Ultrasonic Wall Thickness Gauges - Knowledge Base

Ultrasonic thickness measurement is a non-destructive testing technique that uses high-frequency sound waves to determine the thickness of solid materials. The gauge sends an ultrasonic pulse through the material and measures the time it takes for the echo to return from the back wall. Since the travel time of the sound is directly proportional to the material’s thickness, the instrument can calculate the wall thickness accurately using the known speed of sound in that material.

1.1 Measurement Principle and Sound Velocity:

Accurate results depend on correctly setting the sound velocity for the test material. Each material has a characteristic speed of sound (for example, about 5,900 m/s for steel and 2,700 m/s for aluminium). Most Checkline gauges include automatic or manual calibration functions that allow you to set this velocity precisely, either by measuring a sample of known thickness or using a certified reference block.

If the velocity is not correctly adjusted, readings may be offset, especially when switching between different materials. For example, a gauge calibrated for steel will show an incorrect thickness if used on plastic or glass without recalibration.

1.2 Suitability of Materials:

Ultrasonic thickness gauges perform best on homogeneous materials such as metals, plastics, glass and ceramics, where sound waves travel uniformly. These materials provide consistent and repeatable readings.

Materials that are porous, fibrous or composite—such as wood, concrete or layered plastics—can scatter or absorb ultrasonic energy, leading to unstable results. In such cases, specialised transducers or alternative methods (like magnetic thickness measurement) may be more appropriate.

1.3 Measurement Range and Accuracy:

The measurable range depends on the transducer frequency, the material’s acoustic properties, and the gauge model.

• Higher frequencies (7–10 MHz) offer excellent resolution for thin walls.
• Lower frequencies (1–5 MHz) provide greater penetration for thick or attenuating materials.

For example, steel can typically be measured from 0.5 mm to over 500 mm with a precision of ±0.01 mm, while cast iron may show larger variations due to its granular structure.

1.4 Coupling Agent:

A coupling agent (couplant) is required to transmit sound efficiently between the probe and the material surface. This thin layer eliminates any air gap that would otherwise reflect the ultrasound. Checkline offers standard gels, high-temperature couplants, and special fluids for rough or curved surfaces. Proper coupling is essential for stable and repeatable results.

1.5 Influence of Temperature:

Temperature affects both the sound velocity and the transducer’s performance. As temperature increases, the speed of sound typically decreases, which can influence measurement accuracy.
For high-temperature applications, it’s important to:

• Use delay-line or high-temperature transducers.
• Apply temperature-resistant couplants.
• Calibrate the gauge at the same temperature as the material.

Maintaining a consistent temperature during measurement ensures reliable results and prolongs probe life.

Advantages:

• Measurement possible from one side only
• High accuracy and repeatability
• Suitable for metallic and non-metallic materials
• Portable and simple to operate
• Fast scanning and data logging options

Selecting the right ultrasonic thickness gauge depends on several key factors that influence both accuracy and usability. Choosing a model aligned with your material, environment and reporting requirements ensures precise, repeatable results in any industrial or laboratory setting.

3.1 Key Selection Criteria:

Before choosing a model, consider:

• Material type and expected thickness range – thicker or more attenuating materials (e.g. cast iron) require lower-frequency probes, while thin metals need higher-frequency, delay-line transducers.
• Surface condition – rough, corroded or curved surfaces are best handled by dual-element probes; polished or smooth materials can use single-element probes.
• Presence of coatings – to measure only the base material through paint or protective layers, select a gauge with Echo-Echo or Triple-Echo (E-E-V) capability.
• Data recording and traceability – if measurement logging, statistics or reporting are needed, opt for models with internal memory or USB output.
• Display and analysis features – advanced models offer waveform displays (A-Scan, B-Scan) for visual verification and corrosion profiling.

3.2 Calibration and Accuracy:

Proper calibration is essential for obtaining reliable readings.

Calibration Methods:

• Direct Input: if you already know the exact sound velocity of the material, it can be entered directly on the instrument.
• Sample-Based Calibration: if the sound velocity is unknown, perform a one- or two-point calibration using a reference sample of known thickness. The gauge measures it, compares the result with the known value, and automatically calculates the correct velocity.
• Integrated Zero Plate: most Checkline gauges include a built-in zero reference plate for fast daily calibration and temperature compensation.

Tip: All models from TI-25MX upwards include automatic calibration functions, ensuring precise results across different materials and operating conditions.

3.3 Recommended Model Overview:

The table below provides a simplified guide to help you select the most suitable gauge according to your requirements.

3.4 Choosing by Application:

• General industrial use: TI-25M, TI-25MX
• Coated or corroded surfaces: TI-25DLX, TI-25MXT
• Thin materials or plastics: TI-007X, TI-PVX
• Through-paint and advanced inspections: TI-PVX, HPX-DLP
• Subsea or offshore environments: TI-UMX2

3.5 Final Recommendation:

For routine tasks, the TI-25 Series provides accurate, reliable results at an excellent value.

For more demanding inspection environments—requiring data logging, coating measurement, or waveform visualisation—consider the TI-007X, TI-PVX, or HPX-DLP models.

For underwater or subsea applications, the TI-UMX2 remains the dedicated choice.