Gas metering is a big part of what we offer customers. One product that has proven to be very successful is the Honeywell Smart Multivariable (SMV) Transmitter.  It is an industry leader – there are no others on the market that work as well.  It measures the gas pressure & temperature and measures a differential pressure across a primary element.  The primary element can be an orifice plate or a Preso Ellipse pitot tube.  It creates a differential pressure that we measure and relate to flow (the square root of the pressure drop is proportional to the flow rate).  The SMV calculates a corrected flow by accounting for the compressibility of the gas along with the pressure and temperature that exist in the pipe.  It is quite an involved calculation that is pre-loaded into the transmitter and set up using a software wizard.  Please keep it in mind for gas metering applications – it is an affordable alternative.  For under $4000 you can measure big flows accurately with easy installation.