THE IMPLEMENTATION OF TOROIDAL THROAT VENTURI NOZZLES TO MAXIMIZE PRECISION IN GAS FLOW TRANSFER STANDARD APPLICATIONS

摘要

The measurement of the mass flow of gasses in the range up to 50 g s~(-1) (2500 NI min~(-1)) is important in a variety of industrial processes and research fields. Precise and stable transfer standards are an indispensable part of the system to support these measurements. Filling the transfer standard role effectively requires instruments that are transportable and easy to use with minimal sensitivity to external influences such as ambient pressure, ambient temperature, upstream geometry and upstream or downstream pressure. In the lower part of this flow range, the transfer standard role has been filled effectively by instruments based on laminar flow elements [1, 2]. However, as flow increases, the differential pressure differential pressure across the laminar element increases aggravating temperature influences and eventually the flow regime transitions to turbulent. A well known alternative, particularly for higher flows, is the toroidal throat, critical flow Venturi nozzle. A new flow element based on critical flow nozzles (CFN) has been developed to complement existing laminar flow modules using a common mass flow terminal. The new elements cover the range of flow from 0.02 to 50 g(centre dot)s~(-1) (1 to 2500 NI min~(-1)) and beyond. The laminar flow elements are known under the trade name molbloc-L and the CFN based elements are designated molbloc-S.