SMA DIVERSION TO ALTERNATE ARPT AFTER INFLT ENCOUNTER WITH ICING CONDITIONS.

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I DEPARTED ZANESVILLE AT APPROX XA30 EST AND CLBED THROUGH A THIN BROKEN LAYER. ALTHOUGH I HAD INITIALLY FILED FOR 9000 FT I REQUESTED AND WAS GIVEN 7000 FT AS MY FINAL ALT; SINCE THE ACFT I WAS FLYING IS NOT KNOWN FOR ITS STELLAR CLB PERFORMANCE. AS I CONTINUED EASTWARD THE CLOUD TOPS BEGAN TO RISE. I DID NOT NOTICE THIS AT FIRST AS I WAS DISTRACTED BY A BARRAGE OF QUESTIONS FROM MY PAX WHO WAS A STUDENT PLT 'ENTHRALLED' WITH THE WORLD OF IFR OP. AS I NEARED THE JOHNSTOWN VORTAC ON V12 I NOTICED THAT I WAS VERY NEAR THE TOPS OF THE UNDERCAST AND THAT UP AHEAD THE TOPS SLOPED SLIGHTLY ABOVE MY ALT. I IMMEDIATELY REQUESTED HIGHER FROM ZOB. I WAS TOLD TO STANDBY. AFTER A MIN OR 2 THE CTLR GAVE ME A FREQ CHANGE AND ADVISED ME TO ASK FOR HIGHER ON THE NEW FREQ. I GOT OFF AN INITIAL CALL DURING A MOMENTARY LAPSE IN THE RADIO TFC BUT MY CALL WAS NOT ACKNOWLEDGED BY THE CTLR WHO CONTINUES ISSUING INSTRUCTIONS AT A RAPID RATE TO VARIOUS ACFT. BY THIS TIME I WAS STARTING TO GO IN AND OUT OF THE CLOUD TOPS. I ATTEMPTED CONTACT 2 MORE TIMES WITH NO RESPONSE FROM ATC. I THEN SWITCHED BACK TO THE PREVIOUS FREQ AND TOLD THE CTLR WHAT HAD HAPPENED. I ALSO REPEATED MY REQUEST FOR A HIGHER ALT. A THIN LAYER OF RIME ICE HAD ALREADY BEGUN TO FORM ON THE ACFT. THE CTLR TOLD ME TO TRY CALLING AGAIN ON THE PREVIOUSLY ASSIGNED FREQ AND TOLD ME THAT HE COULD NOT GIVE ME HIGHER AS I WAS ALREADY OUT OF HIS SECTOR. I SWITCHED TO THE NEW FREQ. I WAS TOLD TO STANDBY. ICE WAS NOW FORMING ON THE WINGS AND STRUTS OF THE AIRPLANE AT AN ALARMING RATE. THE CTR CTLR ADVISED ME THAT THE ONLY ALT AVAILABLE WAS 14000 FT. I THEN ASKED FOR A LOWER ALT AND WAS CLRED TO 5000 FT. E OF JST I BEGAN TO ENCOUNTER TURB AND ICE AGAIN BEGAN TO BUILD ON THE AIRPLANE. I MADE 2 CALLS TO ATC REQUESTING A 180 DEG TURN. THE FREQ WAS STILL VERY BUSY AND I RECEIVED NO REPLY. FINALLY; I DECLARED AN EMER AND REQUESTED VECTORS TO JOHNSTOWN. POST-FLT INSPECTION OF THE ACFT REVEALED ABOUT 1/2 INCH OF RIME ICE ON THE AIRPLANE. I DECLARED AN EMER BECAUSE IT WAS THE ONLY WAY I COULD THINK OF; UNDER THE CIRCUMSTANCES; TO GET OUT OF THE SIT I WAS IN BEFORE IT REACHED THE POINT WHERE I HAD NO ESCAPE OPTIONS LEFT. I CAN; HOWEVER; POINT TO 2 POSSIBLE ERRORS ON MY PART. THE FIRST WAS CHOOSING NOT TO CLB TO 9000 FT; THE ALT WHICH I HAD FILED FOR. MY MOTIVATION FOR STOPPING THE CLB WAS A DESIRE TO AVOID THE EXTRA TIME (AND EXPENSE) REQUIRED TO CLB THAT HIGH IN AN ACFT WHICH DOES NOT POSSESS A VERY GOOD CLB RATE AT NEAR GROSS WT. 9000 FT WOULD HAVE BEEN A MUCH 'SURER' BET FOR STAYING OUT OF THE CLOUDS THAN 7000 FT WAS. THE SECOND POSSIBLE ERROR WAS EXHIBITING TOO MUCH DEFERENCE TO ATC WHEN FIRST ENCOUNTERING THE HIGHER TOPS. I SAT QUIETLY AND DID EVERYTHING I WAS TOLD TO DO. I SHOULD HAVE FLATLY REFUSED TO ENTER THE TOPS EVEN FOR A FEW MINS. I COULD HAVE ASKED TO DO 360'S UNTIL I COULD GET HIGHER; OR I COULD HAVE ASKED FOR A VFR ON TOP CLRNC FROM THE FIRST CTLR. I ALSO SHOULD NOT HAVE LET MYSELF BE DISTRACTED BY MY PAX. HAD I NOTICED THE SLOWLY RISING TOPS EARLIER I WOULD HAVE HAD MORE TIME TO ACT PRIOR TO ENTERING THE CLOUD TOPS.