How to choose a magnetic particle inspection system
By Grant, Inspection Tool Specialists. Reviewed November 2026.
Three things decide which magnetic particle inspection (MPI) equipment you need: whether the part is magnetic, where the defects can be (on the surface or just below it), and whether you test on site or in a workshop. MPI works only on ferromagnetic materials such as carbon and low-alloy steel and iron.
How magnetic particle testing works
The part is magnetised. A crack or other discontinuity that breaks the magnetic field leaks flux at the surface, and fine magnetic particles, dry powder or a wet suspension, gather at the leak and show the defect. The field has to run across the defect you expect, so the direction of magnetisation matters as much as its strength.
Yoke, flaw detector or kit?
| Type | Suits | Typical in our range |
|---|---|---|
| AC yoke | Quick surface crack checks on welds and parts, on site | EF-6Y AC yoke |
| Permanent yoke | Portable checks where no power supply is available | Model 310 permanent yoke |
| Yoke kit with probes | Flexible field for different shapes and sizes of part | Yoke flaw detector kits Model-I and Model-II |
| Bench flaw detector | Repeatable tests on smaller parts in a workshop, with the field set and recorded | MT-1A, MT-1B and MT-1C |
AC or DC?
AC magnetisation finds defects open at the surface and is the usual choice for welds and cracks in service. DC magnetisation penetrates further and can find defects somewhat below the surface. Choose by where the defect is likely to be, and confirm the method in the procedure your customer works to.
Check the field every time
A field that is too weak or in the wrong direction hides defects, so check it before you test. A pie-shaped field indicator or a QQI shim shows whether the field is strong enough and in the right direction. A reference test block with known defects confirms the whole method on a sample before you rely on it for the part.
Particles and light
- Dry powder is for on-site work on rough surfaces. Red, green and yellow powders give contrast against different parts.
- Wet suspension gives finer detail and is applied by spray or flow. It comes in visible and fluorescent forms.
- Contrast paint provides a white background for visible particles.
- Fluorescent particles need a UV-A lamp at 365 nm, and the inspection area must be dark enough for the indications to show. Check the lamp output before you start.
Concentrates are sold by the box and mixed with water. Check the mix ratio on the data sheet.
Safety and records
Strong magnets must be kept clear of people with pacemakers and of sensitive instruments. UV light needs appropriate eye protection. Demagnetise parts where they must not stay magnetised. Follow the procedure your customer or standard sets for the method, the particle type, the field checks and the acceptance criteria, and keep the records it requires.
Which equipment should I buy?
| Application | Suggested starting point |
|---|---|
| Quick AC surface checks on site | EF-6Y AC yoke (MR 5-6Y) |
| Portable checks where there is no power | Model 310 permanent yoke |
| Flexible field for different parts | Magnetic yoke flaw detector kit Model-II |
| Repeatable workshop testing | MT-1B magnetic particle flaw detector |
| Checking the field | PIE-type field indicator, QQI shim set, or the MPI reference test block |
| Fluorescent inspection | DG-50 UV-A lamp or the UV LED headlamp |
| Dry powder, wet suspension and contrast paint | MR230 red dry powder, MR76F fluorescent wet suspension, MR72 white contrast paint |
As a guide to our current range, including VAT: AC yokes from about R9,400; permanent yokes from about R16,800; yoke kits from about R28,800; bench flaw detectors from about R64,100; UV-A lamps from about R14,300, and UV headlamps from about R8,800; field indicators from about R880; dry and wet particles from about R1,000. Prices change, so check the product pages and confirm the particle and lamp each quote includes.
Questions we will ask you
Tell us the material and whether it is magnetic, the part's shape and size, where the defects are likely to be, whether you test on site or in a workshop, whether you need visible or fluorescent particles, the procedure or standard you work to, and the records you must keep. With that we can recommend a system and provide pricing.