Plant maintenance relies on non-destructive testing (NDT) to judge whether equipment can stay in service. Three of the most widely used methods are also among the most accessible: ultrasonic thickness gauging for wall loss, magnetic particle inspection for surface cracks in steel, and remote visual inspection with a video borescope for places the eye cannot reach.
Ultrasonic Thickness Gauging
An ultrasonic thickness gauge sends a short pulse of high-frequency sound from a probe into the material and times its echo from the far wall. With the speed of sound in that material known, thickness is half the round-trip time multiplied by the velocity. Measuring from one side means pipes, tanks, boiler tubes, ship hulls and pressure vessels can be checked for corrosion and erosion without being emptied or opened.
Practical points:
- Velocity: accuracy depends on the sound velocity used. Calibrate on a reference block of the same material, or measure velocity on a sample of known thickness. Many gauges offer this two-way mode.
- Couplant: gel or oil is needed between the probe and the surface to carry sound across the gap.
- Dual-element probes are used for corrosion work because they handle rough, pitted back walls better.
- Coatings: standard pulse-echo measurement includes paint thickness in the reading unless the gauge has an echo-to-echo mode.
- Temperature: hot surfaces need high-temperature probes and couplant.
MEWOI's ultrasonic thickness meters cover general-purpose, precision, cast-iron and high-temperature versions. The supplier gives ranges of about 0.75–300 mm in steel (probe dependent), a velocity range of 1000–9999 m/s, and surface temperatures up to 300°C or 450°C on the high-temperature models.
Magnetic Particle Inspection
Magnetic particle inspection (MPI) finds cracks at or just below the surface of ferromagnetic materials. The part is magnetised; where a crack crosses the magnetic field, flux leaks out of the surface. Fine iron particles, dry or suspended in a liquid and often fluorescent, gather at the leakage field and draw a visible line along the crack.
- Direction matters: cracks show best when they run across the field, so each area is inspected twice with the yoke turned through about 90°.
- AC vs DC: AC magnetisation concentrates near the surface and is preferred for surface cracks such as weld toe fatigue cracks. DC penetrates somewhat deeper.
- Limitations: MPI works only on ferromagnetic materials. It cannot be used on austenitic stainless steel, aluminium or copper alloys; liquid penetrant testing is the usual alternative there.
The MEWOI-Z9 is a portable yoke available in AC 220 V, DC 12 V battery and combined AC/DC versions for site work on welds, castings, forgings, shafts, gears and structural steel. The supplier specifies a pole spacing of 0–210 mm and lifting power above 5.5 kg (AC) and 22 kg (DC). It recommends magnetising pulses of 3 seconds or less with at least 5 seconds off during long sessions.
Remote Visual Inspection with Video Borescopes
Visual inspection remains the most common NDT method, and a video borescope (industrial endoscope) takes it inside engines, turbines, gearboxes, heat exchangers, pipes and cavities through an existing opening. A camera at the tip of a probe sends live video to a handheld screen, with lighting at the tip. Articulating tips steer the view, and recorded images document findings.
MEWOI's HD endoscope range includes compact 3.5–5 inch handheld units, long-reach systems with 6.5 mm or 17 mm probes rated IP68, a 360° panning model, and pipe inspection cameras for drains and ducts. Choose by the access hole size, the reach needed, whether tip articulation is required, and the environment (oil, water, temperature).
Choosing the Method
| Question | Method |
|---|---|
| How much wall is left? | Ultrasonic thickness gauge |
| Is there a surface crack in this steel weld or part? | Magnetic particle inspection |
| What does it look like inside? | Video borescope |
| Is there a buried flaw inside a weld? | Ultrasonic flaw detection or radiography (not covered by these tools) |
Frequently Asked Questions
Do NDT results need qualified operators? For code and certification work, inspection is normally carried out by personnel qualified to schemes such as ISO 9712. For routine condition monitoring, trained maintenance staff use these tools widely.
Can a thickness gauge find pitting? It can find wall loss under the probe, but isolated pits can be missed between readings. Close grid scanning or other methods are needed where pitting is expected.