B737-300 FLT CREW DECLARED EMER DUE TO LOW FUEL RESULTING IN DIVERTING TO ANOTHER ARPT THAN THE FLT PLANNED ALTERNATE.

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ON FLT ABCD FLL TO ISP WITH 26800 LBS OF FUEL; WITH TWO ALT ORF/ALB. ATC ADVISED US THE ISP RVR WAS 2200 FT AS WE WERE DSNDING FOR ISP. ATC OFFERED TO KEEP US CLOSE TO ISP INCASE RVR CAME UP. THE CAPT COMMUNICATED WITH DISP WHILE I FLEW THE ACFT AND ATC. THE CAPT COMMUNICATED WITH DISPATCH AT LEAST 3 TIMES WHILE WE HELD IN VICINITY OF ISP. DURING THIS TIME THE RVR WENT DOWN TO 1600 FT AND FLUCTUATED AROUND 1800 FT. DISPATCH CHANGED OUR ALTERNATE TO BWI (SAID WX WAS VFR AT BWI). DISPATCH WANTED US TO HOLD UNTIL WE WERE DOWN TO 9400 LBS OF FUEL. THE CAPT CALLED DISPATCH AT 10400 LBS OF FUEL AND SAID WE WERE GOING TO BWI. WE DEPARTED THE ISP AREA WITH 10200 LBS OF FUEL. WE CLIMBED TO 28000 FT. THE FMC WAS SHOWING ARRIVING AT BWI WITH 2500 LBS OF FUEL. WE CHECKED THE FMC AND CROSSED CHECKED WITH THE VOR AND ATC VERIFIED OUR DIST TO BWI. DURING THIS TIME THE FLT ATTENDANTS INFORMED US OF A MEDICAL EMER. THE CAPT KEPT TRYING TO COMMUNICATE OUR SIT TO DISPATCH; HOWEVER THEY WERE NOT GIVING US ANY SOLUTIONS. WE WERE NOW NORTH OF ACY AND I SUGGESTED LNDG THERE AT PHL. THE CAPT DECLARED A FUEL EMER AND WE LANDED AT ACY WHICH WAS VFR. FUEL ON BOARD AFTER ENG SHUTDOWN WAS 5700 LBS.