How Ultrasonic Testing Works

The concept of utilizing radio or sound waves in order to determine distance began with the invention of sonar and radar during the Second World War.  Today, multiple technologies incorporate this concept in order perform other functions such as, x-ray imaging, medical sonography, flow monitoring, weld inspection, flaw detection and for our purpose – ultrasonic thickness gauging.

While ultrasonic thickness testing is relatively new in terms of widespread use, the technology itself has been around for some time.  The concept of using ultrasonic waves to determine internal structure and thickness were introduced in the early 1920s.  Then in 1940, Professor Floyd Firestone of the University of Michigan developed sonar as an aid to the Allied war effort.  In the early 1950s, ultrasonic material testing was further advanced by the United States Navy in order to detect flaws in the hulls of ships and planes.  Ultrasound then expanded throughout a wider range of applications in order to determine preventative measures and ensure the reliability of mechanical equipment in aerospace, aviation, nuclear power, and other highly critical industries.

Throughout the 1970s and 1980s, innovations to the ultrasonic thickness gauge made it battery powered and reduced it in size to a hand held device, which in turn further extended its use.  Data logging, on-screen waveform verification, greater accuracy, ease of use, the ability to test various materials, and more advanced functions such as the ability negate the thickness of paint or coating, or compensate for temperature, again extended its range as a diagnostic tool.  While more limited devices are available in terms of quality and features, CorrView only employs the best available equipment, which we consider to be the Olympus / Panametrics 38DL Plus line of instruments.

In all of the cases where we have been retained to confirm or refute questionable wall thickness measurements produced by another testing service, it has been the use of low quality instruments having no waveform display and very limited features, combined by instrument operators of limited knowledge and ability, that have produced the errors in question.

• Ultrasonic Theory

Ultrasonic testing or UT as it is called, is the practice of determining the wall thickness of a material by sending a high frequency sound wave through one side of the subject material and measuring the amount of time it takes for the sound wave to pass through and then reflect back to the same beginning location – typically its exterior or accessible surface.

With the frequency of the sound signal known, as well as the velocity by which sound moves through the material tested, the time required for traveling through the test subject will then produce a highly accurate wall thickness measurement.

A visual representation of this process is shown at left, and below the mathematical formula.

Thickness = Velocity x Time

or in more detail

Thickness (in.) = Sound Velocity (in./μs) x Travel Time (μs) / 2

• Method of Use

As the first step to every investigation, the instrument is calibrated to the material of interest through different methods.  This may be by a theoretical calibration to a known value (0.1830 in./μs. for copper for example), but is more accurate using a certified step calibration block for that same material.   Where no reference standard is available or where we can gain greater accuracy by calibrating to a sample of the pipe itself, such as is the case for cast iron and ductile iron pipe, we then utilize that method.  Sound wave travel time is measured in microseconds (μs.), with that information then internally processed by the instrument into a wall thickness measurement accurate to three decimal places.

Equally important to the instrument itself is the selection of probes, with dual element probes more suited for identifying the deeper pitting of condenser water systems or galvanized steel pipe installed for pre-action fire sprinkler systems.  Higher frequencies offer greater resolution and accuracy, although generally unsuitable for thicker and more porous materials such as older cast iron pipe.  For dense materials such as lead, more porous materials such as cast iron, or where wall thickness is high such as a 48 in. Class 350 cast iron tee at 1.25 in., a very low frequency probe is required.

Accuracy in measurement is typically +/- 0.001 in. for most materials, and is entirely dependent upon the instrumentation as well as operator skill.  Properly performed, ultrasound is equally accurate to a digital dial caliper measurement – as we have proven to many skeptical clients, and especially those disagreeing with our findings.

A waveform view of the sound signal is absolutely critical to a reliable thickness measurement since it allows the UT technician to instantly verify a true and accurate wall thickness measurement.  Having a waveform to indicate the intensity, quality, and other important aspects of the sound signal, properly interpreted, eliminates errors in thickness measurement.  A waveform display is also critical for negating a paint or coating using echo-to-echo.  With the waveform also saved with the thickness data, it can be retrieved at any time to verify a questionable thickness value.

In both examples below, we show a waveform picture of the same 2 in. diameter section of schedule 40 chill water pipe having some mild exterior surface rust.  Without adequately removing the surface rust the sound signal is impeded, which results in an inaccurate wall thickness value of 0.104 in., as shown by the left side waveform.  For any investigator using a low cost instrument, only the number 0.104 in. would appear – providing no indication that an error has occurred.  Many other factors will produce the same inaccuracy including too high or low signal gain, metal delamination, or a paint or coating delamination.

The right side waveform is correct, and exhibits steep defined peaks of the correct amplitude and a sharp back wall reflection.  Successive small echos represent the signal attenuating out as it bounces between front and back wall boundaries.  The wall thickness is accurate at 0.148 in.

Of course, a significant difference exists, with the falsely low measurement suggesting the pipe nearing the need for replacement where, having an original wall thickness of 0.154 in. with very little pipe wall loss, long service life still remains.  Once confirmed as accurate, most better quality instruments allow data logging and downloading – thereby eliminating the always present possibility of human error while transcribing thickness values.

• Expanding Applications

Quality control professionals utilize ultrasound in order to inspect a wide range of products for multiple reasons.  For those in the HVAC, real estate and property management and maintenance fields, the ability to measure the wall thickness of a pipe or tank when only one side is exposed becomes invaluable.  Whether to diagnose a problem, define corrosion activity, establish age related conditions, verify pipe schedule, complete due diligence for an acquisition, or any of its other many uses, ultrasound is almost always the most cost-efficient diagnostic tool.

One example of quality control is measuring the shell wall thickness of a water storage tank prior to installation in order to confirm that all design specifications have been met.  Another example, is using ultrasound to check the thickness of paint on Ferrari’s new cars because the paint can only be accessed from one side and it cannot be measured using a caliper.  In its more advanced form known as flaw detection, ultrasound can verify the integrity of a welded joint; eliminating the extended safety concerns of x-ray analysis.  These examples are only a few of the many very beneficial applications of ultrasound which can be applied to quality control.

Predictive inspections are the most critical and frequent applications of ultrasonic testing.  Components and materials are constantly deteriorated by abrasion, corrosion, erosion, stress fractures and flaws caused by manufacturing.  This affects system components and mechanical equipment which then need to be evaluated and maintained in order to reduce further cost and provide uninterrupted service to important buildings and facilities.

For example, a 40 story high rise office building has condenser water piping running from the roof to the basement.  In order to identify weakness caused by corrosion prior to a leak or failure (which can cost millions in lost productivity, water damage, and legal fees) building engineering has an ultrasonic evaluation of the piping performed.  The result is new knowledge of some weakness at its smallest diameter threaded fittings due to a high 7.5 MPY corrosion condition.  In response, the building addresses areas of weakness through pipe replacement and improves their chemical treatment program.  As a result of their ultrasonic investigation, and with the appropriate corrective measure quickly instituted, they have prevented the potential for millions of dollars in losses and helped to extend the service life for the rest of the system.  More information on report content is available Here.

This is just one example of how applying ultrasound can be proactive in improving the reliability of operations and preventing potential catastrophe at a fraction of the cost.  This approach can be applied to any mechanical system and its piping components as deemed necessary.

© Copyright 2024 – William P. Duncan, CorrView International, LLC