Annubar

A Rosemount Annubar primary element is an averaging Pitot tube similar to a single point pitot tube used to measure the flow of gas, steam, or liquid in a pipe. An Annubar primary element or Annubar averaging Pitot tube provides better accuracy than single point Pitot tubes.

Profile view of upstream slots

Profile view of downstream averaging holes

The Pitot tube measures the difference between the static pressure and the flowing pressure of the media in the pipe. The volumetric flow is calculated from that difference using Bernoulli's principle and taking into account the pipe inside diameter.[1]

The biggest difference between an Annubar averaging Pitot tube and a Pitot tube is that an Annubar averaging Pitot tube takes multiple samples across a section of a pipe or duct. In this way, it averages the differential pressures encountered accounting for variations in flow across the section. A Pitot tube will give a similar reading if the tip is located at a point in the pipe cross section where the flowing velocity is close to the average velocity.

The T-Shape cross section of the Rosemount Annubar primary element allows for increased signal strength and reduced signal noise compared to other averaging pitot tube shapes.

Annubar is a registered trade name with Emerson Process Management / Rosemount. [2]

An Annubar averaging Pitot tube is economical to install and has negligible pressure drop but it is unsuitable when dirty or sticky fluids are used.

References

1. Perry, Robert H., Don W. Green. "Perry's Chemical Engineer's Handbook, Seventh Edition". pages 10-8 to 10-9. McGraw Hill. 1997
2. Rosemount Annubar Flowmeter Series