A mechanism was proposed to explain the error of Coriolis mass flowmeter in testing different density.The results showed that the upper segment of tube in U shape had velocity gradient when torsion triggered by Coriolis force, thus produced an adverse torque to reduce time difference and finally created the negative calibration errors.The mathematical model was built to precisely calculate the magnitude of the effect caused by different density and other factors such as the dimension of sensor.A simplified way to choose the value of density for calculation was given due to the limit of calibration method and experiment condition.In addition, it was proved that the error was directly proportional to density, but not related to flowrate.The application of this theory could improve the accuracy grade of Coriolis mass flowmeter in measuring compressed natural gas(CNG) and pave the way to the correction algorithm in the future.
