A mechanical separator for separating a fluid sample into first and second phases is disclosed. The mechanical separator includes a float having a first portion and a second portion, a ballast circumferentially disposed about a section of the float, and a deformable bellows defining an open passageway extending between a first end and a second end. The ballast is longitudinally moveable with respect to the float and engaged with the deformable bellows between the first end and the second end. At least a portion of the float is transitionable from a restraint position to a sealed position through the first end of the bellows. The first portion of the float can be positioned within the interior of the deformable bellows in the restraint position, and the first portion of the float can be positioned at an exterior location longitudinally displaced from the deformable bellows in the sealed position.