Abstract. An interlaboratory comparison (EUROMET Project 852) of four primary standard gas provers was carried out between the Italian and Swiss NMIs (IMGC/INRIM and METAS) from 0.3 m³/h to 25 m³/h using ambient air. The transfer standard was a G25 rotary piston meter. The comparison showed a full compatibility of all four provers above 4 m³/h, whereas at lower flows some discrepancies occurred, whose causes are discussed. The results generally confirm the improved accuracy of the large piston provers pioneered by both laboratories, if compared to bell provers, a couple of which were also tested.

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