C172 PLT MISSETS ALTIMETER; RESULTING IN AN EARLY LEVELOFF.

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IFR FLT. NO PAX ONBOARD. RUNNING ABOUT 1 HR BEHIND SCHEDULE BECAUSE OF WX CONDITIONS. DID ACFT PREFLT; STARTED ENG AND OBTAINED ATIS. ALTIMETER SETTING WAS 29.77. THOUGHT I HAD SET ALTIMETER CORRECTLY NOTING THAT IT INDICATED ABOUT 20 FT OFF PUBLISHED FIELD ELEVATION. OBTAINED CLRNC; D-BAE CLB TO 3000 FT EXPECT 4000 FT AFTER 10 MINS. TAXIED TO RWY 36 CONDUCTED PRE-TAKEOFF CHKS AGAIN NOTING THAT ALTIMETER WAS ABOUT 20 FT OFF OF PUBLISHED FIELD ELEVATION. DEPARTED UES CLBING THROUGH A LAYER OF CLOUDS WITH BASES AT ABOUT 1700 FT MSL AND TOPS AROUND 3000 FT MSL. AFTER CONTACTING MKE ATCT (DEP) WAS CLRED TO CLB TO 4000 FT OSH WHEN ABLE. CLBED TO 4000 FT. A FEW MINS LATER; MKE ATCT ADVISED THAT I WAS SHOWING LEVEL 3000 FT AND GAVE THE ALTIMETER SETTING OF 29.77. I IMMEDIATELY CHKED ALTIMETER WHICH INDICATING 4000 FT. LOOKED AT KOHLSMAN WINDOW AND THOUGHT I SAW 29.77 WHICH I RPTED TO ATC. ATC RESPONSE TO THIS WAS NOT CLR; I AM NOT SURE BUT I THINK HE SAID MAINTAIN 3000 FT. AT THAT TIME I BECAME CONCERNED ABOUT THE POSSIBILITY OF A SERIOUS ALTIMETER ERROR AND WAS CONTEMPLATING PROCEEDING TOWARDS GRB WHERE THE WX WAS RPTED VFR. I THEN LOOKED AT THE ALTIMETER AGAIN; AND SAW THAT THE KOHLSMAN WINDOW WAS MIS-SET TO 30.77. AT THAT TIME I RESET THE ALTIMETER AND CLBED A COUPLE OF HUNDRED FT UP TO AN INDICATED OF 4000 FT. AT ABOUT THIS TIME MKE ATCT INSTRUCTED ME TO CONTACT ZAU ON 127.01 CONTACTED CTR ADVISED LEVEL AT 4000 FT. THE CTR CTLR STATED 'LEVEL FOUR; I THOUGHT MKE SAID 3.' I DON'T RECALL THE CTR CTLR SAYING ANYTHING ELSE ABOUT THE ALT. A FEW MINS LATER HE INSTRUCTED ME TO TURN L 10 DEGS TO INTERCEPT THE ILS 36 AT OSH. AS I GOT CLOSER TO THE LOC; CTR ADVISED THAT HE WAS GOING TO HOLD ME AT 4 BECAUSE OF TFC SHOOTING THE RWY 18 LOC/BACK COURSE INTO OSH. I INTERCEPTED THE ILS AND FLEW INBOUND MAINTAINING 4000 FT. A SHORT TIME LATER; CTR CLRED ME FOR THE ILS 36 APCH; ADVISED RADAR TERMINATED AND CONTACT OSH ATCT. I EXECUTED THE APCH AND LANDED OSH WITHOUT INCIDENT. WX AT THE TIME WAS ABOUT 400 FT CEILING; VISIBILITY 1 1/2 MI IN FOG. IN RETROSPECT; I RECALL SEEING; WHEN CHKING WX PRIOR TO FLT; THAT MANY STATIONS RPTED PRESSURE RISING RAPIDLY. IF I WOULD HAVE BEEN TOTALLY FOCUSED; THIS SHOULD HAVE CAUSED MORE DILIGENCE IN SETTING THE ALTIMETER.