What a magnetic particle testing unit is
Magnetic particle inspection finds cracks in ferromagnetic material by making the part itself part of a magnet. The part is magnetised, the surface is flooded or dusted with ferromagnetic particles, and any discontinuity that interrupts the flux path lets field leak out at the surface, where the particles gather into an indication the inspector can see. It is a surface and near-surface method: it reaches into the material only a few millimetres, and that depth is set by how the part is magnetised rather than by the equipment maker.
Its strength is that it is fast, it covers a whole surface at once, and it finds the tight cracks that a penetrant cannot enter and that a visual inspection misses altogether. Weld toes, fillet welds, forging laps, grinding cracks and heat-treatment cracks are its usual targets. It is also the method most often specified for pressure piping, boilers, lifting gear and welded structures, which is why the acceptance criteria usually come from a code rather than from a drawing.
What the equipment has to deliver is therefore not a reading but a demonstrated field. The part has to be magnetised in the right direction for the defect being looked for, strongly enough to produce a readable indication, with a particle medium and a lighting arrangement that make it visible. Everything else - the format of the unit, the power source, the probe shape - exists to bring that field to the part in the condition the inspection is carried out in.
How a magnetic particle testing unit measures
Magnetisation is directional. A crack is best detected when the flux crosses it at a large angle, so a longitudinal crack needs the field applied circumferentially and a circumferential crack needs it applied along the axis. In practice both directions are applied in turn, because assuming the direction of an unknown defect is how an inspection misses one. A yoke produces the field between its poles, and a bench unit passes current through the part or through a coil wound around it.
The current form decides the depth. Alternating current concentrates the field at the surface because of skin effect and is the right choice for cracks that break the surface; direct current and rectified current penetrate further and are used where a defect may sit just below the surface. Field strength is not a setting to be guessed - it is demonstrated with a reference indication, either a test piece with a known artificial defect or a field indicator that shows when the field is adequate and its direction.
The indication depends on the medium and the light. Dry powder is convenient and suits rough surfaces; wet suspension in a carrier is more sensitive and is what code work usually calls for. For white-light inspection the particles are coloured and contrast against a prepared background; for fluorescent inspection they are coated to fluoresce and are read under ultraviolet light in a darkened area, which finds weaker indications at the cost of a darkened bay and an ultraviolet source that has to be maintained.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Unit format | A portable electromagnetic yoke, a mobile power unit with a yoke, or a bench machine with a headstock and coil | Whether the part can be brought to the machine or the machine has to be taken to the part, which is usually the deciding factor on site work. |
| Current form | Alternating current, direct current, or rectified current from an inverter pack | How deep the field reaches, and therefore whether a surface crack or a near-surface one can be found at all. |
| Magnetising power and pole spacing | The output the unit can deliver, and the span between the yoke poles | The field density achieved on a given section, because the same output spread over a wider span gives a weaker field. |
| Field direction | A single yoke arrangement, or a fixture that can also apply a longitudinal and a circular field | Whether the inspection covers defects of unknown orientation, or only those lying in one direction. |
| Particle medium | Dry powder, or a wet suspension bath, in a choice of colours | The sensitivity and the surface finish it suits. Wet suspension is more sensitive; dry powder tolerates rougher and hotter surfaces. |
| Inspection light | White light over a prepared contrast background, or ultraviolet with fluorescent particles | The faintest indication that can be seen, and whether a darkened inspection area is available at all. |
| Demagnetisation | A separate demagnetising coil, or the capability built into the unit | Whether the part can be released for service without residual magnetism affecting later machining, welding or instrumentation. |
| Field verification | A reference test piece or a field indicator, with the interval at which it is used | Whether the inspection can be shown to have been carried out at a proven field strength, which is what a code-compliant record has to contain. |
How to choose a magnetic particle testing unit
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Whether the material is ferromagnetic | The method does not work on austenitic stainless steel, aluminium, copper or any other non-ferromagnetic material | State the material and grade, not only the part name. If the material is not ferromagnetic, the enquiry belongs with a different method. |
| Where the defect is expected to lie | A surface-breaking crack and a near-surface one need different current forms, and the portable and bench ranges are built around that difference | Describe the defect being looked for and how it is expected to have formed, since the origin usually indicates the depth. |
| The size and shape of the part | A yoke needs access to both poles on the same face; a bench machine needs the part to fit between its heads | Give the part size, the weld or surface to be covered, and how much of it can be reached from one position. |
| Power available at the point of work | A mains unit cannot be used away from a supply, and a battery pack limits how long a field survey can run | State where the work is carried out, whether mains power is available, and how long the survey is expected to take. |
| Whether ultraviolet inspection is required | It changes the lighting, the particle cost and the working conditions, and not every acceptance standard permits it | Quote the acceptance standard and say whether fluorescent inspection is called for or merely preferred. |
| Whether the part must be left demagnetised | Residual magnetism can interfere with later machining, welding, or nearby instrumentation | State what happens to the part after inspection, and whether a residual field limit is specified. |
If any of these are still open, send what is known together with the acceptance standard being worked to. A quotation can be issued against that and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a magnetic particle testing unit usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from magnetic particle inspection |
| Ultrasonic flaw detectors | Finds defects inside the material rather than at its surface, and works on any material that transmits sound. It needs a couplant and a trained operator to interpret the echoes, where magnetic particle inspection shows the indication on the surface directly - which is why the two are often used together on the same weld. |
| Ultrasonic thickness gauges | Measures how much material remains rather than looking for a discontinuity. It answers a corrosion and remaining-life question, and it does not detect cracks at all in its normal mode. |
| Coating thickness gauges | Also relies on a magnetic field on ferromagnetic parts, but measures the depth of a layer above the substrate. The shared principle does not make them interchangeable, and neither instrument reports what the other does. |
| Dye penetrant inspection | Draws a liquid into a surface-breaking defect and develops it into a visible indication. It works on non-ferromagnetic materials and on complex geometries that a yoke cannot straddle, but it only finds defects that reach the surface and that the penetrant can enter. |
| Eddy current testing | Uses an induced field to find discontinuities in conductive material, including non-ferromagnetic grades, and is fast enough for tube scanning. It is more sensitive to small surface defects but less tolerant of rough surfaces and of geometry changes. |
| Visual and radiographic inspection | Visual inspection finds only what is open to view and cannot size a tight crack; radiography images the internal volume and works on any material, but it needs access to both faces and brings its own safety regime. |
Magnetic particle and ultrasonic testing are the two methods most often paired on welded structures - one to find what reaches the surface, the other to find what does not - and the boundary between them is depth.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The material and grade, and confirmation that it is ferromagnetic
- The part or weld to be inspected, with a sketch or drawing of the geometry
- The defect being looked for, and how it is expected to have formed
- The acceptance standard and the level of acceptance to be worked to
- Whether the inspection is carried out in a workshop or on site, and whether mains power is available
- Whether white-light or fluorescent inspection is required
- The size and weight of the parts, and how they will be presented to the unit
- Whether the parts must be demagnetised after inspection
- Whether the field strength has to be demonstrated and recorded
- Quantity, destination port and required delivery date
