Flow cytometry system with stepper flow control valve

A system, method, and apparatus are provided for flow cytometry. In one example, the flow cytometry system includes dual laser devices and dual scatter channels to measure velocity of particles in a core stream of sample fluid. The total flow rate of the sample fluid and the sheath fluid around the sample fluid is controlled, and thus held constant, by a feedback control system controlling a vacuum pump based on differential pressure across ends of a flow channel in the flow cell. A stepper flow control valves are disclosed that apply a physical fluid resistance to a flow of sheath fluid in the flow cytometer. The physical fluid resistance regulates a flow rate of the sheath fluid and thereby regulates a flow rate of sample fluid in the flow cytometer.