Abstract: A resonator (1) is vibrating close to its resonance frequency. The vibration is excited by one or a first transducer (2) connected to an oscillator (11). The vibration is measured by the transducer (2) or a second transducer (3) and stabilized by a phase-locked feedback loop comprising a phase sensitive detector (33), a feedback controller (36), a phase shifter (17) and means (15) to evaluate the measured frequencies. In order to measure the damping of the resonator (1), the phase in the phase shifter (17) is alternately set to two different values. The difference between the frequencies corresponding to these two phase values is a measure for the damping of the system. One or more switches (19, 28, 27) and a gate generator (16) make sure that excitation and measurement do not occur at the same time. Thereby, any cross-talk between driving and sensing transducers is completely eliminated. Also, a single transducer can be used to both excite and measure the vibration.

Abstract: A resonator in the form of a cylindrical body is immersed in a fluid to be measured and excited by a piezoelectric transducer in the vicinity of its natural frequency. The vibration frequency is stabilized by a feedback loop. In that feedback loop a phase shift is introduced which can be switched between to discrete values. The resulting difference in frequency of oscillation of the resonator is proportional to the damping ratio of the resonator and is, therefore, a measure for the viscosity of the fluid. The natural frequency of the piezoelectric transducer is substantially higher than the natural frequency of the resonator, and the oscillation of the resonator is mechanically isolated from the housing by an inertial mass - spring arrangement with low natural frequency.