ASME MFC-26-2011 (R2018)

This Standard applies only to the steady flow of single-phase gases and gas mixtures and applies only to bellmouth inlet flowmeters in which the flow remains subsonic throughout the measuring section and the flow is steady or varies only slowly with time. It also addresses procedures by which calibration of the device can be made to allow for application with consistent conclusions and within known limits of uncertainty.

Bellmouth inlet flowmeters should be used only within the limits for which a given unit is tested, or if additional uncertainty can be tolerated, over a range within which extrapolation is reliable.

This Standard outlines the general geometry and method of use of bellmouth inlet flowmeters to determine the mass or volumetric flow rate of the gas or gas mixture flowing through the device. It also gives necessary information for calculating the flow rate and its associated uncertainty.

A bellmouth inlet flowmeter is a device that provides flow conditioning and flow measurement whose inlet is located or positioned in a large reservoir or supply source. The reservoir can be outside ambient, room, or plenum conditions depending on the application. The bellmouth inlet flowmeter is also referred to as an airbell, nozzle with zero beta ratio, borda tube, etc. Typical geometry consists of a convergent inlet followed by a constant throat area. This flowmeter is a differential pressure type device that allows determination of the flow rate from the differential pressure between the total pressure and static pressure at a single specified axial location in the constant area throat of the bellmouth.