Variable displacement base and container and method of using the same

A hot-fillable plastic bottle, the bottle comprising:

• a finished portion defining an opening;
• a body portion disposed below the finished portion and defining a central longitudinal axis, the body portion comprising a body sidewall defining an interior, the body sidewall having at least one circumferential rib extending about its circumference and configured to resist radial distortion;
• and
• a base portion disposed below the body portion, the base portion comprising:
• a base sidewall,
• a bottom support surface extending radially inward from the base sidewall toward the central longitudinal axis and defining a reference plane,
• an inner support wall extending upwardly from the bottom support surface at an angle less than 90 degrees, and
• a diaphragm extending radially inward toward the central longitudinal axis from the inner support wall, the diaphragm comprising an inner wall and a central conical structure, the inner wall extending from the inner support wall to the central conical structure and including a continuous arcuate shape in a side cross section with a radius r1, wherein the diaphragm is in an initial as-formed position with the conical structure above the reference plane, the diaphragm configured to move from the initial as-formed position toward a second position when an internal pressure relative an external pressure external of the bottle exceeds a threshold value, wherein the diaphragm is configured to move downwardly in response to an increase in the internal pressure and wherein the second position is below the as-formed position, and further wherein the diaphragm is configured to move upwardly and axially inward at least to the initial as-formed position in response to a decrease in internal pressure corresponding to an internal vacuum being created within the bottle due to cooling of a hot-fill liquid provided in the bottle.