BE55 BARON SAT AT ARPT 4 DAYS WHEN PRESSURE DROPPED BY ALMOST 1 INCH SO RPTR PLT SET ALTIMETER TO FIELD ELEVATION; BUT FAILED TO SEE THE ALTIMETER IN THE MINUS RANGE. CLBING OUT DEP CTLR ALERTED RPTR TO GOING ABOVE ASSIGNED ALT AND GAVE THE CORRECT ALTIMETER SETTING.

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ON NOV/WED/97; I DEPARTED BNA ON AN IFR FLT PLAN WITH SGF AS THE DEST. TKOF WAS AT XX55 HRS. PRIOR TO DEP MY IFR CLRNC WAS RECEIVED AND READ BACK CORRECTLY AFTER CHKING CURRENT ATIS. I SET THE ALTIMETER AT BNA ELEVATION AS PUBLISHED ON THE APCH PLATES; WHICH IS 599 FT. WE WERE CLRED FOR TKOF ON RWY 31 AND TO TURN L TO 280 DEGS AFTER DEP. WE WERE INSTRUCTED TO CLB AND MAINTAIN 4000 FT. AS I WAS PASSING THROUGH 3600 FT (AS I READ IT ON MY ALTIMETER); DEP CTL ADVISED ME THEY SHOWED ME AT 4700 FT. I ASKED FOR THE CURRENT ALTIMETER SETTING AND FOUND IT WAS ABOUT 1 INCH DIFFERENT THAN WHAT I HAD SET EARLIER. PRIOR TO TKOF I HAD SET THE ALTIMETER TO -599 FT RATHER THAN 599 FT. THE ACFT HAD SAT ON THE RAMP FOR 4 DAYS AND THE PRESSURE HAD CHANGED NEARLY 1 INCH DURING THAT TIME. AFTER DISCOVERING THE ERROR; I CORRECTED MY ALTIMETER TO THE CURRENT PRESSURE OF 30.27 AND LEVELED AT 4000 FT. THERE WAS NO OTHER TFC IN MY IMMEDIATE AREA AND THE WX WAS VFR. THE PROB AROSE WHEN I SET MY ALTIMETER TO THE FIELD ELEVATION RATHER THAN THE PRESSURE RPTED ON ATIS. SINCE THE PRESSURE HAD RISEN NEARLY 1 INCH DURING THE DAYS THE PLANE WAS ON THE GND I VISUALLY MISSED THE FACT I WAS ACTUALLY SETTING IN A -599 FT READING ON MY ALTIMETER. WHERE BAROMETRIC PRESSURE VALUES ARE AVAILABLE I ALMOST NEVER SET THE ALTIMETER ON FIELD ELEVATION ONLY; AND AM NOT SURE WHY I DID THIS TIME. IN THE FUTURE I'LL SET TO THE CURRENT PRESSURE INSTEAD OF FIELD ELEVATION AND THEN VERIFY THE ELEVATION MATCHES THE READING ON THE ALTIMETER.