A B190 CREW. DURING CLB THROUGH FL210; EXPERIENCED A LOSS OF CABIN PRESSURE; SPAWNING AN INITIAL DSCNT TO FL180.

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ON A CLB TO FL240; ON A FLT FROM COS TO ABQ; A 'CABIN ALT HIGH' ANNUNCIATOR ILLUMINATED AT APPROX FL210. APPROPRIATE MEMORY ITEMS AND 'CABIN ALT HIGH' EMER CHKLISTS WERE COMPLETED; WHICH REQUIRED THE USE OF CREW AND PAX OXYGEN. THE CREW VERIFIED THAT THE PRESSURIZATION HELD CABIN ALT AT 9800 FT AND THERE WAS NO SUDDEN OR RAPID DECOMPRESSION. DSCNT TO FL180 WAS CLRED BY ATC. DURING DSCNT; CABIN ALT INDICATOR SHOWED INCREASING PRESSURIZATION AND 'CABIN ALT HIGH' ANNUNCIATOR EXTINGUISHED. FURTHER DSCNTS WERE APPROVED AS TERRAIN PERMITTED. MASKS REMAINED DONNED FOR PAX AND CREW AS A PRECAUTION UNTIL ACFT DSNDED THROUGH 10000 FT. WE LANDED WITHOUT INCIDENT IN ABQ. EVEN THOUGH THE 'CABIN ALT HIGH' ANNUNCIATOR ILLUMINATED; THE CABIN ALT INDICATOR WAS READING 9800 FT. CREW DID NOT KNOW IF IT WAS A MIS-RIGGED ANNUNCIATOR OR FAULTY GAUGE; SO WE OPTED FOR SAFETY AND USED OXYGEN. A CONTRIBUTING FACTOR TO THIS WAS THAT THE ACFT WE USE IS OLD AND CAN NO LONGER HOLD ADEQUATE ACFT PRESSURIZATION; AND THE ANNUNCIATORS ARE SOMETIMES SET TO ILLUMINATE TOO LOW A CABIN ALT. THE CABIN WAS NOT DEPRESSURIZING; BUT THE 'CABIN ALT HIGH' ANNUNCIATOR HAS ASSOCIATED MEMORY ITEMS AND EMER CHKLISTS THAT REQUIRE THE CREW TO DEPLOY OXYGEN MASKS; REGARDLESS OF THE SIT. INSTEAD; THE CREW SHOULD BE ABLE TO CONFIRM CABIN ALT BELOW 10000 FT BEFORE EXECUTING THE EMER CHKLIST. THIS WOULD HAVE PREVENTED THE WHOLE INCIDENT AND EASED PAX TENSIONS BY NOT HAVING TO DEPLOY THE OXYGEN MASKS. WE DEFERRED THE MALFUNCTION; WITH THE ACFT LIMITED TO FL180 OR BELOW AS PER OUR MEL; AND CONTINUED OPS.