A P210 ACFT ALTIMETER FAILED. THE PILOT CLBED ABOVE WX TO 14500 AND REALIZED HE WAS AT FL183 AFTER READING HIS TRANSPONDER ALTIMETER.

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ALTIMETER NEEDLE WAS BOUNCING RAPIDLY; I THOUGHT. TURNED OUT TO BE THE NEEDLES WERE WINDING DOWN RAPIDLY. I HAD A BRIEF PERIOD OF VERTIGO EVEN IN VFR CONDITIONS. AFTER THE NEEDLES SETTLED DOWN I THOUGHT THAT I WAS AT 12500 FT. AT SOMETIME AFTER PASSING OVER ZZZ1 I REALIZED THAT I HAD BEEN AT 13500 FT; AND AT THAT POINT; I LOOKED AND THE 10000 FT NEEDLE WAS AT JUST ABOVE SEA LEVEL. IT WAS AT THAT POINT THAT I REALIZED THAT THE ALTIMETER HAD FAILED. AGAIN AFTER THE NEEDLE STOPPED BOUNCING; I THOUGHT IT WAS JUST A DROP OF WATER IN THE STATIC SYS; AND I WANTED TO CLB TO 14500 FT. AFTER ABOUT 10 MINS I WAS STILL AT THE SAME INDICATED ALT BUT THE AIRPLANE WAS NOT ACTING NORMAL. LOW MANIFOLD PRESSURE; HIGHER CABIN ALT AND THE CLOUDS IN FRONT OF ME; THAT I EXPECTED TO BE ABOVE ME; WERE BELOW ME. I THEN LOOKED AT THE XPONDER WHICH HAS PRESSURE ALT READOUT WAS SAYING 18300 FT. I WASN'T SURE WHICH INST WAS CORRECT; BUT I DSNDED IMMEDIATELY TO BELOW 18000 FT INDICATION. I COULD HAVE LANDED AT ZZZ1 BUT I WASN'T SURE WHICH INST WAS CORRECT AND FELT THAT I WOULD PREFER TO LAND AT ZZZ2; IF I DIDN'T HAVE ALT INDICATIONS. I ESTIMATE THAT WHEN I WAS ON THE 350 DEG RADIAL FROM ABC RWY 32 DME I WAS OUT OF THE CLASS A AIRSPACE AND WAS THERE FOR 3-8 MINS.