Ultrasonic thickness measurement

Ultrasonic thickness measurement allows a component to be inspected non-destructively, determining its thickness without sectioning it and without needing access to the opposite side. This is particularly useful when checking pipes, tanks, sheets, bulkheads, installed structures or manufactured items where only one side is accessible for measurement.

Ultrasonic thickness gauges are used in production, quality control and maintenance activities to verify material thickness, check for possible thinning and monitor components subject to wear or corrosion. The range includes different models according to the required level of accuracy, material type, presence of paints or coatings, graphic signal analysis and inspection of composite materials.

How ultrasonic thickness measurement works

An ultrasonic thickness gauge uses a probe that sends a high-frequency sound pulse into the material. By means of coupling gel, the signal travels through the component, reaches the opposite surface and returns to the probe. The instrument calculates thickness according to the time taken by the signal and the propagation velocity set for the material being inspected.

The main advantage of this method is the ability to perform the measurement from one side of the part. Unlike mechanical instruments that require access to both opposite surfaces, an ultrasonic thickness gauge allows closed, installed or internally inaccessible components to be checked.

However, correct instrument selection depends on the actual application. Homogeneous materials, such as many compatible metals and plastics, can be measured with standard instruments; painted metal surfaces require models with a dedicated through-coating mode; very thin metallic samples require a high-accuracy thickness gauge; fibreglass and certain composite materials instead require a dedicated probe and more careful application assessment.

Ultrasonic thickness gauges for standard inspections

For standard thickness checks on homogeneous materials that transmit ultrasound, portable models are available for inspecting sheets, pipes, box sections, tanks, vessels, bulkheads and components subject to progressive thinning.

The 0.1 mm-resolution thickness gauge is suitable when a simple and immediate check of residual thickness is required, for example during maintenance activities or rapid production checks. It is a practical solution for standard measurements on components accessible from one side, when advanced analysis or data-storage functions are not required.

The 0.01 mm-resolution thickness gauge is intended for more structured inspections. Its 0.01 mm resolution, Scan mode, internal memory and the possibility of using optional probes with different characteristics and frequencies make it suitable when thickness variations must be detected across an area, several readings must be managed or the measurement must be adapted to different application conditions.

Scan mode is particularly useful when checking sheets or pipes affected by non-uniform corrosion: by moving the probe across the surface, local variations can be detected and areas requiring closer attention can be identified compared with a single point measurement.

Residual thickness and corrosion control

One of the most common uses of ultrasonic thickness measurement concerns the inspection of components that may lose material over time due to corrosion, erosion or wear. In these cases, the instrument allows residual thickness to be measured and compared with the nominal value or with measurements taken previously.

This type of inspection is used on pipes, tanks, vessels, structural steelwork, sheets, bulkheads and structures used in industrial or marine environments. The possibility of measuring from one side makes it possible to work directly on the installed component without dismantling it or making its internal side accessible.

Model selection depends on the level of investigation required: a portable thickness gauge may be sufficient for standard checks; when thickness trends must be monitored, results stored or the signal analysed graphically, instruments with more advanced functions should be considered.

Measurement through paints and insulating coatings

When a metal surface is painted or protected by an insulating coating, a standard ultrasonic inspection may be affected by the presence of the surface layer. To determine the thickness of the base material alone without removing the coating, thickness gauges with Through Coating mode, namely measurement through paint, are available.

This function is useful for maintenance inspections on painted pipes, coated tanks, vessels, boilers, fire extinguishers, bulkheads, watertight compartments and structures protected by anti-corrosion coating systems. By avoiding paint removal at the measuring point, the inspection is more practical and preserves the component’s surface protection.

The model dedicated to measurement through paint is the suitable solution when the main requirement is to check the thickness of the base metal beneath the coating. The advanced graphic model also includes Through Coating mode, but offers a broader application range, the ability to exclude thicker insulating coatings and signal-analysis functions useful for more complex inspections.

0.001 mm measurement of thin, flat metallic samples

When the inspection concerns very thin metallic thicknesses, the choice must be a dedicated high-accuracy instrument. The 0.001 mm-resolution ultrasonic thickness gauge is designed for flat metallic samples, with 0.001 mm resolution and a level of control suitable for thin parts where even small variations can affect component conformity.

In high-accuracy mode, with reference to steel, this model can measure thicknesses starting from 0.15 mm. It is therefore suitable for thin sheets, foils, thin plates and metal parts used in applications where thickness is a critical requirement.

This type of thickness gauge is used in laboratories, quality departments, micromechanics and sectors with demanding inspection requirements, such as aerospace. It is not intended for measurement through paints or insulating coatings and does not replace instruments designed for general checks on pipes, tanks or complex geometries.

Graphic thickness gauge with A-scan and B-scan

The graphic ultrasonic thickness gauge is suitable when a simple numerical thickness reading is not sufficient and the behaviour of the signal must be observed or the measurement trend along a scan must be displayed.

A-scan mode displays the echoes of the ultrasonic signal at the inspected point. This function allows the reading quality to be assessed more effectively and can help identify indications or anomalies compatible with discontinuities, inclusions or internal defects. It is not a universal defect-detection method: interpretation depends on the material, geometry, probe used and inspection procedure adopted.

B-scan mode instead represents the thickness trend along the measurement path. By moving the probe across the component, the operator can view thickness variations, identify localised thinning and more readily recognise areas affected by corrosion or material loss.

The graphic model also includes Through Coating measurement, minimum and maximum value management, alarm thresholds, high memory capacity and the possibility of storing waveforms. It is therefore a complete solution for professional inspections on metallic components, including painted or coated parts, when the completed verification must be documented and examined in greater depth.

Thickness measurement on fibreglass and composite materials

Thickness measurement on fibreglass and composite materials requires a dedicated instrument because these materials may have an internal structure and ultrasonic propagation behaviour that differ significantly from homogeneous materials.

The thickness gauge for composite materials is equipped with a dedicated low-frequency probe and is primarily intended for checking fibreglass products, such as hulls, bulkheads, panels, tanks and marine components. It allows measurement to be performed from one side of the component without cutting or taking destructive samples.

The instrument can also be assessed for certain carbon-fibre components, compatible plastic materials and other low-density materials. However, composition, layering, fibre orientation, the presence of cores and part geometry can affect the result: for particular composites or layered structures, the feasibility of measurement should therefore be assessed in advance on a real sample.

A-scan and B-scan functions also support analysis of the signal and thickness trend. In specific applications, for example on fibreglass hulls, they can help investigate indications compatible with delaminations, discontinuities or internal defects, without turning the instrument into a universal defect detector for every composite material.

Applications of ultrasonic thickness measurement

Ultrasonic thickness gauges are used in numerous production and maintenance settings when thickness must be checked without damaging the component and access is available from one side only.

The main applications include:

– Residual thickness control of sheets, pipes, tanks and vessels

– Inspection of components subject to corrosion, erosion or wear

– Quality control in production and material acceptance

– Measurement on bulkheads, watertight compartments and structures in the marine sector

– Inspection of base metal beneath paints or insulating coatings

– High-accuracy verification of thin, flat metallic samples

– Graphic analysis of thickness variations along a scan

– Inspection of fibreglass hulls, tanks and panels

– Assessment of certain compatible composite and plastic materials, subject to application verification where necessary

– Management of documented inspections in quality departments and industrial maintenance activities

How to choose an ultrasonic thickness gauge

The choice of the correct ultrasonic thickness gauge depends on the material, component geometry, thickness to be checked and level of analysis required.

For rapid and standard checks on homogeneous materials, the 0.1 mm-resolution model is suitable. When 0.01 mm resolution, Scan mode, internal memory and greater application flexibility are required, the 0.01 mm-resolution model is preferable.

When the metal component is painted or coated and only the base material must be measured without removing the covering, a model with through-coating mode must be selected. For more detailed under-coating inspections, with signal analysis and greater documentation capacity, the graphic model is the most complete solution.

When measurement concerns very thin, flat metallic samples, the 0.001 mm-resolution model allows high-accuracy checks with 0.001 mm resolution. When the component is instead made of fibreglass or a compatible composite material, the dedicated thickness gauge with a low-frequency probe must be used, assessing the application in advance in more particular cases.

Identifying the correct model makes it possible to set up an inspection consistent with the actual component, obtain reliable measurements and manage quality and maintenance checks more effectively.