handbook of modern sensors

11.9 Drag Force Flow Sensors 377
With a no-flow state, the tube vibrates identically at its inlet and outlet sides with
the sine-wave motions with the zero phase shift between them. During flow, the tube
twists in response to the flow, and the inlet and outlet sides vibrate differently with a
phase shift between them (Fig. 11.17C). The main disadvantage of the Coriolis sensor
its relatively high initial cost. However, the versatility of Coriolis sensors in handling
multiple fluids makes them very useful for plants where the flow of multiple fluid
types must be measured. There are also an increasing number of the gas applications
for the Coriolis meters.
11.9 Drag Force Flow Sensors
When fluid motion is sporadic, multidirectional, and turbulent, a drag force flow
sensor may be quite efficient. Application of such flowmeters include environmental
monitoring, meteorology, hydrology, and maritime studies to measure the speed of
air or water flow and turbulence close to surface [11]. In the flowmeter, a solid object
known as a drag element or target is exposed to the flow of fluid. The force exerted
by the fluid on the drag element is measured and converted to a value for speed of
flow. An important advantage of the drag sensor is that it can be made to generate
a measurement of flow in two dimensions, or even in three dimensions, as well as
of flow speed. To implement this feature, the drag element must be symmetrical in
the appropriate number of dimensions. These flowmeters have been used by industry,
utilities, aerospace, and research laboratories to measure the flow of unidirectional
and bidirectional liquids (including cryogenic), gases, and steam (both saturated and
superheated) for almost half a century.
The operation of the sensor is based on strain measurement of deformation of an
elastic rubber cantilever, to which a force is applied by a spherical symmetrical drag
element (Fig. 11.18). However, an ideal drag element is a flat disk [12], because this
configuration gives a drag coefficient independent of the flow rate. Using a spherical
drag element, which departs from the ideal of a flat disk, the drag coefficient may
vary with flow speed, and, therefore, the gauge must be calibrated and optimized for
the conditions of intended use. The strain measurement can be performed with strain
gauges that should be physically protected from interaction with moving fluids.
Fig. 11.18. Drag force sensor.