Sonic Nozzles are internationally recognized as the best Flow Transfer Standard for gas flow measurement and Calibration. The Sonic Nozzle (Critical Flow Nozzle, Critical Flow Venturi, Sonic Venturi) is a converging-diverging flowmeter that has become the standard for air flow measurement in the aerospace industry. It consists of a smooth rounded inlet section converging to a minimum throat area and diverging along a pressure recovery section or exit cone. The Sonic Nozzle is operated by either pressurizing the inlet (P1) or evacuating the exit (P3), to achieve a pressure ratio of 1.2 to 1 or greater, inlet to outlet. When used with Air, for example, this ratio maintains the Nozzle in a “choked” or “sonic” state. In this state, only the upstream pressure (P1), and temperature are needed to calculate the flowrate through the Nozzle. The flowrate through the Nozzle becomes primarily a linear function of the inlet pressure, doubling the inlet pressure doubles the flowrate. The simplest flow system would use an inlet pressure regulator to control air pressure and a thermocouple to measure temperature. Adjusting the pressure regulator will change and maintain the flow through the Nozzle.
As a gas accelerates through the Nozzle, its velocity increases and its density decreases. The maximum velocity is achieved at the throat, the minimum area, where it just breaks Mach 1 or sonic. Pressure differences within a piping system travel at the speed of sound and generate flow. Downstream differences or disturbances in pressure, traveling at the speed of sound, cannot move upstream past the throat of the Nozzle because the throat velocity is higher and in the opposite direction. These are relative velocities and they add algebraically. Since the pressure disturbances cannot move past the throat, they cannot affect the velocity or the density of the flow through the Nozzle. This is what is referred to as a choked or sonic state of operation. Normally in a sub-sonic flowmeter (Venturi, ASME Flow Nozzle, or Orifice Plate), any change in downstream pressure will affect the differential pressure across the flowmeter, which in turn, affects the flow. This is not the case in sonic flow and is one of the strongest advantages of using a Sonic Nozzle. If you have a system with pulsating or varying gas consumption downstream and you want to feed it a constant or locked flowrate, a Sonic Nozzle is an excellent way to achieve this. You won’t need a complicated PID control loop system. Adjusting the inlet pressure with a pressure regulator will change the flow to any point within the gas pressure supply available.
