An apparatus and method is provided for establishing a constant flow rate of a fluid, such as a gas or a liquid, into a process system. The apparatus typically includes a fluid pressure regulator, a rotameter, and a back pressure regulator. The system may also include a throttle valve. Establishing a constant flow rate involves established a fixed pressure drop across a fixed flow resistance device, such as a rotameter. The pressure of the output of the rotameter is adjusted to be higher than the backpressure from the process system.
 The U.S. Government has rights to this invention pursuant to contract number DE-AC05-000R22800 between the U.S. Department of Energy and Babcock & Wilcox Technical Services Y-12, LLC.