Abstract: A measurement device is provided for determining relative proportions of at least two materials forming a liquid and having different densities and different responses to light. A transparent capillary size cuvette tube confines the sample liquid, and is attached radially on a rotor wheel, preferably by a holder which seals an outer end of the tube and provides a package for manipulation of the tube without exposure to the sample. A motor controlled by a processor turns the rotor wheel, separating the materials by their densities. A linear light source illuminates the tube, and a linear array of photosensors defining pixels produces a signal which varies where the different materials abut. The pixels are counted, to locate the stratum between materials and the result is displayed as a percentage of total volume, suitable for hematocrit measurements. The cuvette tube has a fin for assisting in manipulation, is shaped as an elongated lens, and is cut to a spherical end surface to assist in sample collection.