Manual No. OM5000
Jan X/04 Ver 1.40
10
Figure 5
Another fundamentally very important electronic control which affects the A-scan appearance is the
receiver gain control. The gain control changes the amplitude scale along the vertical axis of the A-
scan trace. A gain control permits the height of the echos to be either conveniently increased or
decreased.
If a material is somewhat attenuative, it may be necessary to increase the receiver gain until the
echos are readily observable. On the other hand, if the gain is too high for a given material, the
echos may detrimentally saturate the receiver or cause an excessively noisy baseline on the A-scan.
Normally, when a material is not too attenuative or too thick, the A-scan will actually indicate a series
of equally-spaced echos, known as multiples (see Figure 5). These multiples are secondary inter-
reflections of the initial ultrasonic pulse reverberating back and forth between the two surfaces of the
material. The spacing between any two successive multiples represents the echo round-trip time in
the material and, thus, also can be used for gaging thickness (in addition to the initially described
interface to first back surface echo period). The dotted curve across the tips of the echos in Figure
5 shows the rate at which the amplitude of the echo multiples decreases or decays and is related to
the degree of ultrasonic attenuation for a given material. This entire echo pattern of such multiples
is sometimes called a ringing pattern.
4.2.4 Resolving Power and Sensitivity
- One of the performance characteristics always associated
with pulse-echo ultrasonics and A-scan presentations is range resolving power (resolution). It refers
to the instrument and transducer’s ability to clearly separate two sequential echos along the A-scan
trace. Figure 6 shows pairs of completely resolved, partially resolved and unresolved echos.
The resolving power of an instrument is defined as the closest echo separation it can make,
expressed in terms of time (i.e., nanoseconds), or more commonly, in terms of some minimum
specified material thickness. The limiting ultrasonic factor is the pulse width (or pulse length) of the
echos, as inferred in Figures 6a, b & c. Thus, highly damped, high frequency transducers produce
much better resolving power (shorter ultrasonic pulse lengths) than lowly-damped, low-frequency
transducers. It follows that instrumentation with short IPs (main bangs), broadband receiver
amplifiers and high-speed processing circuits have higher resolving power.