What I didn't mention is that one of those (#1 or #2) should cancel itself out on the backside.
Here's a few more links:
http://www.govictory.com/How%20To/FL...FLOWBENCH.html
http://sanyodb.colle.co.jp/product_d...pdf/e_spec.pdf
http://www.specialinstruments.com/pdf_e/28-29.pdf
It then becomes apparent that since air is a compressible fluid, the measurement may do well to include this as a factor.