The invention comprises a multi-chamber module-cap combination useful for measuring the impedance of samples of nutrient media in the chambers thereof, some of which samples include aerobic microorganisms. The module comprises a printed circuit board having a plurality of hollow cylinders upraised from one side thereof. The conductors of the circuitry provide a pair of spaced-apart contacts under each of the cylinders. The cylinders, before being sealed to the boards, are open at both ends so that the spaced-apart contacts are exposed within the chamber formed by the cylinders and the one side of the board. Caps are provided in combination with the board-cylinder structure, the caps serving to fit over the cylinders, the skirts of the caps being sufficiently long to extend against the one side of the board so that a gap exists between the tops of the caps and the tops of the cylinders. Interiorly of the skirts are a plurality of generally evenly spaced longitudinally placed upraised ribs. When the caps are pushed over the cylinders and down against the boards, the ribs engage the outer surface of the cylinders and an air flow channel is thereby provided from the interior of the chamber to the surrounding atmosphere, the channel being defined by the space between the cylinders, the skirt and the ribs.