13 Reasons Ultrasonic Thickness Readings Go Wrong, in Checking Order
Same gauge, same batch of parts, numbers that do not match. Most problems come from coupling, velocity and surface condition. Here they are in the order you should check them.
Start with surface and coupling
1 Rough surface. Poor coupling between probe and part gives a weak or missing echo. On corroded in-service equipment, grind or file the surface, remove oxide and paint, and expose bare metal.
2 Small radius. On small-diameter tube, a flat probe touches the surface only at a point or a line, so transmission is poor. Use a 6 mm small-diameter probe.
3 Couplant choice. Low viscosity for smooth surfaces; high viscosity for rough, vertical or overhead surfaces; high-temperature couplant for hot work. Apply thin and even – on hot parts, apply to the probe, not the workpiece.
4 Worn probe face. The acrylic wear layer on a common thickness probe clouds with use and loses sensitivity. Resurface with 500-grit paper while keeping it parallel; if it is still unstable, replace the probe.
Then parameters and material
5 Wrong velocity. Calibrating on a steel block and then measuring aluminium is guaranteed error. Confirm the material, or reverse-measure velocity on a standard.
6 Temperature. Sound speed in solids drops as temperature rises. Hot in-service equipment needs a 300-600 degrees Celsius probe; standard probes must not be used.
7 Stress. Compressive stress speeds sound up slightly, tensile stress slows it. In-service pipe usually carries stress; correct against a reference in the same condition where it matters.
8 Laminated and composite material. Sound will not cross an unbonded interface, and velocity is not constant in inhomogeneous material. On multilayer wrapped vessels, the gauge reads only the layer in contact with the probe.
9 Oxide or paint layer. Sound speed differs between the two materials, and the error grows with coating thickness.
Finally geometry and internal condition
10 Non-parallel surfaces. The back wall scatters the beam and no echo returns.
11 Internal deposit. When the deposit has acoustic impedance close to the steel, the reading is wall plus deposit.
12 Internal defects. With inclusions or laminations, the reading often falls near 70 percent of nominal thickness. Switch to a flaw detector to locate the defect.
13 Corroded back wall. Pitting attenuates the beam, so readings jump irregularly and may disappear entirely.
Summary
Roughly nine out of ten bad readings resolve in the first three steps: clean the surface, choose the right couplant, confirm the velocity. Only then look at material and geometry.
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