A single port Pitot tube includes an L-shaped tube measuring impact pressure.  This tube is inserted into the flowstream, with the opening facing directly into the flow.  Another tube measuring static pressure has an opening parallel to the direction of flow.  Flowrate is proportional to the difference between impact pressure and static pressure.

A multiport averaging Pitot tube has multiple ports to measure impact pressure and static pressure at different points.  The DP transmitter computes flowrate by taking the average of the differences in pressure readings at different points.

Some companies such as Emerson Rosemount and Veris have introduced proprietary versions of the averaging Pitot tube.  Emerson Rosemount’s proprietary version is called the Annubar, and it was formerly sold by Dieterich Standard, now part of Emerson Process Management.  Veris’ averaging Pitot tube is called the Verabar.