OPERROR.

Narrative

A CMH INBOUND WAS WBOUND DSNDING OUT OF FL240 FOR 10000 FT. I THEN TOOK THE HDOF ON A CAK INBOUND DSNDING NBOUND OUT OF FL210 FOR 15000 FT. I RECOGNIZED THAT THE 2 ACFT WERE IN CONFLICT IN SEVERAL MINS; SO I STOPPED ACR X AT FL180. HE WAS NOW OUT OF FL240. CPA Y WAS NOW OUT OF FL190 DSNDING STILL TO 15000 FT. I STILL HAD A COUPLE OF MINS BEFORE THEY WERE IN CONFLICT. I THEN ISSUED CTL INSTRUCTIONS TO SEVERAL OTHER ACFT. I THEN LOOKED BACK TO ACR X AND CPR Y TO FIND THAT THE CONFLICT ALERT HAD ACTIVATED. AT THIS POINT THE 2 ACFT WERE ABOUT 8-10 MI APART WITH ALTS OF FL184 (ACR X) AND FL182 (CPR Y). I TOLD CPR Y TO EXPEDITE THE DSCNT THROUGH 17000 FT REPORT LEAVING 17000 FT. CPR Y WAS THEN TURNED 30 DEGS R TO INSURE THEY WENT WELL BEHIND ACR X. THE RESULTING PRELIMINARY SEPARATION WAS 3.4 MI AND 600 FT. CPR Y DID RPT LEAVING 17000 FT AND RPTED THE OTHER ACFT IN SIGHT BUT BY THAT TIME SEPARATION WAS LOST. I ATTRIBUTE THE ERROR ON MY INCORRECT ASSUMPTION THAT CPR Y'S DSCNT RATE WOULD REMAIN CONSTANT AND TAKE IT WELL BELOW ACR X'S ASSIGNED ALT. IT WAS NOT BUSY AT THE TIME; ABOUT 6 ACFT ON FREQ. T3 LOW ALT SECTORS WERE COMBINED AT 1 POS. THIS EVENT COULD EASILY HAVE BEEN PREVENTED BY STOPPING ACR X DSCNT AT A HIGHER ALT; AND INCREASING CPR Y DSCNT ON INITIAL CONTACT OR TURNING THE ACFT TO INSURE SEPARATION. ALWAYS BE ALERT AND NEVER ASSUME ANYTHING; ESPECIALLY AT THE END OF A WORK SHIFT.