Simultaneous Measurements at U-tube Density Sensors in Fundamental and Harmonic Oscillation
Edwin Krasser, Helmuth Senn · 2007
An oscillating U-tube varies its resonant frequency depending on its mass and its spring constant. The oscillating mass is the sum of the mass of the U-tube and the mass of the liquid sample. As the volume of the U-tube is fixed, the density of the liquid sample relates to the measured resonant frequency. A feedback loop built by an analog electronic amplifier compensates the mechanical losses and controls the amplitude. This leads to a stable oscillation at the resonant frequency. New, adaptive algorithms implemented on a digital signal processor (DSP) improve this analog excitation system. They allow to operate the U-tube density sensor in its fundamental and first harmonic mode simultaneously. Moreover, they lead to a faster and more accurate determination of the resonant frequencies in both modes, of the quality factor and consequently of the density of the sample.