WRONG ALTIMETER SETTING PLACES ACR MLG IN POS OF JEOPARDY IN DSCNT TOWARDS OCCUPIED ALT. TCASII RA SAVES THE DAY AND PREVENTS NMAC. EVASIVE ACTION CLB.

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VALUABLE LESSON LEARNED AND TCASII WORKS. WE WERE FLYING FLT FROM IAH TO PIT. A MAJOR WX SYS WAS MOVING THROUGH THE EASTERN PART OF THE UNITED STATES. WHILE DSNDING FOR PIT; WE WERE GIVEN HOLDING INSTRUCTIONS FROM A BUSY CTLR AT ZOB. PIT HAD A TSTM ON THE FIELD WITH GOLF BALL SIZE HAIL. OUR HOLDING INSTRUCTIONS WERE GIVEN ALONG WITH A DSCNT TO 16000 FT ALONG WITH AN ALTIMETER SETTING. I WAS THE PNF AND WROTE DOWN THIS INFO. I MISREAD THE ALTIMETER SETTING AND WROTE DOWN JUST '.34' ON MY SCRATCH PAD IN MY HASTE TO COPY ALL THE INFO. I THEN READ BACK ALL THE INFO TO CTR. I AM PRETTY SURE I READ BACK THE ALTIMETER AS 30.34. HOWEVER; IN REALITY; IT WAS 29.34. AS WE WERE APCHING OUR HOLDING FIX AND DSNDING THROUGH 17000; OUR TCASII CALLED A TA. THIS ACFT WAS OBVIOUSLY ALSO IN A HOLD; ALSO AT WHAT APPEARED TO BE 16000 FT. THE TCASII THEN CHANGED TO AN RA. WE FOLLOWED TCASII COMMANDS AND LEVELED OFF. THE OTHER ACFT PASSED AND WE STARTED ASKING CTR 'WHAT WAS GOING ON?' IT TOOK A MIN OR SO TO REALIZE WHO WAS WRONG. WE WERE 1000 FT LOWER THAN OUR ALTIMETERS SHOWED. I WILL NEVER AGAIN JUST WRITE DOWN .34 AND THEN ASSUME IT MUST BE 30.34; NO MATTER HOW MUCH INFO THERE IS TO COPY DOWN!