3. Venturi-tuyères operating in sonic regime

Sonic venturi nozzles (hereinafter referred to as "sonic nozzles") offer significant advantages over other types of flow reference. They are highly stable over time, their uncertainty is essentially linked to that of the associated measuring instruments, and they remain easy to build and simple to use. However, their flow dynamics depend on the generating pressure upstream of the sonic nozzle. So, to vary the flow through a sonic nozzle, the operator must act on this upstream pressure. Only the presence of a stable pressure source can enable sonic nozzles to be used routinely.