Mass flow verifier with flow restrictor

An apparatus for preventing an external element from affecting verification of a flow rate of a fluid by a flow rate verifier, the flow rate verifier including a vessel configured to receive flow of the fluid from a mass flow controller, and an outlet valve configured to control flow of the fluid out of the vessel, the external element located upstream from the flow rate verifier, the apparatus comprising:

• a flow restrictor located along a flow path of the fluid from the mass flow controller into the vessel, the flow restrictor being adjacent to and upstream of the vessel;
• wherein the flow restrictor is configured to restrict the flow of the fluid along the flow path so as to induce a shock in the flow path, after the flow rate of the fluid into the vessel has reached a steady-state and the outlet valve has been closed so that the pressure of the fluid rises within the vessel, and
• wherein the flow restrictor is further configured to sustain the shock during a time period sufficient to render the verification of the flow rate measurement substantially insensitive to the external element.