Ultrasonic Thickness (UT) measurement is one of the most commonly used non-destructive testing techniques in industry, with inspectors in the oil and gas, power generation, marine, and process industries using it to monitor wall thickness and detect corrosion or erosion before it becomes a safety issue.
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One variable is frequently underestimated in the field: temperature. When surfaces are hot, whether because of ambient conditions in tropical climates or the asset's process temperature, UT readings become inaccurate.
Anyone responsible for asset integrity must understand why this happens and how to prevent it.
Why Heat Affects Ultrasonic Measurements
UT thickness gages (UTGs) transmit an ultrasonic pulse through a material using a transducer (probe) and measure how long it takes for the pulse to pass through the material, reflect off the rear wall, and return.
The thickness is then calculated using a known number for the material's speed of sound, or "velocity".
The velocity is not a set constant; it varies with temperature. As a substance heats up, its molecular structure becomes less dense and more elastic, slowing the rate at which sound travels.
If an instrument uses a velocity value calibrated for a component at room temperature, but the actual component is much hotter, the UT meter's thickness measurement will be incorrect.
In most cases, this overestimates wall thickness, which may conceal the same issue the operator is there to discover: unsafe levels of wall loss.
This is not a small effect. On carbon steel, for example, every 55 °C increase in temperature reduces sound velocity by around 1%. On an asset operating at several hundred degrees, this can create a measurement error large enough to miss a thin, at-risk section of pipework or plate.
Beyond Velocity: Other Heat-Related Complications
Temperature-driven velocity change is the most prominent concern, but it is far from the only one. Inspectors confront additional challenges when dealing with high surface temperatures.
- Transducer degradation: Standard piezoelectric transducers are not intended to tolerate prolonged exposure to high temperatures. Prolonged contact with hot surfaces might harm the transducer element, reducing signal quality and lowering probe life. For this reason, specialized high-temperature transducers exist; nonetheless, even these have practical limitations and must be handled with care.
- Couplant breakdown: A couplant, usually a gel or paste, is used in ultrasonic testing to close the air gap between the transducer face and the test surface, allowing sound to transmit more efficiently. Many common couplants are not designed to withstand high temperatures; they can dry up, boil, or lose viscosity rapidly when in contact with a heated surface, disrupting acoustic coupling and producing unreliable or unreadable signals.
- Thermal expansion of the component: At high temperatures, the substance under test physically expands. While this has less influence than velocity change, it adds another layer of measurement uncertainty that precision applications must consider.
- Operator safety and rushed readings: It is important to acknowledge the human factor. Inspectors working on hot surfaces often feel pressure to take readings quickly and move on for their own comfort and safety. This can result in poor probe contact, inadequate couplant application, or inadequate dwell time for a stable reading, all of which reduce data quality regardless of the instrument's technical capacity.
How to Avoid Temperature-Related Errors
The good news is that these errors are well understood and may be avoided with proper techniques and equipment.
- Calibrate at temperature: Whenever possible, velocity calibration should be performed on a calibration block that has been heated to match the temperature of the component under test, rather than utilizing a room-temperature reference. Many current gages also have temperature compensation capabilities that change the computed thickness in response to a detected or input surface temperature.
- Use appropriate high-temperature transducers and couplants: Purpose-built high-temperature probes and couplants are designed to sustain performance and safeguard equipment at elevated surface temperatures, and they should be used if readings exceed the manufacturer's recommended operating range for standard equipment.
- Allow adequate contact and dwell time: Ensure appropriate interaction and dwell time. Rushed readings on heated surfaces are more susceptible to mistakes. Following the manufacturer's contact-time recommendations and taking multiple readings to ensure consistency will help identify unusual findings caused by improper coupling.
- Record and report surface temperature: Documenting the actual surface temperature at the time of inspection enables more accurate interpretation of results later on and for trend analysis over time as conditions change.
Conclusion
Heat is one of the most common and often neglected causes of mistakes in ultrasonic thickness testing. If this source of inaccuracy is not addressed properly, it could lead to misplaced confidence in an asset's condition, with potentially catastrophic implications.
Understanding the underlying physics, employing temperature-appropriate equipment, and adhering to disciplined inspection methods can help operators and inspectors ensure that UT readings remain accurate and reliable even in the most demanding thermal settings.
This information has been sourced, reviewed and adapted from materials provided by Cygnus Instruments Ltd.
For more information on this source, please visit Cygnus Instruments Ltd.