AN SF340 FLIGHT CREW OPERATED WITH THE AUTO PRESSURIZATION CONTROLLER DEFERRED. AFTER LANDING THE FIRST OFFICER DEPRESSURIZED THE AIRCRAFT ABRUPTLY WITH THE MANUAL SYSTEM.

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WE WERE ASSIGNED ACFT X. THE AUTO MODE OF THE PRESSURIZATION SYSTEM WAS DEFERRED BECAUSE THE ACFT WAS NOT DEPRESSURIZING CORRECTLY. THE MECH SAID TO USE THE MANUAL MODE AND TO BE CAREFUL BECAUSE IT WAS SENSITIVE. I ALSO LOOKED UP THE PROC IN MEL BOOK. I MADE AN ANNOUNCEMENT TO THE PAX LETTING THEM KNOW OUR AUTO MODE WAS DEFERRED AND WE WOULD BE USING OUR MANUAL MODE AND THAT IT WAS SENSITIVE. I ALSO CALLED DISPATCH AND REQUESTED TO FLY THE FLT AT 10000 FT MSL. WE MADE THE FLT WITHOUT ANY PROBS. HOWEVER; WHEN WE LANDED WE WERE STILL SLIGHTLY PRESSURIZED. I CONTINUED TO TAXI TO THE GATE TO PARK AND SHUT DOWN. AT THE GATE; I TOLD FO NOT TO TOUCH ANYTHING AND I WAS GOING TO CHK THE MANUAL FOR ANY GUIDANCE. AS I TURNED MY HEAD TO GET THE MANUAL; FO (WITHOUT MY CONSENT) RAPIDLY TURNED THE MANUAL PRESSURIZATION KNOB FROM FULLY CLOSED TO FULLY OPEN. WE DEPRESSURIZED IN AN INSTANT. IT WAS NOT A GOOD SITUATION. IT WAS VERY UNCOMFORTABLE. MANY OF THE PAX COMPLAINED TO THE FLT ATTENDANTS AND THE AGENTS IN ZZZ. SOME OF THE PAX SAID THEY WERE GOING TO WRITE IN A COMPLAINT TO OUR AIRLINE. I THEN MADE A PHONE CALL TO CHIEF PLT; HOWEVER; HE PUT ME ON HOLD AND I HAD TO GET GOING SO I WAS NOT ABLE TO TELL HIM. I DID MAKE A CALL TO HIM AFTER MY TRIP. AFTER DISCUSSING THE INCIDENT WITH A MECH IN ZZZ; I FEEL VERY LUCKY THAT NOBODY WAS INJURED.