Ultrasonic Transducers

Ultrasonic transducers are used for a variety of instrumentation and control applications, most commonly for flow measurement, non-destructive inspection of materials, and level measurement.

In general, an ultrasonic transducer is any device capable of converting electric pulses into mechanical energy, producing sound at frequencies above hearing capability. The frequency selection will change with the application. For flow measurement, frequencies can range from 30 kHz to 5 MHz. In level measurement, they can be expected to be lower, usually between 30 kHz and 200 kHz. For inspection equipment, such as thickness gauges and flaw detectors, frequencies can cover larger frequency ranges, but typically are much higher than those seen in the other instruments.

An ultrasonic time-of-flight flowmeter system uses transducers in pairs to measure fluid flow in a conduit. Each transducer includes a piezoelectric crystal, which converts electrical pulses into mechanical acoustic pulses in the ultrasonic frequency range, and vice-versa. It operates alternately as transmitter and receiver. The transducer pair is positioned such that at one moment an acoustic pulse originating from one transducer is traveling downstream of the flow to the other transducer, and at another moment, the traveling acoustic pulse is traveling in the reverse direction, or upstream of the flow. The difference between the downstream and upstream traveling times of the acoustic pulses is proportional to the flow rate of the medium being measured.

Basic design

A typical transducer for an ultrasonic timeof- flight flowmeter system consists of a piezoelectric crystal, a housing, an impedance matcher, and a backing. Its design is intended to maximize energy transfer into the flow medium while eliminating echoes, damping unwanted ringing in the structure, and broadening the transducer bandwidth.

To quantify the damping of a piezoelectric disc, M.G. Silk in his book on ultrasonic transducers proposes:

W = (ZD-ZA)(ZD-ZB)/(ZD+ZA)(ZD+ZB)

Where:

W = square root of the trapped energy after one oscillation, or amplitude ratio of cycle n to cycle n-1

ZD = acoustic impedance of the piezoelectric crystal disc

ZA = acoustic impedance of the impedance matcher

ZB = acoustic impedance of the backing

Depending on the selection of practical materials, a transducer may be designed to fall into one of three categories:

• Narrow bandwidth: With piezoelectric disc ringing for a long time, when W > 0.75
• Moderate bandwidth: Acceptable for much ultrasonic work, according to Silk, when W~0.3
• Broadband: With very short (broadband) signal, when W < 0.1

Transducer applications:

For a representative flare gas application (see illustration), a quarterwave matching technique can be used with all transducer components housed inside a metal shell to protect them from a corrosive gas medium. Because the metal window through which the sound emits is very small compared to the 100 kHz wavelength, the loss of energy going through it is also small.

For typical liquid application, the piezoelectric crystal is bonded to the metal housing without any impedance matching layer.

For more on ultrasonic measurement, see L. C. Lynnworth, “Ultrasonic Measurements for Process Control” in Theory, Techniques, Applications, pp123-126, Academy Press, (1989); and M. G. Silk, Ultrasonic Transducers for Nondestructive Testing, Adam Hilger Ltd., Bristol UK (1984).

Toan Nguyen (toan.h.nguyen@ge.com) is engineering technical leader, flow center of excellence, transducer group, and Christopher Frail (christopher. frail@ge.com) is product marketing manager, flow technologies, GE Sensing, Billerica, MA; www.gesensing.com