AN MD80 FLT CREW; AFTER LEVELING OFF AT FL310 IN ZFW'S AIRSPACE; EXPERIENCED DEPRESSURIZATION; CAUSING THE CAPT TO PERFORM AN EMER DSCNT AND LNDG AT THE DEST ARPT; DFW.

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JUST AFTER LEVELOFF AT FL310; THE XFER LOCK-OUT LIGHT AND STANDBY LIGHT ON THE PRESSURIZATION CTL PANEL CAME ON. THE CABIN BEGAN TO CLB. THE RED CABIN ALT ANNUNCIATOR CAME ON; AND THE HORN SOUNDED. WE DONNED OUR OXYGEN MASKS; BEGAN AN EMER DSCNT; AND DECLARED AN EMER. I CALLED THE FLT ATTENDANTS TO BRIEF THEM ABOUT THE ITEMS SAYING WE WOULD BE LNDG IN DFW WITHIN 10 MINS AND THAT AN EVAC WAS UNLIKELY; WE WERE NOT WORRIED ABOUT THE INTEGRITY OF THE ACFT. I MADE A PA TO THE PAX TO USE THEIR OXYGEN AND TO FOLLOW FLT ATTENDANT INSTRUCTIONS. A FLT ATTENDANT THEN CALLED TO SAY THE MASKS WERE NOT DEPLOYED SO THE FO MANUALLY DEPLOYED THEM. DURING DSCNT; I ASKED ATC TO PLEASE NOTIFY ACR ABOUT OUR DIVERT AND EMER. ON DOWNWIND; ONCE CHKLISTS WERE COMPLETE; I WAS ABLE TO NOTIFY DISPATCH ON THE RADIO. THE FO CONTINUED TO FLY AND LAND. HE DID A GREAT JOB AND FOLLOWED ALL PROCS. THE FLT ATTENDANTS TOOK CARE OF ALL CABIN DUTIES; CALMED THE PAX; AND WERE PREPARED TO LAND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS ACFT'S PRESSURIZATION SYS; PER DISCUSSION WITH MAINT AFTER LNDG; WAS FOUND TO HAVE A HISTORY OF SIMILAR PROBS. THIS CREW HAD SIMILAR INDICATIONS DURING CLB THROUGH 6000 FT BUT THEY WERE ABLE TO CLR THEM THROUGH NORMAL PROCS. THESE INDICATIONS ARE QUITE NORMAL AND; IF CLRED BY PROC; DO NOT REQUIRE A MAINT LOG ITEM. AT THE TIME OF THE SECOND LOCKOUT LIGHT AND STANDBY LIGHT THE CABIN CLB RATE WAS 1500-2000 FPM BUT NOT CONSIDERED EXPLOSIVE. THE PAX OXYGEN MASKS WERE DEPLOYED MANUALLY BELOW THE CABIN ALT THAT WOULD HAVE ACTIVATED THEM AUTOMATICALLY.