FO OF MLG ACR ACFT MIS-SET ALTIMETER DURING DSCNT RESULTING IN OVERSHOOTING LEVEL OFF AT AN INTERMEDIATE ALT.

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AT APPROX 65 MI SE OF ORD; AT AN ALT OF 23000 FT; WE WERE ISSUED A CLRNC TO CROSS 45 DME SE OF ORD AT 11000 FT. THIS XING RESTRICTION WOULD BE DIFFICULT; SO WE INITIATED A HIGH VERT SPD DSCNT AT 320 KTS. DURING THE DSCNT WE ENCOUNTERED MODERATE TURB AND SLOWED TO 280 KTS. THE CAPT TRIED TO GET ALT RELIEF FROM ATC BUT WAS UNABLE TO DUE TO FREQ CONGESTION. WHEN PASSING THROUGH 18000 FT I MIS-SET MY ALTIMETER TO 30.53 RATHER THAN THE CORRECT SETTING OF 29.53. PASSING THROUGH 11000 FT; THE CAPT TOLD ME TO LEVEL OFF; WHICH I DID. HIS ALTIMETER SHOWED APPROX 10700 FT. THE APCH DSCNT CHKLIST; WHICH WOULD HAVE CAUGHT THE INCORRECT ALTIMETER SETTING WAS NOT YET COMPLETED DUE TO THE HIGH WORKLOAD AND DISTR WITH ATC. ADDITIONAL FACTORS: NEW TRANSPONDER CODE GIVEN BY ATC JUST PRIOR TO DSCNT; REQUIRING A COMPANY MESSAGE THROUGH ACARS; WHICH WAS SENT DURING DSCNT. TRANSPONDER FAILURE DURING DSCNT OR SHORTLY THEREAFTER (MEMORY UNCLR) REQUIRING TROUBLESHOOTING. TRANSPONDER REPLACED BY MAINT AT ORD. COMPANY POS RPT REQUIRED VIA ACARS AT 50 MI (I.E.; DURING DSCNT).