Gas Flow Calculation Software

I am looking to purchase some software to calculate gas flows with orifice plates. I need something easy to use and has some reference to a known standard such as ISO 5167-1. I have in the past compared calculation results from three different programs and found the results to be further and further apart as the velocity and differential pressure increases. I must assume this is due to how each program deals with compressability of gases.

A program with built in databases for pipe diameters and gas properties would be best.