Ultrasonic wall-thickness testing delivers fast non-destructive measurement and plays a vital role in metal inspection workflows. However, inaccurate readings frequently occur due to improper operation or inherent properties of test workpieces. This article outlines eight common root causes for measurement errors and corresponding practical solutions for ultrasonic thickness gauge users.
Common Issues & Solutions for Ultrasonic Thickness Measurement
1. Excessively Rough Workpiece Surface
Rough surfaces weaken coupling performance between probe and workpiece, produce low-amplitude return echoes, and may result in no valid readings at all. Prior to measurement, remove oxides and paint layers to expose bare metallic surfaces. Thoroughly wipe away dust, dirt, grease and other contaminants to keep the measuring area clean.
2. Severe Corrosion on Test Objects
A portable thickness meter is widely deployed for pipeline inspection. Corrosion, sediment and corrosion pits (especially on hidden inner or back walls) cause acoustic attenuation, erratic readings or complete signal loss. Grind and polish corroded zones before testing. Carry out regular maintenance for measured components.
3. Too-Small Radius of Curved Workpieces
Standard flat-faced probes only achieve point-contact or line-contact against curved surfaces. For small-diameter tubes, poor coupling reduces acoustic transmission and leads to missing readings. Select special small-diameter-tube probes to achieve sufficient contact and acquire reliable thickness data for curved pipeline parts.
4. Non-parallel Test Surface and Back-wall Surface
Based on ultrasonic pulse-echo principle, the target back wall should stay parallel or coaxial with the testing surface. Non-parallel surfaces scatter ultrasonic echoes so the probe cannot receive valid bottom-wall signals, bringing measurement deviation or blank readings. When selecting test locations, confirm that the measuring face and back wall remain parallel or coaxial.
5. Limitations on Composite Multi-layer Materials
Ultrasonic signals cannot travel across uncoupled gaps and propagate unevenly within heterogeneous composite materials. For multi-layer wrapped assemblies, readings from digital ultrasonic tester only reflect the thickness of the layer directly contacting the probe. Screen suitable test samples ahead of inspection for composite structures.
6. Incorrect Sound Velocity Setting
Sound velocity is a critical parameter for ultrasonic thickness measurement. If you test a new material while retaining sound-velocity values calibrated for another material, wrong measurement results will appear. Identify workpiece material and set proper sound velocity before testing. Three available setup methods include selecting preset material parameters, calculating sound velocity against reference blocks of known thickness, and manually inputting exact sound-velocity figures. These simple operations effectively avoid sound-velocity-related mistakes.
7. Temperature-induced Measurement Drift
Sound velocity within solid materials decreases as temperature rises. Test data shows sound velocity drops roughly 1 % for every 100 °C temperature increase. Use high-temperature dedicated probes for hot-state workpieces. Dual-element probes work reliably under non-extreme thermal conditions.
8. Improper Use of Couplant
Couplant eliminates air gaps between probe and workpiece so ultrasonic energy can penetrate into test parts. Missing couplant, wrong couplant type or incorrect application will trigger errors or failed measurement. Choose couplant according to actual conditions: low-viscosity couplant for smooth surfaces; high-viscosity couplant for rough, vertical or overhead surfaces; high-temperature-grade couplant for hot components. Apply couplant moderately and spread evenly. Normally apply couplant onto workpiece surfaces; for high-temperature measurement, apply couplant on the probe instead.
Conclusion
The above covers eight typical problems encountered during daily ultrasonic thickness testing and their troubleshooting approaches. Following these operational guidelines helps field technicians reduce measurement errors and obtain repeatable, trustworthy thickness data.
If you are looking for reliable non-destructive testing hardware and professional technical guidance, Mikrosize, founded in 2009, is a professional manufacturer of measuring and testing instruments. We deliver one-stop testing and measuring solutions for global users. Supported by our worldwide distributor network covering more than 40 countries, we provide dependable instruments and accessible local technical support for your on-site inspection tasks.
