Centrifuged material layer volume measurements are made in a transparent capillary tube containing an elongated float which expands the layers being measured. The extent of layer expansion will vary depending on the tube bore diameter and the float diameter. The tube bore diameter and float diameter for each tube/float pair are measured, and an indication of the difference between the two is marked on the tube. When the volume determinations are made, the measured difference between the tube bore diameter and the float diameter is taken into account before calculating actual layer volume malues.