The gas flow bells BP100 and BP250 are primary standards for flow calibration with air. The cylindrical, closed top hollow body (bell cylinder) sinks to generate a flow in a filled with barrier liquid jar and displaces the trapped air volume. Depending on the sinking speed of the bell, a volume flow is generated, which in turn can be attributed to the SI units length and time. The system is based on a proven technology that is used worldwide in national metrology institutes for the calibration of nozzles and other standards. The bells BP100 and BP250 are of identical design, but differ in size and thus in flow. In the PTB variant, measurement uncertainties <0.06% (k=2) are possible, making Ehrler Prüftechnik's gas measuring bells one of the most accurate calibration systems in the world.