Abstract
Oscillation type (vibrating tube) density meters are widely used for quality control and custody transfer in petrochemical processes. In service, corrosion and wear of the tube wall change the effective mass of the resonator. The measured density drifts, and the drift accumulates with time. A calibration methodology for process oscillation type density meters is used, traceable to the national primary density standard of the National Institute of Metrology (NIM) of China. The analytical relationship between the oscillation period and the liquid density was derived and extended with temperature and pressure correction terms, and a complete calibration system was established to reproduce pressure and temperature conditions. For three meters of two types, in service from lightly to severely corrosive duty, out of date coefficients produced a density error of 3.35 kg/m3 at the density of water, where the internal corrosion of the tube has its greatest effect. After a full coefficient calibration the residual at the reference liquids fell to 0.01–0.10 kg/m3, with an expanded uncertainty of 0.1 kg/m3 (k = 2). The correction model and the traceable procedure therefore restore the required accuracy. Periodic, traceable full coefficient calibration is essential to correct corrosion induced drift, and preliminary full coefficient calibration intervals of 6 to 36 months follow from those instruments according to service severity.