pito:
Your diy altimeter needs a certification I would say (ie. FAA - Aircraft Certification Service is concerned with the approval of software and airborne electronic hardware for airborne systems (e.g., autopilots, flight controls, engine controls), as well as that used to produce, test, or manufacture equipment to be installed on airborne products).. Will you pass it with arduino based system?
This is an old thread but it's worth noting for future readers that it's a common misperception is that all things for aviation must be high-dollar "certified".In the U.S.A. FAA only cares about such things when used in so-called "Certified" class aircraft - not "experimental" or ultralight class except in the case where the equipment will be used for "IFR" flight (instrument flight rules - i.e. flying by reference to instruments only in low/no visibility).