as possible.  A couple hours later, when they took
the first arterial blood sample, her CO levels were still more than half of
what is normally fatal.  The 80 cf tank I presently use meets my need for O2
aboard, but will not last as long as the unit I saw and is a whole lot less
convenient to store.  The longer supply was particularly appealing since
some of our boating is done in the Bahamas, where time to evacuation may be
considerably longer than it would be from local waters.

On the other hand, the 80 cf tank could be used for in water decompression
if/when that seemed to be the best chioce.  The DAN unit with scrubber was
not designed for underwater use.

Lee