Annubar reading: air character change


Thread Starter

Clyde September

Hi all,

What effect would air under compression have on the reading of a differential type probe e.g.. Annubar, as opposed to air flowing at the same
velocity, but not having been compressed. How can one accurately measure the air velocity of the output of a compressor, taking into consideration all possible changes in air density,
temp etc. ?

Clyde September
Instrumentation Technical Officer
SANS Fibres
Belville Site
+27- 21- 9594443
+27- 21- 9594621 (fax)
[email protected]

Hall, Christopher W

Annubar type flow measurement devices are a development of the Pitot Tube and will work quite happily on dry clean air at elevated pressures. You just have to apply the ideal gas laws or some other equation of state, preferably with temperature and pressure measurements of the fluid close to the flow element. There are a variety of such devices on the market. One
that I have come across in the past is the Tribar which takes an average reading across the duct or pipe and seems to give good repeatability.

The old British standard BS-1042 (now superseded by BS-5197, not too sure on the number I'm afraid) will give you all the information for the design and calibration of such an installation.

Chris Hall - Control Engineer
Speaking for myself!
While changing pressure or temperature would change the mass flow rate, the velocity rate(volumetric flow rate) would stay the same. If you are interested in the mass rate, use pressure and temperature compensation to correct. Look into using the Fisher-Rosemount multivariable flow
transmitter model 3095FB or equivalent. Fisher-Rosemount also provides a mass flow meter package for an Annubar called Mass Probar. If you don't
like Fisher-Rosemount, there are plenty of flow computers that will do job. Some of these are Daniel, Elliot, Foxboro, Omega, Solatron, and tons of others. Just do a search on the Internet on flow computers.

Bill Mostia
William(Bill) L. Mostia, Jr. PE
Independent I &E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
[email protected]
These opinions are my own and are offered on the basis of Caveat Emptor.