Bev
If there are dashes on the screen this means that no deco time is
not being shown as it will never be reached. You did say that you were
shallow. So that is probably the reason. You don't say how shallow, but
if less than 6 metres then this would be the case.

Steve

I think what others said is right about the dashes are just because you
aren't deep enough to take on any significant compression (or whatever you'd
call it).  A friend of mine did the Nitrox course last weekend, and on his
assessment dive he said his Vyper just went blank and didn't show anything.
Whether he means the whole screen or just the deco bit i'm not sure, so it
could be that the Vyper doesn't like switching to Nitrox (which seems
unlikely) or it might be that the Nitrox display isn't obvious to people who
are used to the Air display.
I haven't used Nitrox myself so can't really comment on what I think - just
what someone told me!

David

if the vyper display -- as nodeco time that means that you have more
than 199 minits left on that depth. (page 67 of the manual) for a
overview of nodeco times on varius depth on air look at the table on
page 63. With A0 and P0 the time below 9m is more that 199 minits for
AIR. With Nitrox you are bound to get into this a loot more. On my
stinger I freqvently gets dives that gives just -- and nodeco time.
Especially in the end of the dives, making a resly nice slow acesnt
along a cliff or someting.

Using the Suunto Divemanager you can simulate dives to get an
impression of what the computer will display for you during the dives.

/ Balp