Ultrasonic thickness gauges operate based on the pulse-echo principle, where an ultrasonic wave is emitted from a probe and travels through the material until it reaches an internal interface, such as a back wall or defect. Upon encountering this boundary, the wave reflects back to the probe, and the thickness of the material is calculated by measuring the time it takes for the wave to travel through the material. This method works effectively with materials that allow consistent sound wave propagation. The design of these instruments enables accurate measurement of plates, machined components, pipes, and pressure vessels, making them widely applicable in industries like oil and gas, chemical processing, metallurgy, shipbuilding, aerospace, and more.
**Measurement Techniques:**
1. **General Measurement Method:**
- Perform two measurements at the same point using the probe, with the probe faces oriented at 90° to each other. The smaller value is considered the actual thickness of the component.
- For unstable readings, use the 30mm multi-point method: take multiple readings within a 30mm diameter circle around the original point, and select the minimum value as the thickness.
2. **Accurate Measurement Method:**
- Increase the number of measurements around key points, and mark thickness variations with thick lines for better visualization.
3. **Continuous Measurement Method:**
- Conduct continuous measurements along a specified path using single-point measurements, ensuring intervals do not exceed 5mm for precise tracking.
4. **Grid Measurement Method:**
- Create a grid pattern over the area of interest and record thickness at each intersection. This technique is especially useful for corrosion monitoring in high-pressure equipment and stainless steel linings.
**Factors Affecting Thickness Gauge Readings:**
1. **Surface Roughness:**
If the surface is too rough, it can lead to poor coupling between the probe and the material, resulting in weak or no echo signal. For surfaces with rust or paint, sanding or grinding can improve coupling. Applying a coupling agent also helps achieve better contact.
2. **Small Radius of Curvature:**
When measuring thin-walled tubes or curved surfaces, the flat probe may only make point or line contact, reducing sound transmission. Using a small-diameter probe (e.g., 6mm) can enhance accuracy in such cases.
3. **Non-Parallel Surfaces:**
If the top and bottom surfaces are not parallel, the ultrasonic wave may scatter, preventing the probe from detecting the bottom echo.
4. **Material Structure:**
Castings or austenitic steels with coarse grains can cause significant ultrasonic scattering, leading to inconsistent or no readings. A low-frequency, coarse-grain probe (e.g., 2.5MHz) is recommended for such materials.
5. **Worn Probe Surface:**
Over time, the probe’s acrylic surface can become rough, reducing sensitivity. Sanding with 500# paper can restore smoothness. If issues persist, consider replacing the probe.
6. **Corrosion Pits on the Backside:**
Corrosion or pitting on the opposite side of the material can weaken the signal, causing irregular or no readings.
7. **Sediment in Pipes:**
If there's sediment inside a pipe, the gauge might display the combined thickness of the wall and the deposit, leading to inaccurate results.
8. **Internal Defects:**
Inclusions or interlayers within the material can cause the displayed thickness to be about 70% of the actual value. Additional inspection using an ultrasonic flaw detector is advised.
9. **Temperature Effects:**
The speed of sound decreases with increasing temperature, typically by about 1% for every 100°C rise. High-temperature environments require special probes rated for 300–600°C.
10. **Laminated or Composite Materials:**
These materials can complicate measurements due to their layered structure, requiring careful calibration and specialized techniques for accurate readings.
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