SMA PLT IMC EXCEEDS ASSIGNED ALT.

Narrative

I WAS BEING VECTORED BY ATLANTA DEP CTL DURING A CLB TO AN ASSIGNED ALT OF 11000 FT. THE DEP SID IS IN ESSENCE 'RADAR VECTORS ON COURSE.' THE CTLR WAS ATTEMPTING TO VECTOR ME TO JOIN A JET AIRWAY. SHE MADE A COUPLE OF ATTEMPTS BY ADDING 10 OR 15 DEGS AT EACH ATTEMPT INSTEAD OF JUST SAYING 'JOIN JXX ON COURSE.' WHEN I REALIZED SHE WAS HAVING A PROBLEM PICKING AN INTERCEPT ANGLE THAT WOULD WORK; I STARTED LOOKING AT THE MAP TO CALCULATE AN INTERCEPT THAT WOULD ALLOW ME TO JOIN THE JET AIRWAY. I WAS USING THE AUTOPLT TO HOLD HDG AND CLB ATTITUDE; BUT I DID NOT HAVE THE ALT PRESELECT ARMED FOR CAPTURE. I WAS IMC AND MONITORING MY INSTS WHILE LOOKING AT THE MAP. I HAVE AN XX MODEL TRANSPONDER THAT DISPLAYS THE MODE C ALT THAT IS BEING XMITTED. I WAS USING LOOKING AT THE MODE C DISPLAY AS AN EASY WAY TO QUICKLY CHK MY ALT IN THE CLB. WHEN I NOTICED I WAS APCHING 11000 FT I XCHKED WITH MY ALTIMETER AND REALIZED I WAS PASSING THROUGH 11400 FT. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND STARTED A DSCNT. I DSNDED APPROX 100 FT BEFORE THE CTLR CALLED AND CLRED ME FOR A NEW CLB TO A HIGHER ALT. SHE SAID NOTHING OF THE DISCREPANCY BTWN MY ALTIMETER AND WHAT WAS BEING DISPLAYED ON THE MODE C DISPLAY. THE TOTAL TIME BTWN STARTING TO LOOK AT THE MAP AND CLBING THROUGH 11000 FT WAS APPROX 2 MINS. AFTER LNDG I READ THE SALES AND OPERATING LITERATURE ON THE TRANSPONDER AND FOUND OUT THAT IT IS PRESET TO 29.92 INCH HG AND MAY ONLY DISPLAY THE ACTUAL ALT ABOVE FL180 WHEN I RESET MY ALTIMETER. I HAVE ALSO LEARNED THAT THE COMPUTERS ON THE GND THAT INTERPRET THE MODE C COMPENSATE FOR THIS DISCREPANCY AUTOMATICALLY. I COMPLETED MY FACTORY INITIAL TRAINING FOR THIS ACFT ONLY A COUPLE OF WKS PRIOR; WITH NO MENTION OF THIS AS A POTENTIAL CONSIDERATION. AS A NEW PLT INTO THE FLT LEVELS; IT WOULD HAVE BEEN HELPFUL TO KNOW THIS INFO. I REALIZE THAT THE ALTIMETER IS MY PRIMARY SOURCE OF ALT REF BUT IN A NEW AIRPLANE FLYING SINGLE PLT IFR IN IMC WITH A CTLR THAT SEEMED UNSURE OF WHAT SHE WAS DOING; INTERRUPTING THE ALTIMETER DURING THE CLB SEEMED HARDER THAN LOOKING AT A DIGITAL DISPLAY.