A mechanical separator for separating a fluid sample within a tube into a higher density phase and a lower density phase comprising: a float having a head portion (84, 280, 384) and a body portion; a ballast circumferentially disposed about a section of the float and longitudinally moveable with respect to the float; and a deformable bellows defining an open passageway extending between a first end of the deformable bellows and a second end of the deformable bellows, wherein the upper first end of the deformable bellows includes a deformable sealing portion circumferentially disposed about the open passageway for providing a sealing engagement with the sidewall of the tube, wherein the ballast is secured to_the deformable bellows between the first end and the second end, and at least a portion of the float is transitionable from a restraint position in which the open passageway is open to a sealed position, in which the open passageway is sealed, wherein the first end of the deformable bellows _engages with an engagement protrusion (86, 632) of the float _to maintain the float _in the sealed position, wherein, in the restraint position, the head portion (84,_280, 384) of the float is positioned within the interior of the deformable bellows, and, in the sealed position, at least a portion of the head portion (84, 280, 384) of the float extends through the deformable bellows and is positioned at an exterior location longitudinally displaced from the deformable bellows _through the open passageway of the deformable bellows wherein the float has a first density, the ballast has a second density which is greater than the first density of the float, and the components of the separator are dimensioned and configured to achieve an overall density for the separator that lies between the densities of the phases of the fluid sample. a float having a head portion (84, 280, 384) and a body portion; a ballast circumferentially disposed about a section of the float and longitudinally moveable with respect to the float; and a deformable bellows defining an open passageway extending between a first end of the deformable bellows and a second end of the deformable bellows, wherein the upper first end of the deformable bellows includes a deformable sealing portion circumferentially disposed about the open passageway for providing a sealing engagement with the sidewall of the tube, wherein the ballast is secured to_the deformable bellows between the first end and the second end, and at least a portion of the float is transitionable from a restraint position in which the open passageway is open to a sealed position, in which the open passageway is sealed, wherein the first end of the deformable bellows _engages with an engagement protrusion (86, 632) of the float _to maintain the float _in the sealed position, wherein, in the restraint position, the head portion (84,_280, 384) of the float is positioned within the interior of the deformable bellows, and, in the sealed position, at least a portion of the head portion (84, 280, 384) of the float extends through the deformable bellows and is positioned at an exterior location longitudinally displaced from the deformable bellows _through the open passageway of the deformable bellows wherein the float has a first density, the ballast has a second density which is greater than the first density of the float, and the components of the separator are dimensioned and configured to achieve an overall density for the separator that lies between the densities of the phases of the fluid sample.