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2.3 PRINCIPLE OF OPERATION
The operating principle of the sensor is that of a torsion pendulum. The sensor consists of a stainless
steel driver / sensor head (1), attached to the base plate (2) by means of a tubular torsion spring (3).
In the head, one pair of piezo elements (driver piezos) (4) actuates the pendulum at its torsional
resonance frequency, while another pair (the receiver piezos) probes 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.
A flow tube (5) is placed around the pendulum, to protect it against mechanical damage.
The flow tube is machined with a smooth surface, as well as with a triplet of oval slits. These slits
increase the refreshing rate between sensor and flow tube.
Figure 2 ViscoSense
®
3 sensor (VS3)
