Shear Rate Sensor

In an agitator mixer tank, particles move around the shaft with different velocities depending on their radial positions. Interacting between themselves they experience shear force that may affect sensitive components of the fluid such as proteins. The effect is characterized by the shear rate, \dot γ.

The shear rate is the rate of change in velocity at which one layer of fluid passes over an adjacent layer. Knowing difference in velocities Δu  at two points along the line perpendicular to the flow direction (e.g. along the radius in the mixer with a cylindrical vessel) and the distance between these points, Δr, the shear rate is

\dot γ = \frac {Δu} {Δr}

The velocity difference Δu is measured by a pair of Drag Force Flow (DFF) sensors placed along the radial direction and measuring the difference of flow forces they experience. The measurement concept is illustrated in the diagram.

The principles of flow velocity measurement by a DFF sensor are described here.