Page 1177 - The Mechatronics Handbook

Ultrasound flowmeters of the transmission type [Fig. 45.11(a)], which are based on the principle that
the sound transmission speed will be increased by the flow rate of the fluid, are used in all types of clean,
subsonic flows. Doppler flowmeters [Fig. 45.11(b)] rely on echoes from within the fluid, and are thus
only useful in dirty flows that carry suspended particles or turbulent flows that produce bubbles. Ultra-
sound flowmeters are nonintrusive devices, which can often be retrofitted to existing duct or pipe systems.
Vortex shedding flowmeters (Fig. 45.12) introduce a shedding body into the flow to cause production
(shedding) of vortices. The sound accompanying the production and collapse of the vortices is monitored
and analyzed. The dominant frequency of the sound is indicative of the rate of vortex production and
collapse, and hence an indication of flow rate. Vortex shedding flowmeters are useful in low-velocity,
nonturbulent flows.
Thermal anemometers (Fig. 45.13) are used in low-velocity gas flows with large cross-sectional area,
such as in heating, ventilation, and air conditioning (HVAC) ducts. Convection cooling of the heating
element is related to flow rate. The flow rate measurement is based either on the current required to
maintain a constant temperature in the heating element, or alternatively on the change in temperature
when the current is held constant.
Electromagnetic flowmeters (Fig. 45.14) are useful for slow moving flows of liquids, sludges, or slurries.
The flow material must support electrical conduction between the electrodes, and so in some cases it is
necessary to ionize the flow upstream from the measurement point in order to use an electromagnetic
flowmeter.
Variable-area in-line flowmeters (Fig. 45.15), or rotameters, are sometimes referred to as sight gauges
because they provide a visible indication of the flow rate. These devices, when fitted with proximity sensors
(such as capacitive pickups) that sense the presence of the float, can be used in on–off control applications.

Liquid Level Transducers

Liquid-level measurements are relatively straightforward, and the transducers fall into the categories of
contact or noncontact. Measurements may be continuous, in which the liquid level is monitored contin-
uously throughout its operating range, or point, in which the liquid level is determined to be above or
below some predetermined level.
The contact transducers encountered most frequently are:
• Float
• Hydrostatic pressure
• Electrical capacitance
• Ultrasound
The noncontact transducers encountered most frequently are:

• Capacitive proximity sensors
• Ultrasound
• Radio frequency
• Electro-optical
Float-type liquid level transducers are available in a wide variety of configurations for both continuous
and point measurements. One possible configuration is depicted in Fig. 45.16 for continuous measure-
ment and for both single- and dual-point measurements.
Hydrostatic pressure liquid level transducers may be used in either vented or pressurized applications
(Fig. 45.17). In either case the differential pressure is directly proportional to the weight of the liquid
column, since the differential pressure transducer accounts for surface pressure.
Capacitance probes [Fig. 45.18(a)] are widely used in liquid level measurements. It is possible, when
the tank walls are metal, to use a single bare or insulated metal rod as one capacitor plate and the tank
walls as the other. More frequently, capacitance probes consist of a metal rod within a concentric cylinder

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