Most blind encoders used in GA aircraft are heated , but some of the older ones such as those built by Transcal and most encoding altimeters use a notched wheel and optical reader to encode the altitude data these type do NOT need a walm up.
The more modern encoders have a baro unit that is individualy calibrated and corrected by an eprom and these units must work at the calibration temp and so have a heater unit built in and must have time to walm up.
Most of them can be fitted in or out side the pressurised cabin.
The types that I have found (in todays research ) use this system are Ameriking , Narco , King and the Shadin ADA200 air data computor (if you are going mode S).
Last edited by A and C; 22nd Jan 2003 at 17:14.