The operating principle of the sensor is that of a torsion pendulum. The sensor consists of a stainless steel driver/sensor head, attached to the base plate by means of a tubular torsion spring. In the head, one pair of piezo elements (driver piezos) actuates the pendulum at its torsional resonance frequency, while another pair of receiver piezos detects the actual movement of the head. In a low viscosity medium, like air, the resonance frequency is in the range of 1600 Hz. In a high viscosity medium the movement of the head is damped by the liquid.
Consequently, the resonance frequency slightly shifts towards lower frequencies, whilst the width of the resonance peak increases, which is a measure for the viscosity.
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