Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 207; Flight Crew Total 1265; Flight Crew Type 705
While maneuvering with a student who is rated in the aircraft we got in close proximity to bravo airspace. Never got less than 500 FT below the shelf; but our transponder altitude was reading about 200 FT off of what the actual MSL value was.
A C172 instructor reports flying near but not in PHX Class B airspace. The aircraft transponder however was reading higher than the actual aircraft altitude.