Abstract: An apparatus for compensating for variations in pressure exerted by a fluid within a container against the container to maintain the pressure below a pressure limit within a predetermined pressure range. The pressure range has an upper range limit above the pressure limit. The apparatus includes an insert member which is substantially non-absorbent with respect to the fluid and which is at least substantially immersed within the fluid. The insert member is compressible over a volumetric range, which volumetric range is established to permit sufficient compression of the insert member to relieve the pressure against the container by a pressure difference. The pressure difference is at least equal to the difference between the upper range limit and the pressure limit. In its preferred embodiment, the insert member is freely floating within the container, though it may be tetheredly attached to a cover or other portion of the container to allow it to freely float within a tethered range of its attachment point.