Modes of Vibration

There are two primary types of oscillations often associated with vibrational sensors: lateral and torsional. The mode of vibration is important because each type of wave responds differently when immersed in a fluid. A lateral oscillation generates a P wave, or sound wave, while torsional oscillation generates an S wave, or shear wave.

Each wave type has distinct properties to consider when measuring viscosity. 

Lateral Oscillation

The presence of gas bubbles or debris in the fluid can have disastrous effects on the sound waves generated by laterally vibrating sensors. Anything in the sound path, including bubbles, solid matter or reflective surfaces, will act as a damper.

Torsional Oscillation The microscopic movement of the sensor back and forth at high frequency produces a pure shear wave. This technique is unaffected by any debris or suspended solids, bubbles, or turbulent flows.  Viscosity is a shear measurement so it should be measured under shear conditions. All Hydramotion sensors use a torsional oscillation principle.