Do have you the data ( mass or area ) stored, at moment of the calcul or you should calculate it ? I think it's the same things.

You can find a discussion on the subject here:

http://www.cfd-online.com/Forums/main/8151-area-weighted-mass-weighted-average.html

http://www.cfd-online.com/Forums/main/8151-area-weighted-mass-weighted-average.html

Volumetric flow should be area-weighted and then the mass flow is on top of this, not instead of this. Just get the profile and integrate it. Sometimes the test points are selected so as to represent equal areas, in which case, you're already there. See also these test codes:  PTC-19.5, MFC-2M, MFC-3M, MFC-8M, PFM-143, STD-146.

Mass-weighted average should be used for any extensive property, correspondingly area-weighted average should be used for intensive property. This is by definition.