6.1 MT Equipment, Media, and Lighting

ASNT's magnetic-particle general exam still treats Equipment as official topic 7 on the outline administered prior to 15 December 2026. That heading is not a catalog of brand names. It asks whether you can match a stationary wet bench, a mobile power pack, a yoke, or prods to the part, the access, and the particle system, and whether you can light the examination so a leakage-field pattern is actually visible. ASTM E1444 (practice), ASTM E709 (guide), and ASME Section V, Article 7 are the usual industry references for equipment, media, and lighting numbers. They are not ASNT exam publications. Attribute the number to the standard. If a procedure tightens a value, the procedure wins.

After 15 December 2026, ASNT's revised MT general outline rolls work into five duty domains: principles and theory; use and care of equipment; techniques; interpretation, evaluation, and documentation; and codes and procedures. Lighting, particle condition, and daily care sit more visibly under use-and-care and techniques. This guide still teaches the current ten-topic outline. Use the five-domain map only so a late-2026 or 2027 appointment does not surprise you.

Stationary wet benches

A stationary wet horizontal (or wet vertical) bench is a production machine. Typical features include:

• A headstock and tailstock that clamp a conductive part so a high-amperage direct or rectified current can be passed through it (circular magnetization).
• A coil (often a multi-turn coil on a rail) that slides over the part for longitudinal magnetization.
• A recirculating bath that agitates wet particles in water or oil and applies them by flood, spray, or immersion.
• A black-light (UV-A) booth or hood for fluorescent viewing, with a darkened interior.
• A magnetizing-current control and an ammeter that the Level II reads and records as an essential variable.

Benches earn their keep on lots of similar ferromagnetic parts — shafts, fasteners, gears, spindles, and small fittings — where contact or coil shots can be repeated with the same amperage recipe. They are the wrong mental picture for a single pipe-fillet weld on a scaffold. The specific exam likes that contrast: if the stem describes a production lot of pins, think bench; if it describes one in-service toe, think yoke.

The bench is only as good as the bath and the contacts. Agitators must keep particles in suspension. Copper pads or lead contacts must seat without burning the part. The coil window must actually surround the zone of interest. A Level II who floods fluorescent particles and then inspects under a desk lamp has used a production machine as if it were a flashlight.

Mobile power packs, cables, and coils

A mobile power pack is a high-amperage magnetizing source on wheels or a cart. The Level II attaches cables, wraps a cable coil, or fits prods. The same unit can produce a circular field (current through the part or through a central conductor) or a longitudinal field (current through a coil surrounding the part). Mobile packs serve large weldments, castings, and field repairs that will not fit a bench.

Cable condition is an equipment control, not housekeeping. Frayed insulation, loose lugs, and undersized cables drop amperage at the work and can shock or burn. The Level II confirms the ammeter reading against the procedure's amperage-versus-spacing or ampere-turn rule, usually drawn from ASTM E1444 or ASTM E709, then records that current. A pack that can push thousands of amperes is useless if the indicated current is wrong.

Electromagnetic yokes and permanent-magnet yokes

The portable electromagnetic yoke is the default one-person tool for local weld examination.

• An alternating-current (AC) yoke produces a longitudinal field between the legs. Because of the magnetic skin effect, AC flux concentrates at the surface. That is why an AC yoke is the usual choice for weld-toe fatigue cracks and other surface-breaking openings.
• A direct-current (DC) or half-wave DC (HWDC) yoke drives flux deeper and can reveal some near-surface conditions. It is heavier, can leave more residual magnetism, and still does not turn magnetic particle testing into a mid-wall volumetric method.
• A permanent-magnet yoke needs no line power. It is useful where electricity is restricted, but lift strength changes with wear, temperature, and pole face condition, so process-control lift checks still matter.

Legs must seat on the surface. Articulating legs follow contours. Pole spacing is a procedure variable: too wide and the field between the poles is weak; too narrow and you examine only a postage stamp. Paint, scale, or a gap under one leg kills the field just as surely as an unplugged yoke.

Prods

Prod magnetization drives current into the part through two contact prods and produces a circular field around the current path. Industry guides discuss amperage as a function of prod spacing (ASTM E1444 / E709). The exam-relevant hazards are arc burn, local overheating, and two-person handling of heavy cables. Do not put prods on a finished pressure-boundary face, a thin aerospace skin, or a bearing journal unless the procedure explicitly allows it. If the stem offers a yoke or a coil as an alternative on a finished surface, that is usually the safer magnetization method.

Prods still appear on large ferromagnetic castings and rough weldments when a deeper circular field is required and a small arc-burn risk is accepted by the procedure. They are equipment, not a personality preference.

Particle systems: dry visible, wet visible, and wet fluorescent

Magnetic particles are finely divided ferromagnetic powder with high permeability and low retentivity. They must move to a leakage field and then release when the field is removed. Three industrial systems dominate the exam:

Dry powder is the usual field choice on a rough, still-warm, or locally ground carbon-steel weld. Wet fluorescent is the usual shop choice when sensitivity and a controlled booth are available. Wet visible sits in the middle: production parts viewed in white light, or customers who do not want a dark booth.

Carriers: water with conditioner versus oil

Wet particles do not float in untreated tap water. A water bath needs a conditioner package: a wetting agent so the bath sheets instead of beading, a corrosion inhibitor, and often antifoam and a fungicide. Water is cheaper, easier to clean up, and preferred where oil residue would interfere with later painting or liquid penetrant testing.

An oil carrier is a low-viscosity petroleum distillate named by the particle manufacturer. Oil suspends particles well, does not rust ferrous parts, and is common on some aerospace and precision benches. It adds fire load, slip hazard, and disposal controls. Do not dump leftover water-conditioned powder into an oil bath or the reverse. The procedure names the carrier family; the Level II verifies that the drum and the bench match.

Particle size, shape, and fluorescent versus color-contrast

Shape. Elongated or rod-like particles bridge a leakage field more readily than spheres. Quality powders are a controlled mix of sizes and aspect ratios, not shop-floor iron dust.

Size. Finer particles follow weaker leakage from tight, shallow cracks. If they are too fine, they create background that masks the pattern. Coarser dry particles have the mobility to move on a rough weld but will skip a hairline grinding crack on a ground journal. Match the media to the expected discontinuity and the surface finish.

Fluorescent versus color-contrast. Choose fluorescent when you can control darkness and UV-A and you need the highest practical sensitivity — machined aerospace parts, pressure-equipment welds, fatigue-critical toes. Choose color-contrast (visible) when ambient light is high, a UV booth is not available, the surface is rough, or the customer specification calls for visible dry powder. Visible particles must contrast with the surface: dark powder on a light grind, light or red powder on dark mill scale, sometimes after a thin contrasting background if the procedure allows it.

Lighting — attribute ASTM and ASME, not ASNT

Lighting is an equipment-and-media control, not a preference. Learn the common industry values as code and practice numbers:

ASNT does not publish these numbers as exam secrets. A procedure may require more UV-A, less ambient, or a higher white-light minimum. Measure with a calibrated UV-A radiometer and a visible-light meter at the examination surface, not at the lamp face and not in the aisle.

UV-A lamps — historically filtered mercury-vapor "black lights," now often LED — must warm up if the manufacturer requires it, must keep their UV-pass filters intact, and must not flood the booth with visible violet that washes out contrast. White-light examinations need a handheld or fixed lamp that actually delivers 1000 lux on the weld toe.

Realistic exam scenarios

A production shaft lot runs on a wet fluorescent bench. The Level II verifies bath agitation, reads the ammeter for the circular shot, then inspects in a booth that meets 1000 μW/cm² UV-A and 20 lux ambient. That is a complete equipment-and-lighting setup.

A field crew on a painted carbon-steel girder uses an AC yoke and dry red powder after local coating removal. They measure white light at the toe (at least 1000 lux / 100 fc). Fluorescent wet particles without a hood would be the wrong media in noon sunlight.

A finished stainless-clad carbon-steel vessel is not an MT part on the cladding. If the procedure examines the carbon-steel backing, the equipment choice still has to produce a field in ferromagnetic material.

Equipment, media, and lighting are one system. A correct yoke with the wrong powder, or a correct powder under 50 lux of white light, is a failed examination even if the magnetizing current was perfect.