Model-based density measurement with Coriolis flowmeter

Today, Coriolis meters offer the best performance in the mass-flow measurement. For those applications where a high accuracy and a low pressure drop together with the easy clearance conditions are needed, the single oscillating straight pipe will be the best choice. The tube oscillates in resonance, and the resonance frequency encodes the information about the density of the fluid, which is, together with mass flow, an important parameter for the quality and process control. A shortcoming of the density measurement with today´s Coriolis mass flowmeters (CMFs) is the lack of the sophisticated compensation schemes that take into account the influence of the temperature, flowrate, pressure, viscosity, etc., on the resonance frequency. Nevertheless, several heuristic methods to correct the density readings are in use and lead to satisfying solutions. However, these methods are complex, device specific, and rather expensive. In this paper, a method is presented to overcome these drawbacks. It is based on a dynamic model of the mass flowmeter and allows for the parameter identification of the measuring tube during operation. To be more specific, the effective spring constant of the oscillating tube is detected online. Using this result, the vibrating mass and, subsequently, the density of the fluid are calculated by taking into account the resonance frequency