MD80 DIVERTS TO ALTERNATE AND DECLARES EMER DUE TO LOW FUEL IN ORD AIRSPACE.

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FLT BUF-ORD DEPARTED BUF WITH RVR RPTED AT ORD APPROX 3000V3500 FT. ENRTE ORD RPTED RVR DROPPING TO AROUND 2000 FT. WE CONTINUED IN AND WERE GIVEN PROFILE DSCNT ON PULLMAN FOUR ARR. AT PIVOT INTXN; WE WERE TOLD TO SLOW TO 170 KTS AND DSND. WE CONFIGURED AND WERE GIVEN LOWER ALT. THE RVR WAS NOW BEING RPTED AS 1800V1600 FT. I LOOKED AT THE GAS AND TOLD THE CAPT THAT WE SHOULD DIVERT NOW AS THE RVR WAS UNRELIABLE AND WAS FLUCTUATING BTWN 1800 FT AND 1200 FT. CAPT CHOSE TO CONTINUE. AS WE DID THE RVR CONTINUED TO FLUCTUATE MOSTLY TO THE 1600V1200 FT RANGE. I CONVEYED TO THE CAPT WE SHOULD DIVERT AS THE GAS WAS LOWER THAN EXPECTED DUE TO THE LONG TIME SPENT LOW AND SLOW. THEY WERE VECTORING EVERYONE TO ABOUT A 30 NM FINAL FOR ILS RWY 9R. THE CTLR KEPT SAYING THAT THE RVR WOULD COME BACK UP. I EXPRESSED MY DOUBT ABOUT THAT TO THE CAPT AND EVEN REMARKED THAT 'HE IS SUCKING US IN.' THE CAPT SAID TO CONTINUE AND TAKE THE VECTORS TO FINAL. ATC HAD JUST RPTED THE RVR AS 1200 FT. WHEN THE CAPT TOLD ATC WE WANTED TO DIVERT; ATC IMMEDIATELY RPTED RVR AS 1800 FT. I CONVEYED TO THE CAPT MY DOUBTS ABOUT THE VALIDITY OF THAT AND SAID WE WERE GETTING SUCKERED IN. THE CTLR EVEN MADE A COMMENT TO THE EFFECT THAT THEY WANTED TO TAKE CARE OF US GUYS. I TOLD THE CAPT THAT IF ONE MORE RVR LESS THAN 1800 FT WAS RPTED WE WERE GOING MISSED. ONCE ON TWR; RVR WAS RPTED AS 1200 FT AGAIN AND I WENT MISSED. OUR DIVERT OPTIONS WERE RATHER LIMITED DUE TO THE WIDE SPREAD WX SYS. WE DID NOT LIKE FNT AS AN ALTERNATE BECAUSE OF ITS PROX TO DTW WHICH WAS WEATHERED IN; AND THE COMBINATION OF THE WINDS; RWY LENGTH; AND LACK OF SVCS. WE WERE ALREADY AT FL310 AND CAPT HAD ALREADY DECLARED AN EMER TO AVOID FURTHER VECTORING WHICH IF EXCESSIVE WOULD PUT US IN YYZ BELOW RESERVE. I FIGURED THE ON DECK FUEL AT 4.2 IN YYZ. I CONTINUED TO YYZ AND DID A PROFILE DSCNT AND LANDED AT YYZ WITH 4.1. NOTE; WE HAD A JUMP SEATER ONBOARD. HIS ASSISTANCE WAS MUCH APPRECIATED AND VALUABLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT IT WAS THE FLC'S FEELING THAT THE TRACON CTLR WAS TRYING TO 'KEEP THEM COMING' TO THE ARPT; RATHER THAN DIVERT. HE SAID THAT THEY LANDED AT THEIR ALTERNATE ARPT WITH MORE THAN MINIMUM FUEL.