A DC9-30 FLT TEMPORARILY LOSES ITS PRESSURIZATION; DROPPING MASKS; WHEN THE FO INADVERTENTLY MOVES THE OUTFLOW VALVE TO THE OPEN POS INSTEAD OF THE CLOSED POS. THE AUTOMATIC SYS HAD BEEN MEL'ED. FLT CONTINUED TO DEST ARPT FROM 85 MI W OF IIU; KY.

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WE WERE LEVEL AT FL260 IN CRUISE FLT; DEVIATING AROUND A WX CELL. THE ACFT WE WERE FLYING HAD THE AUTOMATIC PRESSURIZATION INOP SO WE WERE MANUALLY PRESSURIZING THE ACFT. THE CABIN ALT WAS CURRENTLY LEVEL AT 6000 FT AND EVERYTHING WAS GOING FINE. WE ENTERED SOME CLOUDS AND THE CAPT TURNED ON THE ENG ANTI-ICE; VERBALIZING HIS ACTIONS SO I COULD MAKE THE PROPER ADJUSTMENT TO THE PRESSURIZATION OUTFLOW VALVE. MY INITIAL MOVE DID NOT CORRECT THE CABIN CLB RATE; SO I KEPT MOVING THE VALVE IN WHAT I THOUGHT WAS THE CORRECT DIRECTION TO CLOSE THE VALVE TO STOP THE CABIN CLB RATE. INADVERTENTLY THE VALVE WAS MOVED FULL OPEN CAUSING THE CABIN TO DEPRESSURIZE AT A FAST RATE. THINKING WE HAD A PRESSURIZATION PROB; WE DONNED THE OXYGEN MASKS AND IMMEDIATELY REQUESTED LOWER. ATC WOULD NOT GIVEN US ANY LOWER THAN FL240; SO WE DECLARED AN EMER AND WERE SUBSEQUENTLY CLRED TO 10000 FT. WITHIN 30 SECONDS; THE ERROR WAS NOTICED AND I IMMEDIATELY CLOSED THE OUTFLOW VALVE TO START DSNDING THE CABIN. THE CABIN HAD CLBED TO 12500 FT AND THE OXYGEN MASKS IN THE CABIN DID DROP. SINCE WE NOW HAD THE CABIN DSNDING AND UNDER CTL WE STOPPED THE DSCNT AT FL190. THE CABIN WAS AT 6000 FT AND DSNDING WHEN WE REMOVED OUR OXYGEN AND THE CAPT INFORMED THE CABIN THAT IT WAS SAFE TO REMOVE OXYGEN. WE DSNDED THE CABIN TO ALMOST SEA LEVEL AND CONTINUED TO OUR DEST AFTER CHKING WITH THE FLT ATTENDANTS TO SEE IF WE HAD ANY MEDICAL CONCERNS WITH ANY PAX. THEY DID INFORM US EVERYONE WAS FINE. THE FLT CONTINUED UNEVENTFULLY TO OUR DEST AND ALL PAX DEPLANED WITHOUT FURTHER ASSISTANCE. I FEEL THIS SIT COULD NEVER HAVE OCCURRED HAD I BEEN MORE FAMILIAR WITH THE MANUAL PRESSURIZATION PROCESS. MY OVERALL TRAINING ON THIS ACFT WAS EXCELLENT AND I MADE EVERY EFFORT TO TRY AND UNDERSTAND WHY THE PROB OCCURRED AND WAS SUBSEQUENTLY CORRECTED WITH THE TRAINING I WAS GIVEN. I THINK A COUPLE OF BASIC GUIDELINES ON THE OP OF MANUAL PRESSURIZATION SHOULD HAVE BEEN DISCUSSED PRIOR TO THE DEP OF THE FLT AND WE AS A CREW WOULD HAVE PROBABLY NOTICED THE ERROR BEFORE IT BECAME AN EVENT. WE DID DISCUSS THE OP OF IT; BUT WE DID NOT SET ANY WORSE CASE SCENARIOS UP TO BE READY FOR. SUPPLEMENTAL INFO FROM ACN 472296: AFTER LEVELING OFF TEMPORARILY AT FL230; WE REQUESTED DEV L OF COURSE TO FL190. PASSING FL255 WE REQUESTED LEVELOFF AT FL260 TO STAY UNDER AN OVERHANG AND THE REQUEST WAS APPROVED. APPROX 90 MI W OF IIU WE ENTERED THE BOTTOM OF THE OVERHANG AND I ANNOUNCED THAT I WAS TURNING ON THE ENG ANTI-ICE AND I DID. AT THIS TIME THE CABIN PRESSURE STARTED TO CLB (NOT OBSERVED BY ME). THE FO'S INITIAL ACTION WAS TO MOVE THE PRESSURIZATION CTL KNOB (OLD STYLE LOLLIPOP) AFT (OPEN) AND THE RATE OF CLB FOR CABIN PRESSURE INCREASED. THINKING HE NEEDED MORE MOVEMENT; THE FO AGAIN MOVED THE KNOB FURTHER AFT (MORE OPEN). AFTER CHKING ON THE CONDITION OF THE PAX AND CREW; I DECIDED THE SAFEST COURSE OF ACTION WOULD BE TO CONTINUE TO DEST.