This International Standard specifies the geometry and method of use (installation in a system and operating conditions) of critical flow Venturi nozzles used to determine the mass flow-rate of a gas flowing through a system. It also gives the infor-mation necessary for calculating the flow-rate and its associated uncertainty.
It applies to Venturi nozzles in which the gas flow accelerates to the critical velocity at the throat (this being equal to the local sonic velocity). At the critical velocity, the mass flow-rate of the gas flowing through the Venturi nozzle is the maximum poss-ible for the existing upstream conditions.
This International Standard is applicable only where there is steady flow of single-phase gases. The critical flow Venturi nozzles dealt with can only be used within specified limits, e.g. limits for the nozzle throat to inlet diameter ratio and throat Reynolds number. It deals with Venturi nozzles for which direct calibration experiments have been made in sufficient number and quantity to enable inherent systems of application to be based on their results and to enable coefficients to be given with certain predictable limits of uncertainty.
The Venturi nozzles specified in this International Standard are called "primary devices". The other instruments necessary for the measurement of the flow-rate are known as "secondary devices". This International Standard principally covers primary devices; secondary devices are discussed only occasionally.
Information is given in this International Standard for cases where
a) the pipeline upstream of the Venturi nozzle is of circular cross-section, or
b) it can be assumed that there is a large space upstream of the Venturi nozzle.