GE PT878GC Manual De Usuario

April 2004

All ultrasonic thickness gauging involves timing the round trip of 
a sound pulse in a test material. Because solid metal has an 
acoustic impedance that differs from that of gases, liquids, or 
corrosion products such as scale or rust, the sound pulse will 
reflect from the far surface of the remaining metal. The test 
instrument is programmed with the velocity of sound in the test 
material, and computes the wall thickness from the simple 
formula 

                        Distance = Velocity 

× Time

Single element transducers use one element as both transmitter 
and receiver. Dual element transducers incorporate separate 
transmitting and receiving elements. These elements are mounted 
on delay lines that are usually cut at an angle to the horizontal 
plane (the roof angle), so that the transmitting and receiving beam 
paths cross beneath the surface of the test piece. This crossed-
beam design of duals provides a pseudo-focussing effect that 
optimizes measurement of minimum wall thickness in corrosion 
applications. Duals are more sensitive than single element 
transducers to echoes from the base of pits that represent 
minimum remaining wall thickness. Also, duals may often be 
used more effectively on rough outside surfaces. Couplant 
trapped in pockets on rough sound entry surfaces can produce 
long, ringing interface echoes that interfere with the near surface 
resolution of single element transducers. With a dual, the receiver 
element is unlikely to pick up this false echo. Finally, duals may 
be designed for high temperature measurements that would 
damage single element contact transducers.