A capacitive manometer assembly (500, 600, 800, 900A, 900B, 1000, 1100, 1200) comprising: a diaphragm (502, 602, 802, 902, 1002, 1202) including a conductive material; a circular electrode structure (504, 604, 700A, 700B, 804, 903, 1004, 1104, 1204) including an inner electrode (506, 606, 702A, 702B, 806, 908, 1006, 1106, 1206), an outer electrode (508, 608, 704A, 704B, 808, 904, 1008, 1108, 1208), and a conductive guard structure (510, 610, 706A, 706B, 810, 906, 1010, 1012, 1110, 1112, 1114, 1210, 1212) disposed between the inner and outer electrodes, wherein the diaphragm is movable relative to the electrode structure between (i) a zero position when the pressure on each side of the diaphragm is the same and (ii) a maximum differential position when the maximum measurable differential pressure is applied to the diaphragm; wherein the guard structure (510, 610, 706A, 706B, 810, 906, 1010, 1012, 1110, 1112, 1114, 1210, 1212) forms a zone or region of the electrode structure (504, 604, 700A, 700B, 804, 903, 1004, 1104, 1204) that separates the inner electrode (506, 606, 702A, 702B, 806, 908, 1006, 1106, 1206) and outer electrodes (508, 608, 704A, 704B, 808, 904, 1008, 1108, 1208) and does not contribute to capacitance measurements; a support structure arranged so as to support the diaphragm so that the diaphragm is constrained relative to the electrode structure, and the diaphragm is spaced from and axially aligned with the inner and outer electrodes relative to a longitudinal axis of the manometer; and a circular baffle (514, 612, 812, 912, 1014, 1120, 1214) with one or more apertures (516, 614, 814, 914, 1016, 1122, 1216) configured to admit gas to a region adjacent the diaphragm, wherein the baffle is configured to reduce effects of surface deformation of the diaphragm due to reactive gas, wherein the one or more apertures are aligned with the guard structure in a direction parallel to the longitudinal axis or aligned with a gap between the guard structure and the inner or outer electrodes in a direction parallel to the longitudinal axis. a diaphragm (502, 602, 802, 902, 1002, 1202) including a conductive material; a circular electrode structure (504, 604, 700A, 700B, 804, 903, 1004, 1104, 1204) including an inner electrode (506, 606, 702A, 702B, 806, 908, 1006, 1106, 1206), an outer electrode (508, 608, 704A, 704B, 808, 904, 1008, 1108, 1208), and a conductive guard structure (510, 610, 706A, 706B, 810, 906, 1010, 1012, 1110, 1112, 1114, 1210, 1212) disposed between the inner and outer electrodes, wherein the diaphragm is movable relative to the electrode structure between (i) a zero position when the pressure on each side of the diaphragm is the same and (ii) a maximum differential position when the maximum measurable differential pressure is applied to the diaphragm; wherein the guard structure (510, 610, 706A, 706B, 810, 906, 1010, 1012, 1110, 1112, 1114, 1210, 1212) forms a zone or region of the electrode structure (504, 604, 700A, 700B, 804, 903, 1004, 1104, 1204) that separates the inner electrode (506, 606, 702A, 702B, 806, 908, 1006, 1106, 1206) and outer electrodes (508, 608, 704A, 704B, 808, 904, 1008, 1108, 1208) and does not contribute to capacitance measurements; a support structure arranged so as to support the diaphragm so that the diaphragm is constrained relative to the electrode structure, and the diaphragm is spaced from and axially aligned with the inner and outer electrodes relative to a longitudinal axis of the manometer; and a circular baffle (514, 612, 812, 912, 1014, 1120, 1214) with one or more apertures (516, 614, 814, 914, 1016, 1122, 1216) configured to admit gas to a region adjacent the diaphragm, wherein the baffle is configured to reduce effects of surface deformation of the diaphragm due to reactive gas, wherein the one or more apertures are aligned with the guard structure in a direction parallel to the longitudinal axis or aligned with a gap between the guard structure and the inner or outer electrodes in a direction parallel to the longitudinal axis.