Flow Nozzles

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications.

The flow nozzle is relative simple and cheap, and available for many applications in many materials.
The TurnDown Rate and accuracy can be compared with the orifice plate.

The Sonic Nozzle - Critical (Choked) Flow Nozzle:

When a gas accelerate through a nozzle, the velocity increase and the pressure and the gas density decrease. The maximum velocity is achieved at the throat, the minimum area, where it breaks Mach 1 or sonic. At this point it's not possible to increase the flow by lowering the downstream pressure. The flow is choked.
This situation is used in many control systems to maintain fixed, accurate, repeatable gas flow rates unaffected by the downstream pressure.

Article Source : www.engineeringtoolbox.com