For which purpose a sonic nozzle is used?
There are situations where flow meters need to provide very accurate and reliable value, this is the case when they are used for commercial transactions. For example, on a gas meter station, the data they report is the basis for billing gas quantities. Given the quantities transported, a small measurement error can have important consequences. Thus, to ensure correct counting, it is necessary to use a flow meter truly calibrated according to the intended conditions of use. The measuring instrument must therefore be compared to a standard to be able to verify its performance. This calibration must be carried out before commissioning and during periodic checks according to the regulations in force.
The nozzle in sonic mode (choked flow achieved) is then used to provide the standard value of flow. Indeed, under critical flow conditions, the flow depends only on the temperature and the pressure upstream of the measuring element. If the conditions upstream of the sonic nozzle can be kept constant, the flow will remain constant. This is the major interest of these measuring devices as a means of calibration.