A mechanical separator for separating a fluid sample into first and second phases is disclosed. The mechanical separator includes a float, a ballast assembly longitudinally moveable with respect to the float, and a bellows structure. The bellows structure includes a first end, a second end, and a deformable bellows therebetween. The float is attached to a portion of the first end of the bellows structure, and the ballast is attached to a portion of the second end of the bellows structure. The attached float and bellows structure includes a releaseable interference engagement therebetween. The float has a first density, and the ballast has a second density that is greater than the first density of the float.