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The Evolution of Acoustic Inspection Based on Acoustic Pulse Reflectometry

June 13, 2011
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Considering the many uses of APR in the academic world, it was only a matter of time until it was discovered by the industrial world.



Shown is a schematic depiction of (a) the UT sound wave, perpendicular to the tube wall vs. (b) the APR sound wave, parallel to the tube wall. UT inspects one point at a time, whereas APR scans the entire tube with one measurement. Source: AcousticEye



Time domain reflectometry (TDR) is nothing new. Google it, and you get 117,000 hits. TDR involves sending a pulse of some type into a medium and measuring any reflections created somewhere in this medium. It has been used to examine electric cables, waveguides and optical fibers. Acoustic pulse reflectometry (APR) is a particular case of TDR, where the pulse is an acoustic signal, usually injected into one type of tube or another. APR is not as well known as TDR in general. Googling it gets all of 627 hits.

APR has been used in the past for some applications that might appear unusual. For example, one major application has been seismic explorations. This, in fact, is not so surprising. Drilling deeply into the ground is a very expensive operation. If important information about ground strata can be gleaned from carrying out several explosions on the surface and measuring the waves reflected from changes in rock and ground density, so much the better. This type of APR has been around for a long time-since the 1960s at the very least.

Another application of APR has been to examine the human airways: the nose, mouth and trachea. These are much shorter than the distances to layers deep in the ground; however, there is understandably a great interest in probing these tube systems noninvasively, to the benefit of doctors and patients alike. This also is a rather old application of APR, with papers appearing on the subject since the 1970s.

Finally, during the past few decades, APR has been used quite extensively for the study of musical instruments. The study of wind instruments poses problems that are both similar and different to examining human airways: testing them by invasive means is qualitative and inconvenient, due to bends and valves. Thus, the idea of probing their bore by acoustic means is very appealing.

In this context, APR is most often used for “bore reconstruction,” for example, reconstructing the geometrical structure of the bore from the reflected acoustic signals. This has been used for probing delicate historical instruments, or even for quality control in the manufacture of modern instruments.

In the same context, several in-depth studies have been the characterization of the reflections caused by through-wall holes. Though the main motivation might have been to detect leaks in historical musical instruments, these methods serve equally well for through-holes in heat exchanger tubing.

In essence, the basic idea behind APR also has been applied to the NDT world: ultrasound testing (UT) is based on a very similar principle. In UT, a pulse of high-frequency acoustic energy is sent into the object being inspected, rather than into space, and reflections are created wherever there is a change in the characteristic impedance. In simple words, the reflections are created by any nonuniformity in the object, which is often an indication of a fault or an edge.

The capabilities of these techniques in general are strongly related to the typical wavelengths they employ. Ultrasound, for example, has very short wavelengths, enabling it to distinguish slight variations in tube wall thickness. However, it propagates poorly in most media, severely limiting its range. It is therefore used by focusing the acoustic wave into a narrow beam perpendicular to the object’s surface. Thus, when used to examine pipes or tubes, it can only inspect one small spot at a time, depending on the beam width. UT has been adapted to perform tube inspection through the use of a rotating beam pulled slowly through the tube.

APR, however similar in principle to UT, employs much lower frequencies, with the result that it can propagate to relatively long distances, but with lower resolution. Most commonly it is used to inspect various types of tubular systems, but instead of propagating the acoustic energy into the tube wall, as in UT, it is propagated into the medium filling the tubes-air, or less frequently, water.

Naturally, the first question that comes to mind is: how can APR be used to inspect the tube walls if it propagates the acoustic energy in the air enclosed by the tube, rather than the tube wall itself?

The answer is quite simple. If a pulse is injected into a perfectly uniform tube, no reflections will be created until the pulse hits the end of the tube. However, any changes in cross section, whether intentional or caused by imperfections on the inner surface of the tube, will create reflections. These can then be recorded and analyzed.

Considering the many uses of APR in the academic world, it seemed only a matter of time until it was discovered by the industrial world. The question is, of course, how does it measure up?



This APR inspection tool is composed of a handheld probe and an electronics box with an embedded computer for storage, analysis and display. Source: AcousticEye

From Science to the Marketplace

Bringing a viable scientific principle out of the academic lab into the field is always a challenge, certainly so in the crowded field of heat exchanger tube inspection. Several different issues, not all of them technical, must be addressed at the same time for such an endeavor to succeed:

1. Performance

2. Usability

3. Standardization

4. Market acceptance

As APR is a tool gaining acceptance in the tube inspection market, it is interesting to examine how its implementation deals with the above issues.



  • Performance. Clearly, no technology is a panacea. The relative advantages of APR must be identified in comparison to other technologies, along with the drawbacks, to encourage proper use and avoid unrealistic expectations. Looking at these points in detail:

  • Internal vs. external defects. As discussed, the acoustic pulse utilized in APR propagates in the medium inside the tube. Since it detects changes on the internal diameter (ID) only, it cannot detect defects on the outer diameter (OD). Through-holes, however, present a “short circuit” to the external atmosphere, therefore creating strong reflections even when they are quite small, from approximately 0.5 millimeter in diameter. Such “pinholes” are quite difficult to detect using other means.

  • Speed of inspection. APR does not require traversing the tubes with a physical probe, sending a sound wave instead. As such, speed of inspection can be increased considerably, with no danger of probes getting stuck or becoming worn. Inspection is typically several seconds per tube, regardless of tube length.

  • Tube material and configuration. Another advantage of propagating the acoustic wave in the medium filling the tubes is that it is not influenced by tube wall material, be it ferromagnetic or not, or even graphite or any other nonmetallic material. It also can propagate through u-bends, regardless of the tightness of the bend.

  • Automatic interpretation. APR signatures created by various types of defects are well defined, enabling them to be detected automatically by appropriately designed software. Though there have been some attempts to carry out such automatic detection on eddy current signals, they have not yet achieved widespread acceptance.

  • Cleanliness. The ability of APR to detect defects is subject to tube cleanliness. Pits or holes covered by deposits will not affect the acoustic wave and will therefore not be detected. On the other hand, APR is very sensitive to blockages, even when they are very small. Various types of fouling, such as sludge, sedimentation and scaling will show up as a multitude of blockages. In this case, APR can be used to quantify fouling, giving extremely useful information on the current status of the tubes and the successfulness of the cleaning process. This is a “blue ocean” application: one for which no other solution existed previously.

  • Usability. Aside from the performance issue, in an industry in which thousands of tubes must be inspected in as short a time as possible, usability is a major and multifaceted issue. Taking a fresh view from the ground up has enabled APR manufacturers coming out with products to innovate in more than the basic technology. An example is tube sheet mapping: the operator takes a photo of the tube sheet on-site, loading it into the software, which then detects and numbers all the tube-ends automatically. This process takes seconds, avoiding the need for manual mapping or procurement of hard to obtain manufacturer schematics.

  • Furthermore, the software presents this map on screen during inspection, indicating the next tube to be inspected, thus enabling the technician to synchronize his location on the tube sheet with the software. Any contribution to speeding up the inspection process is important-saving several seconds per tube can add up to a savings of several hours for an entire job.

  • In a similar vein, automatic flagging of defects also can contribute to making the inspection process more accurate and effective. Software that can run through measurements of an entire heat exchanger, flag the potential defects and present them to the analyst, makes the process more objective, faster, and far less fatiguing for the operator.

  • Standardization and market acceptance. In the regulated world of NDT, standardization and market acceptance go hand in hand. In fact, this is a typical chicken and egg problem. Many potential users will hesitate to adopt a technology that does not appear in the ASME/ASTM/ASNT standards, and on the other hand, these organizations are hesitant in initiating a standardization process for a technology that is not in widespread use, particularly if it is backed mainly by a single manufacturer, as new technologies often are.



    For the reasons above and many others, the NDT market tends to be a conservative one, so there is probably no fast track to success in this respect. Patience and persistence have proved fruitful, however, and APR is gaining acceptance in traditional applications and more innovative ones such as analysis of tube cleanliness. The heterogeneity of the worldwide NDT market is an aspect that newcomers can take advantage of: current technologies are strongly entrenched in the developed markets, such as the United States and Europe, which are heavily invested in equipment, training and standardized procedures, and therefore often resistant to change. Developing countries are often more receptive to the adoption of newer technologies. APR has been very successful in these markets, arming it with a plethora of proven case studies that are very useful in overcoming resistance in other markets.

    From its long incubation period in the academic world to becoming an off-the-shelf NDT tool, APR has undergone a process of refinement and adaptation readying it for the rigors of industrial use. It is now gaining recognition and acceptance as a useful tool for heat exchanger tube inspection and heat exchanger tube cleanliness verification, with a steady increase in market-share and a good outlook for further expansion into boiler inspection. NDT


  • Tech Tips

  • Acoustic pulse reflectometry (APR) is a particular case of time domain reflectometry, where the pulse is an acoustic signal, usually injected into one type of tube or another.

  • APR, similar in principle to ultrasound testing, employs much lower frequencies, with the result that it can propagate to relatively long distances, but with lower resolution.

  • APR is gaining recognition and acceptance as a useful tool for heat exchanger tube inspection and heat exchanger tube cleanliness verification.

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