ACR MLG MADE APCH AND LNDG WITH WX REPORTED BELOW MINIMUMS.

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RECEIVED WX FROM FSS PRIOR TO DSCNT OF 1/2 AND 300 OVCST WITH A VARIABLE RVR OF 16 TO 2400'. THE APCH PLATE INDICATED WX OF 1/2 MI OR RVR OF 1800' WAS REQUIRED FOR THE ILS APCH. I ELECTED TO FLY THE APCH BASED ON THE 1/2 GIVEN AND THE VARIABLE RVR ABOVE 1800'. DURING DSCNT CENTER ASKED IF WE HAD THE WX AND WE SAID YES. JUST PRIOR TO THE FAF CENTER HANDED MY FLT OVER TO THE LCL FSS THAT HAD ALREADY GIVEN ME THE WX. WE CONTACTED RADIO TO RPT THE FAF INBND. AT THAT TIME NEW WX WAS GIVEN THAT WAS BELOW MINIMUMS BUT THE REGS ALLOW US TO CONTINUE IF WE ARE AT G/S INTERCEPT OR PAST FAF WHICH WE WERE. THE WX WAS BETTER THAN RPTED AND A LNDG WAS COMPLETED. THE FOLLOWING DAY I DID SOME RESEARCH ON VARIABLE RVR'S AND FOUND REG 121.655 THAT INDICATES TO ME I WAS IN ERROR FOR FLYING THE APCH WITH THE GIVEN WX AND I SHOULD HAVE REQUESTED A CONSTANT RVR READING BEFORE FLYING THE APCH. ALSO SINCE MY FIRST WX REQUEST WAS PRIOR TO THE HOUR AND MY ARR WAS AFTER THE HR NEW WX WAS RPTED THAT I DID NOT HAVE UNTIL CONTACTING THE FSS AT THE FAF. IN THE FUTURE I WILL REQUEST A CONSTANT RVR BEFORE ANY APCH.