A flow nozzle or flow venturi (differential head meter) with a multivariable transmitter (d/p, pressure and temperature) would provide a mass flow with a turndown of 5:1. You could use just a d/p transmitter to get volumetric flow. In either case, the sensing lines need to be purged with an inert gas such as nitrogen to keep particulates out of the lines. I favor a flow nozzle or venturi over an orifice plate to avoid the problems of orifice edge wear and buildup of particulates on the plate. Also, the straight run requirements are less and the turndown (rangeability) and accuracy is slightly better.
 
If much greater flow turndown (rangeability) or accuracy is needed, you could go to a Coriolis flow meter, but this meter is much more expensive than a differential head meter for your line size. Additionally, a Coriolis meter requires relatively high gas pressures (high gas densities) to perform properly and the pressure drop across the meter is much larger than the pressure drop across a flow nozzle or venturi tube.