A centrifuge tube is used to hold a mixture of several constituents, and also contains a generally cylindrical float. The float settles, after centrifugation, into the zone occupied by the constituent whose volume is to be measured. The constituent layer will settle, after centrifugation, into the annular space between the tube bore and the outside of the float, and will be expanded axially due to the restricted volume of the annular space. The degree of expansion is dependent upon the respective sizes of the float O.D. and the tube bore ID, both of which must be closely controlled for accurate results. A known volume of a control material is placed in the tube to settle into the annular space during centrifugation in an area thereof outside of the constituent layer zone. The length of the band of the control material is measured after centrifugation and is compared to a known length which will result if the annular space is of the proper target volume. A correction factor is thus obtained and applied to all of the other constituent layers which were measured. The band thus forms a varying reference which reflects the actual volume of the annulus.