Generally, a needle valve is provided for small sized glass tube rotameters (Purgemeters) for flow control purpose. This is to explain the recommended location (Inlet / Outlet) of valve by application.

For Liquid measurement application

P-520 P-620 The location of valve may not affect the accuracy and any location(Inlet / Outlet) is acceptable.

For Liquid measurement application
For gas measurement, the location of control valve may differ depending on the application.

In case of Atmospheric discharging ( 0MPa pressure outlet) P-820 Valve at Inlet (Lower) is recommended. In this case, the flowmeter is calibrated according to 0MPa (1atm). In case of pressurized gas measurement P-100 Valve at Outlet (Upper) is recommended. In this case, the flowmeter is calibrated according to customer specified pressure. In case of Vacuum application (Vacuum in outlet) P-710 Valve at Outlet (Upper) is recommended. If the valve is located Inlet (Lower), the pressure in tapered tube becomes vacuum and the movement of float will be unstable.

The valve provided for purgemeters is for flow control purpose, not for process shut down, Excessive rotation for closing may cause breakage of the valve.
Install separate closing valve upstream when complete shut down is needed.