ARTCC RADAR CTLR USED VISUAL SEPARATION TO DSND A B737 THROUGH THE ALT OF AN ATR42 WHILE ACFT WERE IN PCA. AFTER CONFLICT ALERT CAUGHT THE LTSS; THE CTLR REMEMBERED VISUAL SEPARATION CAN ONLY BE USED BELOW FL180.

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ACR X (ATR42) AT FL200 WAS ENRTE FROM HOUSTON TO BROWNSVILLE ABOUT 60 MI NE OF CRP. ACR Y (B737) WAS AT FL260 FROM HOUSTON TO CRP. I COULD EASILY SEE THAT ACR Y WOULD OVERTAKE THE ACR X ABOUT THE TIME ACR Y WOULD WANT TO DSND FOR LNDG. I CALLED THE PREVIOUS RADAR SECTOR REQUESTING CTL FOR DSCNT ACR Y. MY INTENTION WAS TO FORCE ACR Y UNDER ACR X EARLIER TO FACILITATE ARR DSCNT. BY THE TIME ACR Y CAME TO MY FREQ; THAT PLAN WOULD NO LONGER WORK. I DECIDED JUST TO HOLD ACR Y HIGH OVER THE TFC AT FL210 UNTIL HE OUTRAN HIM AND THEN LET HIM DOWN. AS REQUIRED; I ISSUED TFC INFO TO BOTH ACFT. THEY RPTED EACH OTHER IN SIGHT. THEN I HAD TO CATCH UP ON SOME OTHER REQUIRED; BUT LOWER PRIORITY DUTIES THAT HAD STACKED UP. WHEN I RETURNED TO THE RADAR SCOPE ACR Y WAS ABOUT 3 MI IN FRONT OF ACR X. I ASKED ACR X IF HE STILL HAD THE B737 IN SIGHT AND IF HE COULD MAINTAIN VISUAL SEPARATION FROM HIM. WHEN ACR X RESPONDED 'I HAVE HIM IN SIGHT AND COULDN'T CATCH HIM IF I TRIED;' I INSTRUCTED HIM TO MAINTAIN VISUAL SEPARATION AND ISSUED DSCNT CLRNC TO ACR Y. THE VISUAL SEPARATION STANDARD I EMPLOYED WAS MISAPPLIED DUE TO THE ALTS INVOLVED. THE STANDARD IS FL180 AND BELOW. I HAD BEEN AWARE OF CONFUSION IN THE STANDARD WITHIN THE FAA AND HAD READ A BRIEFING CLARIFYING THE STANDARD; BUT I ALSO HAD READ OF FACILITIES TESTING AND USING FL230 AS THE STANDARD. IN MY HASTE TO PROVIDE ACR Y A COMFORTABLE DSCNT AND HAVING BOTH PLTS RPT SEEING EACH OTHER; MY OWN CONFUSION LED ME TO USE AN INAPPROPRIATE STANDARD. MINIMUM SEPARATION WAS 3.5 MI AND 500 FT VERTLY. WHILE THIS CANNOT EXCUSE MY ERROR; I DO FEEL THAT THE SEEMINGLY CONTINUOUS CHANGES IN STANDARDS HAVE A TREMENDOUS IMPACT IN THOSE DECISIONS MADE INSTANTANEOUSLY FROM MEMORY. IN MY 23 YRS AS A CTLR; SOME STANDARDS HAVE FLUXED BACK AND FORTH; SOME HAVE CHANGED ALMOST YRLY; AND FEW HAVE REMAINED STATIC. THESE CHANGES PUT VERY MUCH INFO INTO THE BRAIN; BUT NO ONE HAS TRAINED US HOW TO PURGE THE BAD INFO.