Annubar is a Pitot tube, multiport design.
The ports are located based on Chebychev statistical distribution for an improved averaging
You don't calculate the flow formula.
The flow can be calculated from the differential pressure reading, and it is based on the formula given by Annubar (based on test).
This device is a differential pressure producer. Same principle as the orifice plate but of different geometry.
Untill recently, it's accuracy was not normalised, but a good installation can be trusted to near the accuracy of the orifice plate. Read the informations from Annubar and obtain their catalog. Nice source of information in Instrumentation.
Does that reply answer your question ?
On the following web page you will find an explanation of the annubar principle
To use this system as a mass flowmeter, you should convert the volumic flow to a massic flow by multipling it with the density. If in your application the temperature of the liquid change you must add the calculation of the density in function of the temperature. I have heard about transmitter which can handle this calculation but never used (does someone on the list know a
The flow is proportional to the square root of the deltaP multiplied by a constant which depend of the geometry of the sensor itself (take care about the fact that this geometry will change by corrosion or wearing and that will effect the precision of the sensor -> need to be recalibrated).
Steve Monnet suggests you also need a density signal to convert volumetric to mass flow.
For gas or for liquid, if you are using an annubar it is large bore pipe work so you should look for an insertion type density meter for liquids. For gases, the trick is to use the guidelines for natural gas flow to install the density meter. This is because to get to the mass flow you need the density at as near the gas temperature and pressure conditions for the density to be accurate enough. Vibrating spool gas density meters are low mass and can be mounted in a "thermowell" in the pipeline. With oriface plates, a samll gas sample flow is induced through the density sensor using the vena contracta method (which avoids bypassing gas around the flow meter)This method is far more accurate than using PTZ calculations to compensate the density for any temperature and pressure differences between the conditions at the density meter from the conditions at the flow meter. For more information on this you should contact one of the gas density meter suppliers or consult the standards authorities. (a visit to www.Solartron,com will let you download the manuals for their gas density transmitters and gas flow computers which will let have a look at installations, methods and calculations as these are extensively described there. (gas density is the 7812, the flow computers are the 795X)You could try the same with Peek ( abit of a net search needed here, you might try under Thermal Instruments or Onyx and track them from there.) who offer gas and liquid density devices also.
Incidently, the comment about temperature correction is relevant to standard volume flow measurements where the line condition volumetric flow is converted to the standard volume flow (at 15degC and 1 bar) using either PTZ or density (though you still need the line temperature and line pressure readings). Incidently, the Solartron site also shows another flow device, the K-bar which is an alternative to the Annubar.