AIR FREIGHTER REQUESTS ANOTHER RWY WHEN RVR GOES BELOW MINS ON ORIGINAL ASSIGNED RWY. MAKES LOC BACK COURSE APCH.

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WE WERE BEING VECTORED FOR APCH; ILS 10R AT PDX THE WX WAS MOSTLY CLR WITH SOME LCL GND FOG FORMING AS THE SUN CAME UP. THE RVR WAS 6000 PLUS FOR 10R. ON BASE; PDX APCH INFORMED US THAT THE RVR WAS NOW 1200 AND WE NEEDED 1800 RVR. I THEN ASKED APCH IF THE WX WAS BETTER ON THE OTHER RWY. AFTER A SHORT PAUSE; APCH ASKED IF WE COULD ACCEPT AN APCH FOR THE LOC BACK COURSE 10L. I CHKED WITH MY COPLT; THEN TOLD APCH THAT WE COULD AND WAS CLRED FOR THE LOC BACK COURSE 10L AND TO CONTACT PDX TWR AT THE FINAL FIX. DURING THE NEXT FEW MINS WE RECONFIGURED THE ACFT FOR THE APCH; RESET THE RADIOS AND REVIEWED THE APCH. AT THE FINAL APCH FIX WE CALLED PDX TWR AND WERE CLRED TO LAND. AT ALL TIMES WE COULD SEE THE RIVER AND ARPT AREA. SHORTLY AFTER PASSING POORTA (FINAL FIX) WE COULD SEE SOME RWY LIGHTS AND 1.5 TO 2 MI FROM THE ARPT WE SAW THE RWY AND LANDED. AT NO TIME DURING THIS APCH DID WE FLY IN INST METEOROLOGICAL CONDITIONS UNTIL THE FINAL DSCNT ONTO THE RWY WHICH WAS RESTR SOMEWHAT BY FOG. DURING THIS APCH AND LNDG I DO NOT REMEMBER ASKING FOR OR RECEIVING THE WX FOR RWY 10L. I ACCEPTED THIS APCH BASED ON THE FACT IT WAS OFFERED TO ME AFTER I ASKED ABOUT THE WX BEING BETTER THAN THE OTHER RWY AND THE FACT THAT WE COULD SEE THE PDX ARPT AREA AND THE FOG WAS LCL IN NATURE. THIS ALLOWED US TO SEE THE RWY LIGHTS AND RWY WELL BEFORE THE 1 MI REQUIRED FOR THE LOC BACK COURSE 10L APCH. THIS SITUATION MOST LIKELY CAME ABOUT BECAUSE OF FAST CHANGING WX AND MY DESIRE TO GET ON THE GND WHILE THE WX WAS GOOD. ALSO I GOT VERY BUSY RECONFIGURING THE ACFT AND RADIOS FOR A DIFFERENT RWY AND I WAS A LITTLE TIRED AFTER FLYING MOST OF THE NIGHT.