A DC9 FLT CREW EXPERIENCE A LOSS OF CABIN PRESSURE RESULTING IN AN EMER DSCNT AND LNG.

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WHILE FLYING AT FL290 IN INST WX CONDITIONS; WITH THE ICE PROTECTION SYS ON; WE OBSERVED THE CABIN ALT CLBING 1000 FPM. THE L AND R PACKS WERE ON AUTO WITH A PRESSURE READING OF 12 PSI. THE L DUCT WAS COLDER THAN THE R; BUT EACH WAS WITHIN LIMITS. WITH THE OUTFLOW VALVE FULLY CLOSED AND THE CABIN CONTINUING TO CLB; WE DONNED THE OXYGEN MASK. THE FO CONTINUED TO FLY THE ACFT; WHILE TALKING WITH ATC. I OPENED THE CABIN DOOR AND TOLD THE LEAD FLT ATTENDANT TO 'SIT DOWN AND PUT ON AN OXYGEN MASK.' THE COM PROCS OF 'CABIN PRESSURE LOSS' AND 'RAPID DSCNT' WERE COMPLETED. AS WE WERE PASSING FL200 THE RED CABIN PRESSURE ANNUNCIATOR LIGHT AND MASTER CAUTION LIGHT ILLUMINATED. PASSING 15000 FT; THE CABIN STARTED TO PRESSURIZE. WITH THE CABIN PRESSURIZING; I ELECTED NOT TO DEPLOY THE PAX OXYGEN MASK. AN EMER WAS DECLARED WITH ZTL. I ALSO TRIED TO CONTACT DISPATCH ON THE COMPANY RADIO; BUT WAS UNSUCCESSFUL. BEING UNABLE TO CONTACT DISPATCH ON THE RADIO; A MESSAGE WAS SENT VIA ACARS. DISPATCH NOTIFIED KNOXVILLE OPS THAT WE WERE ENRTE. THE PAX WERE INFORMED OF OUR PRESSURIZATION PROB AND THAT WE WERE DIVERTING INTO KNOXVILLE. THE WX AT TYS WAS 400 FT BROKEN; WITH LIGHT RAIN AND MIST WITH 2 MI VISIBILITY. A NORMAL LNDG WAS MADE AT TYS. AFTER PARKING; THE PAX WERE INFORMED OF THE PROB AND WERE DEPLANED INTO THE TERMINAL. MAINT CTL; DISPATCH; AND A CHIEF PLT WERE CALLED. EACH FLT ATTENDANT WAS DEBRIEFED TO DETERMINE THEIR EMOTIONAL CONDITION; AND ASKED IF THEY WOULD BE WILLING TO CONTINUE. AFTER A LCL MECH MEL'ED THE PRESSURIZATION SYS; THE ACFT WAS FLOWN WITH PAX TO RDU. THE ARR TIME AT RDU WAS 3 MINS PAST THE FAR SCHEDULED DUTY PERIOD.