PLT OF SMA MOTORIZED GLIDER INADVERTENTLY ENTERED THE TCA DUE TO AN INCORRECT ALTIMETER SETTING.

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I TOOK OFF FROM BISTATE ARPT ACCOMPANIED BY (NAME OF PAX; PLT LICENSE CFII); SR. EDITOR OF MAGAZINE; FOR A DEMO FLT OF THE ONLY SMA MOTORGLIDER IN THE COUNTRY IN ANTICIPATION OF A FEATURE ARTICLE. WE DEPARTED THE AREA TO THE NE PLANNING TO CLB ABOVE THE BROKEN CLOUD LAYER FOR AIRWORK. THE PLANE WAS EQUIPPED WITH A (BRAND) GPS AND (BRAND) 3000 MOVING MAP DISPLAY. LEAVING THE ARPT ATA; WE CONTACTED ST LOUIS APCH FOR ADVISORIES. THE CTLR INFORMED US OUR MODE C READOUT INDICATED OUR ALT WAS 4800 FT. OUR GPS/(BRAND) SHOWED US UNDER A 4500 FT FLOOR AND OUR ALTIMETER INDICATED 4400 FT. IN FACT; WE HAD PURPOSELY LEVELED AT 4400 FT TO BE BELOW THE TCA. THE CTLR DIRECTED AN ACR MLG TO TURN S TO AVOID A CONFLICT. THE MLG WAS BELOW US AND RPTED HIS ALT AS 4500 FT. THE CTLR GAVE US THE BAROMETER SETTING MUCH HIGHER THAN OUR SETTING -- WE HAD THE WRONG BAROMETER SETTING. ALTHOUGH USING A CHKLIST I EITHER SET THE WRONG VALUE IN PRIOR TO TKOF OR CHANGED IT INADVERTENTLY WHILE CAGING THE ADI WHICH TOOK A LOT OF EFFORT -- PULLING AND TUGGING ON THE KNOB IMMEDIATELY ADJACENT TO THE BRAND ALTIMETER. IN ANY EVENT; I TOOK OFF WITH THE BAROMETER SETTING 400 FT LOW; EITHER DUE TO OVERSIGHT OR BECAUSE IT 'LOOKED RIGHT' TO TKOF WITH THE ALTIMETER AT 0 ELEVATION (WHICH IS CUSTOMARY FOR GLIDER OPS). THE SECOND ALTIMETER IS CURRENTLY CALIBRATED IN METERS AND WAS NOT BEING USED.