TABLE OF CONTENTS Lawrence C. Lynnworth
Lynnworth Technical Services 77 Graymore Road, Waltham Massachusetts 02451-2201 USA www.lynnworthtechnicalservices.com
Waveguide sensors and buffers have been applied to measuring three of the four principal measurands of interest in industrial process control, namely, liquid level, flow, and temperature. Buffers are used when the measurand is at high temperature, which sometimes occurs in NDE/NDT situations. Waveguide sensors can also sense liquid density and viscosity. Problems include isolating the sought measurand from interfering variables, and eliminating acoustic or ultrasonic noise conveyed by a pipe or other structural element that acts as an unwanted waveguide, channeling interference to a transducer and reducing the signal to noise ratio at the transducer.
This chapter is mainly concerned with practical aspects of using guided waves as part of acoustic sensing systems. Emphasis is on industrial process control applications. Readers seeking theoretical foundation for the various waves to be discussed below are referred to texts such as [61] and the ultrasonic delay line literature. Numerous pre-1967 innovative solutions to buffering a transducer from high temperature are illustrated in [13]. Additional discussions of these thermal buffers and pre-1989 techniques and applications appeared in [38]. In a sense, as the Curie point or maximum use temperature of transducers increases above today's limits, the need for some of these thermal buffers diminishes. Where laser or other noncontact (emat) means evolve for launching or receiving acoustic/ultrasonic waves in hot inhospitable environments, thermal buffers may become obsolete [53].
My personal introduction to ultrasonic waveguides began in...
