CE210 PILOT DEPARTS TVL WITHOUT SETTING THE LOCAL ALTIMETER; WHICH CAUSES A CONFLICT WITH VFR TRAFFIC DURING THE DEPARTURE.

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I DEPARTED TVL ARPT ON JAN/MON/07 AT APPROX XA20 LCL. DURING THE CLBOUT ON THE RICHY 5 SID; I WAS IN RADIO CONTACT WITH ZOA BUT WAS NOT IN RADAR CONTACT. I WAS LISTENING TO ZOA TALK WITH A PA28 THAT WAS NW OF THE SWR VOR AT 12500 FT HEADING S. I WAS ASSIGNED 12000 FT. I HEARD ZOA CALL THE PA28 AND TELL THEM THAT I WAS AT 12000 FT. WHEN I HEARD THAT FROM ZOA; I WAS INDICATING 11300 FT. I THOUGHT THAT I HAD MISSED PART OF THE RADIO XMISSION AND THAT HE WAS TELLING THE PA28 I WAS STOPPING AT 12000 FT. ZOA GAVE ME AN ADVISORY FOR TFC AT 1-2 O'CLOCK POS AND 2 MI AT 12500 FT AND ASKED WHAT ALT I WAS PASSING. I CALLED THAT I WAS PASSING THROUGH 11800 FT WHEN I SAW THE PA28 AT MY 2 O'CLOCK POS AND CLOSING QUICKLY AT MY ALT. I MADE A 30 DEG TURN TO THE R AT THE SAME TIME THE PA28 SAW ME AND MADE A TURN TO HIS R. I WOULD ESTIMATE THAT WE PASSED WITHIN ABOUT 400-500 FT HORIZLY. I CALLED THAT I HAD THE TFC AND WAS MANEUVERING TO THE R. I THEN ASKED FOR A CURRENT ALTIMETER SETTING WHICH SHOWED ME AT 12700 FT; NOT 12000 FT AS I HAD THOUGHT. I TOLD THE CTR OPERATOR THAT I WAS RETURNING TO 12000 FT AS DIRECTED AND APOLOGIZED OVER THE RADIO FOR ANY INCONVENIENCE. I WAS NOT ASKED TO CONTACT ANY ATC AUTH. THE REMAINDER OF MY FLT WAS WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT HE SIMPLY FORGOT TO SET HIS ALTIMETER. TVL IS A NON TOWER AIRPORT; WHICH IS WHERE HE USUALLY OBTAINS THE ALTIMETER SETTING.