FO UNFAMILIAR WITH ALT ALERTER IN DH8 102 MODEL DOES NOT NOTICE ALT EXCURSION.

Narrative

FLYING EUG-SEA. ON THIS LEG'S INITIAL TKOF FROM EUG; ABORTED TKOF (PER COMPANY PROC) BECAUSE AUTOFEATHER FAILED TO ARM. RETURNED TO GATE; SWAPPED INTO INCOMING ACFT. INITIAL ACFT WAS DHC-8-202; SWAPPED INTO DHC-8-102 MODEL. DEPARTED EUG 1 HR LATE. SLIGHTLY HARRIED DSNDING INTO OLM; I REALIZED I DID NOT HAVE MY APCH PLATES FOR THE ARR BRIEF. ON DEP FROM EUG; I HAD STUFFED THE PLATES IN MY FLT BAG. AT ABOUT 11000 FT; 240 KIAS; 1500 FPM; CLRED TO 10000 FT; I TOLD MY FO I WAS STICKING MY HEAD IN MY FLT BAG TO FIND THE PLATES. I LOOKED UP SHORTLY AFTER TO SEE US DSNDING THROUGH 9200 FT. ADVISED ATC WE WERE LOW. HE TOLD US TO STAY AT 9000 FT. NO SHRILL VOICES. CONTRIBUTING FACTORS: 1) FO HAS BEEN WITH THE COMPANY 16 MONTHS. MOST OF HIS FLYING IN THE -202 MODEL. ON -202; ALT ALERTER HAS AURAL AT 1000 FT PRIOR AND 300 FT OFF. -102 HAS LIGHT ONLY. THAT SET HIM UP. 2) FO IS VERY COMPETENT. THAT SET ME UP. 3) I DEVIATED FROM MY USUAL HABIT OF ONLY OBSERVING THE LEVELOFF AND THAT PREVENTED ME FROM CATCHING HIM.