Hey Murph
I don't have alot info on altimatures, however these are real popular
with the high power model rockets. Maybe a model rocket site will have some
links on how to build your own. In fact I saw one my friend had.
On his altimature there was a 16F874 pic. Also you might want to
consider using a 16F628 pic instead of the 16f84 (more program memory,
more features and cheaper).

Sounds like a real cool project. Hope you find an answer.

Gordon