A SAAB SF340 CAPTAIN REPORTED HIS AIRCRAFT FAILED TO PRESSURIZE IN THE CLIMB. HE FOUND THE MANUAL PRESSURIZATION OUTFLOW VALVE IN THE OPEN POSITION.

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WE WERE CLRED TO CLB TO 12000 FT AND WERE AROUND 11000 FT WHEN THE CABIN ALT WARNING WAS ILLUMINATED ON THE CWP. I INSTRUCTED THE FO TO STOP THE CLB AND STARTED THE QRH. CABIN DIFFERENTIAL WAS AT .5 AND CABIN ALT WAS AT 9500 FT. I REQUESTED 10000 FT FROM CTR. I ASSESSED THE SITUATION AND DETERMINED THAT THE MANUAL OUTFLOW VALVE WAS OPEN AND THE AUTO PRESSURIZATION TOOK OVER. THE FLT PROCEEDED AND LANDED WITHOUT INCIDENT. THE MANUAL OUTFLOW VALVE WAS LEFT OPEN BY SOMEONE PREVIOUS TO US. ON PREFLT; THE FO MISTOOK FULL OPEN FOR FULL CLOSED. THIS CAN HAPPEN AS THE MANUAL KNOBS ARE SOMETIMES RIGGED SO THAT FULL CLOSED IS ACTUALLY BEHIND THE WHITE LINE ON THE PRESSURIZATION PANEL. AS THE KNOB IS TURNED TO FULL OPEN; IT IS ABOUT A 330 DEG TWIST THAT WILL ALSO BE SOMEWHERE BEHIND THE WHITE LINE. ON CLBOUT; NEITHER OF US DETECTED THAT PRESSURIZATION WAS NIL OR NEGLIGIBLE. FLT CREW SUGGESTIONS/NARRATIVE: BOTH THE FO AND I WERE UNNERVED THAT SUCH AN ERROR COULD HAPPEN. WE BOTH PLAN TO EXERCISE EXTRA DILIGENCE. I THINK IT COULD ALSO BE HELPFUL IF THE LAST ITEM ON THE AFTER TKOF CHKLIST WAS A CHK OF THE PRESSURIZATION PANEL TO BE SURE THAT CABIN DIFFERENTIAL IS GOING UP AND THE CABIN ALT ISN'T RISING TOO RAPIDLY.