AFTER ALT IS QUESTIONED; A FLT CREW FINDS THEY ARE LOWER THAN ASSIGNED DUE TO MISREADING THEIR PRINTED COPY OF THE CURRENT ATIS. A TIMELY DSCNT CLRNC RESOLVES THE ALT ISSUE. THE CREW RAISES FATIGUE AND DUTY TIME ISSUES.

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WHILE ON ARR TO ORD AND LEVEL AT 11000 FT INDICATED ALT; THE APCH CTLR ASKED US TO CONFIRM OUR ALT. AFTER I STATED 'LEVEL AT 11000 FT;' THE CTLR STATED THAT OUR MODE C WAS INDICATING 10600 FT. WE THEN; AS A CREW; REALIZED THAT WE HAD THE ALTIMETERS INCORRECTLY SET AT 29.95 INCHES INSTEAD OF THE CORRECT 29.48 INCHES. ROUGHLY 5 SECONDS AFTER STATING THAT OUR MODE C WAS INDICATING 10600 FT; THE CTLR ISSUED A DSCNT CLRNC TO 7000 FT. WE RESET THE ALTIMETERS TO 29.48 INCHES AND THE FLT CONTINUED NORMALLY TO A LNDG AT ORD. NO LOSS OF SEPARATION OCCURRED. THIS FLT WAS COMPLETING A 12 HR 15 MIN DUTY DAY FOR THE TWO OF US. THE ENTIRE DUTY PERIOD WAS CHARACTERIZED BY LENGTHY ATC DELAYS; BOTH ON THE GND AND INFLT VIA HOLDING AND EXTENSIVE VECTORING. SIGNIFICANT TSTM ACTIVITY; LOW CEILINGS AND TURB THROUGHOUT THE MIDWEST AND NE WERE THE CAUSES OF THESE DELAYS. AS A RESULT; CREW FATIGUE WAS A MAJOR CONTRIBUTING FACTOR. BOTH CREW MEMBERS MISREAD THE ALTIMETER SETTING OFF THE DIGITAL ATIS RECEIVED VIA ACARS BECAUSE OF THIS FATIGUE. THE WX ALONG THE ARR RTE AND IN THE TERMINAL AREA WAS AGAIN CHARACTERIZED BY MODERATE TSTM ACTIVITY WITH CONTINUOUS LIGHT TURB AND INTERMITTENT MODERATE TURB AT ALL ALTS BELOW FL250. THIS REQUIRED ME TO PERIODICALLY BE OFF THE PRIMARY VHF FREQ WHILE DISCUSSING CABIN SAFETY ISSUES WITH THE FLT ATTENDANTS VIA THE FLT INTERPHONE. I WAS CONDUCTING ONE OF THESE DISCUSSIONS WHEN WE WERE ISSUED THE FIRST DSCNT CLRNC BELOW FL180; THEREFORE; MISSING THE STATED ALTIMETER SETTING. THIS WAS ALSO A CONTRIBUTING FACTOR. CONSTANT VIGILANCE IS REQUIRED BY ALL CREW MEMBERS TO COMBAT FATIGUE'S POTENTIALLY DANGEROUS CONSEQUENCES. NON COMPLACENT; CAREFUL REVIEW OF THE ATIS INFO IS REQUIRED OF ALL PLTS DURING EACH AND EVERY FLT. PLTS AND CTLRS SHOULD ALWAYS BE PREPARED TO CORRECT READBACK AND HEARBACK ERRORS. DISCUSSIONS WITH THE CABIN CREW SHOULD BE KEPT TO THE MINIMUM NEEDED; PARTICULARLY DURING DSCNT. FINALLY; REALISTIC FLT AND DUTY TIME REGS FOR ALL AIRLINE FLT CREW MEMBERS ARE REQUIRED FROM THE FEDERAL GOV.