Precision Ultrasonic Gauging: Factors That Can Impact ...

Quality ultrasonic thickness gauges can offer highly accurate testing on metals, plastics, and other materials. However, several factors related to the test material, equipment, part geometry, and user skill and care can affect the degree of accuracy achieved in an application. Read on to learn factors that can impact ultrasonic testing results.

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Material-Related Factors

The physical properties of the test material is one factor that affects an ultrasonic thickness gauge’s measurement range and accuracy. This includes both acoustic and geometrical factors.

1. Acoustic Properties of the Test Material

Several conditions found in some engineering materials can limit the accuracy and range of ultrasonic thickness measurements:

• Sound scattering: In cast stainless steel, cast iron, fiberglass, and composite materials, sound energy will scatter from individual grain boundaries in castings or boundaries between fibers and matrix in the fiberglass or composite. Porosity in any material can have the same effect. Make sure to adjust the gauge sensitivity to prevent the detection of these spurious scatter echoes. This compensation can limit the gauge’s ability to detect a valid return echo from the back wall of the material, restricting the measurement range.
• Sound attenuation or absorption: In many polymers like low density plastics and in most types of rubber, sound energy is attenuated very rapidly at the frequencies used for ultrasonic gauging. This attenuation typically increases with temperature. The maximum thickness that can be measured in these materials is often limited by attenuation.
• Velocity variations: An ultrasonic thickness measurement is accurate only to the degree that the material sound velocity is consistent with the gauge's velocity calibration. Some materials exhibit major variations in sound velocity from point to point. This happens in some cast metals due to the changes in grain structure that result from varied cooling rates and the anisotropy of sound velocity with grain structure. Fiberglass can show localized velocity variations due to changes in the resin/fiber ratio. Many plastics and rubbers show a rapid change in sound velocity with temperature, requiring that operators perform the velocity calibration at the same temperature as the measurement.
• Phase reversal or phase distortion: The phase or polarity of a returning echo is determined by the relative acoustic impedance (density × velocity) of the boundary materials. Ultrasonic gauges assume the customary situation where the test piece is backed by air or a liquid, both of which have a lower acoustic impedance than metals, ceramics, or plastics. However, in some specialized cases, such as measurement of glass or plastic liners over metal, or copper cladding over steel, this impedance relationship is reversed and the echo appears phase reversed. To maintain accuracy in this situation, make sure to change the appropriate echo detection polarity. An even more complex situation can occur in anisotropic or in homogeneous materials, such as coarse-grain metal castings or certain composites, where material conditions result in multiple sound paths in the beam area. In these cases, phase distortion can create an echo that is neither positive nor negative. Carefully experiment with reference standards in these cases to determine the effects on measurement accuracy.

2. Physical Properties of the Test Material

The size, shape, and surface finish of the test piece must also be considered to establish the limits of measurement range and accuracy.

• Surface roughness of the test piece: The best measurement accuracy is obtained when both the front and back surfaces of the test piece are smooth. If the contact surface is rough, then the minimum thickness that can be measured will be increased because of sound reverberating in the increased thickness of the couplant layer. Inefficient coupling may reduce the echo amplitude. Also, if either the top or bottom surface of the test piece is rough, it can cause distortion in the returning echo due to the slightly different multiple sound paths seen by the transducer, resulting in measurement inaccuracies.

  For corrosion measurements, loose or flaking scale, rust, corrosion, or dirt on the outside surface of a test piece will interfere with the coupling of sound energy from the transducer into the test material. For this reason, clean any loose debris from the sample with a wire brush or file before measuring. Generally, performing corrosion measurements through thin layers of rust is possible as long as the rust is smooth and well bonded to the metal below. Keep in mind that some very roughcast or corroded surfaces might need to be filed or sanded smooth to ensure proper sound coupling. You may also need to remove paint if it is flaking off the metal.

• Curvature of the test piece: A related issue is the transducer alignment with the test piece. When measuring on curved surfaces, it is important to place the transducer around the centerline of the part and hold it as steadily as possible on the surface. A spring-loaded V-block holder can be helpful for maintaining this alignment. In general, as the radius of curvature decreases, the size of the transducer should be reduced, and transducer alignment becomes progressively more critical. For very small radii, an immersion approach with a focused transducer is required. In some cases, it helps to use a waveform display as an aid to maintain the best alignment. Also, on curved surfaces it is important to use only enough couplant to obtain a reading. Excess couplant will form a fillet between the transducer and the test surface where sound will reverberate and might create spurious signals that can trigger false readings.
• Taper or eccentricity: If the contact surface and back surfaces of the test piece are tapered, eccentric, or otherwise angled or misaligned with each other, the return echo will be reduced in amplitude and might be distorted due to the variation in sound path across the width of the beam, reducing the measurement accuracy. Typically, the measured thickness will represent an approximate integrated average of the changing thicknesses in the beam diameter. In cases of significant misalignment, measurement is impossible because the reflected beam will form a V-path away from the transducer and cannot be received. This effect becomes greater as the material thickness increases.

Operator-Related Factors

Calibration: The accuracy of any ultrasonic measurement is only as good as the accuracy and care taken during calibration. Make sure to perform the velocity and zero calibrations described in Section 4 when the test material or transducer is changed. We also recommend periodic checks with samples of known thickness to verify that the gauge is operating properly.

• Beam alignment: Always hold the transducer flat when testing on flat surfaces and normal to the radius of curvature when testing on curved surfaces. When testing on curved surfaces, always center the transducer on the curve. Misalignment will cause echo distortion, which will negatively affect the accuracy.
• Coupling technique: In Mode 1 measurements with contact transducers, the couplant layer thickness is part of the measurement and is compensated by a portion of the zero offset. To achieve maximum accuracy, the coupling technique must be consistent. For consistent measurements, use only enough couplant to achieve a stable reading and apply the transducer with uniform pressure. Practice will show the degree of moderate to firm pressure that produces repeatable readings. Also, never scrape or drag transducers across rough surfaces. In general, smaller-diameter transducers require less coupling force to squeeze out the excess couplant than larger diameter transducers. In all modes, tilting the transducer distorts echoes and causes inaccurate readings.

  For corrosion gauging on small-diameter pipes and tubes, hold the transducer so that the sound barrier material visible on the probe face is aligned perpendicular to the center axis of the pipe, as shown below.

Equipment-Related Factors

While instrument design factors like digital sampling rate will set the limits of range and accuracy for an ultrasonic gauge, the range and accuracy in an application is ultimately determined by the combination of gauge, transducer, and setup, as well as material-related factors. For information on the typical materials and thickness ranges that can be measured with ultrasonic gauges using specific transducers and appropriate instrument setups, visit Section 9.0 Appendices – Transducer Range Charts.

Note that precision gauges using single element transducers typically have higher inherent accuracy than corrosion gauges using dual element transducers.

Does it often feel frustrating when choosing a suitable ultrasonic thickness gauge to meet your inspection requirements?

Understanding your inspection requirements is one thing but choosing the suitable ultrasonic thickness gauges is a whole different ball game. It often requires a completely different set of skills from determining quality assessment requirements. However, choosing the best suited ultrasonic thickness gage is not impossible. All it requires is following a set of carefully mapped out plans and considering critical factors that can make your decision process easier.

This post will show you all the requirements and essential factors you should consider when purchasing an ultrasonic thickness gauge. 

So, if you’re ready, let’s dive into it. 

 

 

Ultrasonic Thickness Gauge Explained

Like all non-destructive testing tools, ultrasonic thickness gauge is a device that ensures the test sample is intact after the test. However, its primary function is to measure the thickness of materials against a specified standard thickness. Invariably, material thickness changes over time due to the effects of such things as corrosion or erosion. And so, most technicians try to ascertain the level of change in this thickness using this device. 

Other thickness-measuring devices like a caliper or a micrometer gauge usually measure thickness from two sides. On the other hand, an ultrasonic thickness gauge measures thickness with access to one side only. Yet, it has proven faster, more versatile, and more reliable than most non-destructive thickness measuring devices. 

Primarily, it works using the laws of ultrasonic sound wave projection. It uses standard waveforms for sound traveling in a medium like steel or other materials to compare the generated waveform for the test sample. 

 

Types of Ultrasonic Thickness Gauge

While the application of ultrasonic thickness gauges is common for ascertaining the level of corrosion damage to metal structures like pipes, tanks, and tubes, the gauges employed come in one of two types, corrosion gauges and precision gauges. 

Naturally, the choice of which ultrasonic thickness gauge you make will be influenced by the test material and the number of transducer elements required to achieve the inspection. 

• Corrosion Gauges

  – With a combination of signal processing and internal detection techniques, corrosion gauges are very effective for measuring the level of internal corrosion in metal structures like pipes and vessels. This measuring effectiveness stems from the ability of the ultrasonic thickness gauge to measure the thickness with access to just one side of the test sample. One other thing is that corrosion gauges use dual element transducers to achieve thickness measurements. 

• Precision Gauges

  – For other materials like fiberglass, rubber, ceramic, and plastics, precision gauges are more effective. Though they use a single element transducer, they can identify the reduction in thickness of the material walls with access to just one side of the material. 

 

Choices of Transducer Types for Ultrasonic Thickness Gauge

Generally, the transducer types available for ultrasonic thickness include contact transducers, delay line transducers, immersion transducers, and dual element transducers. The choice of transducer that you choose will depend on the level of accuracy you desire for a given thickness measurement.

• Contact transducers

  : By far the simplest to implement, contact transducers often contact the test sample when carrying out the thickness measurement. And, because of its simplicity, it has become the most preferred choice for most technicians.

• Delay Line transducers

  : Delay line transducers are often the most effective for measuring very thin materials. The reason lies in their ability to interrupt external pulse excitation from back wall echoes using a cylinder known as a delay line. This disruption is necessary so the echoes don’t interfere with the main pulse coming from the test material to the receiving element. The delay line is often made from epoxy, silica, or plastic material. Also, hot test samples act as a protection for the temperature-sensitive transducer element. 

• Immersion transducers

  : This transducer is often very effective for applications involving moving parts or in-process measurements. It employs a water column to transmit sound waves into the test material. Apart from in-process measurements, immersion transducers also choose materials with channels or grooves. 

• Dual element transducers

  : Often employed for rough corroded surfaces, dual-element transducers are very effective. These transducers are only used with corrosion gauges. 

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How to Select the Right Ultrasonic Thickness Gauge?

Choosing a suitable ultrasonic thickness gauge requires considering certain critical factors to make your decision-making process smoother. Here is a list of essentials you should consider in your quest to choose the right ultrasonic thickness gauge.

• Material

   – This factor is by far the most important one on this list, and the reason is this. Materials absorb or transmit ultrasound differently. For example, certain materials, including glass, metals, and ceramic, transmit ultrasound effectively with materials like rubber absorbing ultrasound. Hence, choosing a reliable ultrasonic thickness gauge will require that you understand the reaction of the test material to ultrasonic waves. 

• Thickness

  – The thickness range specified during measurement is also as important as the material type. The reason is that the thickness range determines the thickness gauge required to achieve a satisfactory result. It is also important for the accuracy anticipated for the thickness measurement. For instance, you might use a delay line transducer to get a highly accurate measurement for thin materials. However, thicker material may require contact or dual-element transducers to measure accurately. The high frequency required for thin materials and low frequencies for thick materials also largely determine the required thickness gauge type. 

• Surface

  – The smoothness or roughness of a material surface is highly important in choosing a thickness gauge suitable for a thickness measuring inspection. Generally, ultrasonic thickness gauge with a dual-element transducer is effective for rough surfaces. In contrast, other transducers are generally suitable for other forms of thickness measurements. 

• Geometry

  – The ease of coupling the test material and the transducer significantly reduces. Hence, considering the geometry will help you choose the suitable transducer that will give an accurate reading based on the level of irregularity or the radius of curvature of certain parts of the test sample. Curvature naturally limits the choice of a transducer to either delay line transducer or the immersion transducer. Additionally, materials with channels and grooves are also often measured with immersion and delay line transducers. 

• Temperature

  – The sensitivity of transducers for most ultrasonic thickness gauges varies significantly as temperature increases. It is generally adequate for primary contact transducers to be accurate within a temperature range of up to 50 degrees Celsius. Once the temperature exceeds this range, only special transducers will be suitable for these measurements as the delicate elements within the transducers will be permanently destroyed. These special transducers may include immersion, delay line transducers with heat resistance, and dual element transducers. These special transducers will help you achieve an accurate thickness measurement in this case. 

• Phase Reversal

  – The bonding between materials of different acoustic impedances in special cases like the insulation of metals with rubbers or plastic bushings on metal pipes significantly affects your choice of a suitable ultrasonic thickness gauge. The acoustic impedance is calculated by multiplying the velocity of the sound with the density of the material and can greatly influence the accuracy of the thickness measurement if the right thickness gauge is missing. The echoes at the boundary of the connection between the materials often introduce a phase reversal with inaccuracies for the thickness measurement. Nonetheless, a simple setting before the device setup can adequately accommodate this irregularity to deliver an accurate result. 

• Measurement Range

  – The range of thickness measurements anticipated is also important. You don’t choose an ultrasonic thickness gauge that is far above the actual measurement range or one that is far below it. Choosing a thickness measuring device without adequate consideration for measurement range can have significant cost consequences. 

• Accuracy

  – This factor forms the basis for all quality assessments as technicians and quality assurance officers ensure the accuracy of measuring devices using tools like calibration. Consequently, you need to consider the level of accuracy required for the specific thickness measurement to choose the suitable ultrasonic thickness gauge. As such, technicians utilize other tools along with device calibration, including material sound velocity, sound scattering, attenuation, and sound couplings to ensure the high accuracy of the measuring device. Also, the process of calibration should follow standard practice using acceptable reference standards and known thickness. Hence, measurements that require an accuracy of 0.01mm will require a different ultrasonic thickness gauge and transducer than if the accuracy requirements were 0.001mm. Nevertheless, immersion and delay line transducers are adequate for generally high accuracy requirements.  

• Operation

  – Given certain conditions, your choice will greatly affect how easy it is to operate an ultrasonic thickness gauge. For instance, where there is limited access to the test sample, some thickness gauges might be more suitable than others. Identifying these constraints gives you an idea of the best thickness gauge for that operation. 

• Durability

   – What applies to highly durable and sturdy test samples will differ considerably from a fragile test sample. Careful consideration of this factor will allow you to choose the best ultrasonic thickness gauge without destroying the sample object. 

 

Knowing how to choose your right ultrasonic thickness gauge, I bet you also want to know where you can find the best thickness gauge manufacturers, and which one is the best. Here’s the top 10 ultrasonic thickness gauge manufacturers guide for you.  

Also, click to continue to read how to use ultrasonic thickness gauge, if you are interested in.

 

 

Conclusion

To wrap up, choosing the right ultrasonic thickness gauge for your thickness inspection operation is essential and possible if adequate consideration is given to critical factors that can adversely affect the result of the thickness measurement. While these factors include material thickness, measurement range, accuracy required, ease of operation, and durability, it is equally important to know the type of transducers available for thickness measurement operations. 

Ultimately, following the information given in this post will ensure that you choose the best suited ultrasonic thickness gauge for your exceptional thickness measuring needs with consistently accurate results every time.

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