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 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 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 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 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 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 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. <EOS>