A DC9 FLC LOST PRESSURIZATION AND DSNDED NEAR CMH.

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AT FL290; WE HAD A PRESSURIZATION PROB. OUR CABIN ALT STARTED TO SPIN. MAINT HAD CHANGE GAUGE DAY PRIOR I THOUGHT IT WAS ONLY AN INDICATION PROB. DECIDED TO TAKE PRECAUTION AND DSND BELOW FL250 JUST IN CASE. WHEN FO REDUCED PWR TO START DSCNT 'CABIN PRESSURE' LIGHT ILLUMINATED -- TOLD FO TO REQUEST DSCNT TO 10000 FT. I WAS ALREADY ON SELCAL WITH COMPANY TO NOTIFY THEM ABOUT GAUGE SO THEY COULD FIX IN MYR. THE OXYGEN MASK THEN DEPLOYED WHILE IN OUR DSCNT. WE FOUND THAT IF WE PUSHED THE PWR UP WE GOT OUR DIFFERENTIAL BACK. I NEVER DECLARED AN EMER EVEN THOUGH I SHOULD HAVE. I ALSO DID NOT DSND AS QUICKLY AS WE SHOULD HAVE. REASONS FOR THIS IS; BECAUSE WE STILL HAD DIFFERENTIAL AND I KNEW THERE WAS A PROB WITH THE GAUGE THE DAY BEFORE I DID NOT REALLY BELIEVE THERE WAS A TRUE TOTAL DEPRESSURIZATION. ALSO THERE WERE A LOT OF TSTMS IN THE AREA AND MY FLT ATTENDANTS WERE IN THE AISLE STILL. I SHOULD HAVE DONE A HIGH DIVE ANYWAY; BUT I DID NOT. IN THE FUTURE; IF I ENCOUNTERED THE SAME SIT I WOULD DECLARE THE EMER AND DO A HIGH DIVE AND NOT SECOND GUESS THE ACFT.