A method for determining whether microbial growth is occurring within a specimen container includes steps of incubating the specimen container and obtaining a series of measurement data points while the specimen container is incubated and storing the data points in a machine-readable memory. The first and second heat sensors and the heat source are arranged in a non-linear orientation. A handle assembly is attached to the hub that includes an actuator member slidable between the spacer elements and the outer surface of the sheath. An embodiment according to the invention provides a plurality of annular structures with staggered, longitudinally overlapping apices of undulating structures. The protruding perforations comprise protrusions increasing surface area and generating turbulent flow, and small openings through the shield to allow convection air flow to pass through.