Buffer waveguides for flow measurement in hot fluids

To obtain reliable ultrasonic flow measurements in the 200-300 degrees C range continuously, with possible excursions to 450 degrees C, buffer waveguides can be used to separate the piezoelement from the hot fluid. It is important that the buffer waveguides do not corrupt the signals. Until recently, this goal has been elusive because of dispersion, multipaths. mode conversion and beam spread (diffraction loss) in the buffer(s). In this paper, we describe two buffer constructions that avoid these problems. The buffers can be made of virtually any standard engineering material, provided the material is compatible with the pipe or fluid. One design, primarily intended for clamp-on applications, guides shear waves at oblique incidence relative to the radiating face. The other design, primarily intended for wetted applications, guides compressional waves at normal incidence to the radiating face. Once the piezoelement is far enough from the heat, it becomes an easy matter to enclose it in an explosion-proof housing. That housing can be made separable and removable from the clamped-on or permanently installed buffer. This removability feature simplifies transducer repair. recalibration or replacement in the field. Laboratory and field test results are presented for liquid and gas flowmeter applications in the 200-300 degrees C range. (C) 1998 Elsevier Science B.V.