High Precision Density Measurement with Coriolis Mass Owmeters

Today Coriolis meters offer the best performance in mass flow measurement. For those applications, where accuracy, low pressure drop and good cleanability is needed, the single straight tube is the best alternative. The tubes oscillate in resonance and the resonance frequency provides information about fluid's density, which is besides mass flow also an important parameter for quality and process control. A shortcoming of Coriolis mass flowmeters for density measurement is the lack of a sophisticated compensation method taking into account the dependency of the resonance frequency on temperature, flowrate, pressure, viscosity etc. In fact several heuristic methods are in use to achieve good solutions to correct the density readings. However, these methods are complex, device specific and 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 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 taking into account the resonance frequency