The rotary positive displacement (RPD) reference gas meters GM4 and GM16 meet the highest demands on reliable and accurate reference gas flow measurement down to 0.01 m³/h. Due to their special, patent-pending impeller design pulsation free operation is realized. They reach an excellent measurement accuracy of 0.5 % MV + 0.005 % EV. Both versions are suited for operating pressures up to 6 bar gauge pressure. As the flow measurement is continuous, absolutely pulsation free and highly accurate, this RPD reference meters are the gold standard for low flow reference measurement.