GULFSTREAM IV PF INTERPED FOREIGN ALTIMETER SETTING AS EQUAL TO INCHES OF MERCURY INSTEAD OF MILLIBARS; CAUSING INTERMEDIATE APCH ALT TO BE OVERSHOT.

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CHIEF PLT; PF WITH LIMITED INTL EXPERIENCE; LIMITED EXPERIENCE IN TYPE. PNF CAPT WITH EXTENSIVE INTL (EXPATRIATE WORK) 5000 HRS IN TYPE. DURING DSCNT FOR APCH TO SHANNON; IRELAND UNDER RADAR CTL; WE WERE CLRED TO 4000 FT WITH LCL ALT SETTING 975 HECTOPASCALS; TRANSITION LEVEL 5000 FT. PF SEEMED CONFUSED ABOUT HECTOPASCALS. I EXPLAINED THAT IT WAS THE SAME AS MILLIBARS. I CALLED RADAR TO VERIFY THAT HECTOPASCALS AND MILLIBARS WERE THE SAME FOR HIS INFO. I SET 975 MILLIBARS ON THE R SIDE ALTIMETER AS WE APCHED 5000 FT. I REMINDED HIM OF THE CORRECT ALTIMETER SETTING. HE INFORMED ME I DID NOT KNOW WHAT I WAS TALKING ABOUT AND THE PRESSURE SETTING COULD NOT BE THAT LOW. HE SAID THAT HE HEARD ATC SAY THAT HECTOPASCALS AND INCHES WERE THE SAME. I EXPLAINED THAT I HAD BEEN TO IRELAND MANY TIMES AND THAT HE WAS WRONG; AND IT WAS COMMON TO HAVE LOW PRESSURE AREA OVER IRELAND. HE BECAME AGGRESSIVE; OFFERING TO BET ME $100; ETC; THAT I WAS WRONG. AT THAT TIME WE WERE IN VISUAL CONDITIONS WITH NO OTHER TFC IN THE IMMEDIATE AREA. I DECIDED IT BEST NOT ARGUE AT THIS TIME AS WE WERE STILL ABOVE THE MSA. HE HAD SET 29.75 INCHES ON HIS SIDE. 4000 FT SET IN THE ALT ALERT. THE AUTOPLT LEVELED THE ACFT INDICATING 4000 FT BASED ON HIS ALTIMETER AND THE ALT ALERT SYS. MY ALTIMETER (COPLT SIDE) INDICATED 3000 FT. ATC ASKED TO VERIFY ALT; I QUICKLY SAID 3000 FT. ATC SAYS YOU WERE CLRED TO 4000 FT. AT THAT TIME THE CAPT ANSWERED ATC ASKING TO VERIFY ALTIMETER SETTING AND THAT HECTOPASCALS AND INCHES WERE THE SAME. ATC QUICKLY CONFIRMED SETTING WAS NOT INCHES; BUT HECTOPASCALS OR MILLIBARS. FINALLY HE SET IT CORRECTLY AND BEGAN A CLB BACK TO 4000 FT; BY NOW ATC SAID STAY AT 3000 FT. A SHORT TIME LATER; WE WERE CLRED FOR THE VISUAL APCH AND SWITCHED TO THE TWR CTL FREQ. NO MENTION WAS MADE ABOUT THE ERROR. I SAW NO POINT TO BRING IT UP AS I HAD 10 DAYS AND MANY COUNTRIES STILL AHEAD; INCLUDING A FLT TO RUSSIA WHERE FLT LEVELS ARE IN MILLIMETERS. I SUSPECT THE IRISH ATC WILL NOT FILE A RPT. THEY WERE PROBABLY THINKING JUST MORE INEPT AMERICANS; AND THEY SEEMED ANXIOUS TO HELP US ON OUR WAY TO FRANCE AFTER REFUELING. FORTUNATELY; THE WX WAS GOOD AND ATC WAS ALERT. IF NECESSARY I WAS PREPARED TO TAKE CTL OF THE ACFT.