Abstract: A fluid diluter has a hydrodynamic focusing device with a circular mixing chamber opening into a receiving chamber via a sensing aperture. Diluent is supplied from a source and mixed with a sample of blood, for example, and the sensing aperture counts the presence of particles such as hemoglobin in the sample using the impedance method. The receiving chamber is subsequently emptied and the cycle repeated. In one embodiment, through-flow is achieved via a pair of pumps having different pumping capacities, connected together with a common actuator rod between the two pistons. In a second embodiment, four pumps are employed, three of which are ganged together. The system is substantially less bulky than known diluters yet permits accurate attainment of high dilution ratios. Particles may be analyzed in the aperture one at a time without coincidence. Altering relative capacities of the pumps permits adjustment of the dilution ratio.