Detection Media, Contrast and Viewing

Outcomes

After this chapter, the learner should be able to:

• describe the properties required of magnetic particles;
• compare dry and wet media;
• compare visible and fluorescent systems;
• select a carrier and contrast aid for the job conditions;
• establish and verify viewing conditions; and
• recognise media degradation, contamination and application errors.

6.1 What particles must do

Detection media convert an invisible leakage field into a visible indication. Useful particles need:

• high permeability, so a weak leakage field magnetises them readily;
• low retentivity, so they do not remain as permanent clusters;
• a controlled mixture of shapes and sizes;
• sufficient mobility on the actual surface;
• density compatible with the application method and carrier;
• colour or fluorescence that contrasts with the background;
• physical and chemical stability within the stated temperature range; and
• acceptable health, environmental and corrosion properties.

Elongated particles form chains effectively, while more rounded particles improve flow. A balanced mixture usually provides better behaviour than a single extreme shape. Very fine particles respond to weak fields but can cling to rough surfaces. Oversized particles move poorly and may not be retained at a fine leakage field.

6.2 Dry powder

Dry powder is applied as a light cloud while the component is magnetised. Visible dry powders are commonly coloured to contrast with the test surface; fluorescent dry products also exist. Dry powder is particularly useful for:

• rough surfaces where a liquid would drain rapidly;
• local site work;
• elevated temperatures within the product rating;
• HWR or DC near-surface applications; and
• situations where wet-media recovery is impractical.

The operator must create a gentle, uniform cloud and remove excess without blowing away a valid indication. Heavy dumping masks weak patterns. Wind, overhead position and confined geometry make application difficult. Dry powder generally requires greater operator skill and is less sensitive than wet media to very fine surface discontinuities.

The supplied training source describes a legacy/product-control expectation in which most dry particles exceeded 40 micrometres and a typical upper size was about 200 micrometres. This draft does not adopt that as a current acceptance rule. Product certification and the approved procedure shall control.

6.3 Wet media

Wet particles are suspended in a low-viscosity liquid and applied by flow, spray or immersion. The carrier transports small particles across the surface, giving high mobility and strong sensitivity to fine surface conditions. The suspension must be agitated so particles do not settle.

Water-based carriers reduce flammability but need appropriate wetting agents, corrosion inhibitors, anti-foam control and water-break performance. Evaporation and contamination can change the concentration.

Oil-based carriers provide good wetting and slower evaporation but introduce combustible-liquid, vapour and disposal controls. Only a qualified carrier should be used; workshop kerosene is not a substitute for a formulated product.

Wet media are unsuitable above their rated temperature and can be messy on vertical or overhead surfaces. Open recirculating baths require more in-service monitoring than sealed aerosols.

The supplied source describes a wet-particle distribution with most particles between approximately 1.5 and 40 micrometres. Treat this as coverage from the source, not a current procurement limit. The manufacturer's certificate and controlling standard must be checked during approval.

6.4 Visible colour-contrast systems

Visible particles are examined under white light. Black or red particles are common, and a thin white contrast coating may be applied when the component does not provide enough background contrast. The coating must be uniform, dry and within the qualified thickness. Excessive thickness separates particles from the leakage field and can hide fine surface response.

The supplied source uses 50 micrometres as a maximum non-ferromagnetic coating thickness unless sensitivity is demonstrated. That value must be confirmed against the current controlling document and product instructions before it becomes a training or job limit.

Advantages of visible systems include portability, simple viewing equipment and use in normal work areas. Limitations include lower contrast than fluorescence for very fine indications and sensitivity to glare, shadow and poor colour contrast.

6.5 Fluorescent systems

Fluorescent particles absorb UV-A energy, commonly near a 365 nm peak, and emit visible yellow-green light near the region where human vision is very sensitive in a dark environment. The method can reveal small quantities of particles against a low-background field, giving excellent sensitivity.

Fluorescent examination needs:

• a darkened inspection area;
• a suitable UV-A source and compatible meter;
• control of ambient visible light and glare;
• time for the operator's vision to adapt;
• a clean viewing area and clean gloves; and
• verification that the lamp/filter is stable and undamaged.

Mercury-vapour lamps need warm-up and a sound external filter. LED units behave differently and may stabilise rapidly, but their beam uniformity, visible-light output and product-specific safe-use distance still require verification.

6.6 Nominal viewing values from the supplied source

The following values preserve the source coverage and are useful for calculation practice. They shall not be treated as current controlled requirements until formal approval verifies the applicable ISO 3059 adoption and procedure.

Measure at the test surface in the working geometry, not beside the lamp. Avoid direct or reflected UV-A in the operator's eyes. Tinted or photochromic lenses can reduce indication visibility unless specifically evaluated and permitted.

6.7 Electromagnetic spectrum and vision

Visible light occupies roughly 380 to 780 nm. UV radiation has shorter wavelengths; UV-A occupies the longer-wave ultraviolet region used for fluorescent NDT. UV-B and UV-C are not required for MT and must be controlled by the source design and filter.

Photopic vision is cone-dominated vision in normal illumination and supports colour discrimination. Scotopic vision is rod-dominated low-light vision and is more responsive to brightness than colour. Practical fluorescent inspection occurs through a transition influenced by background light, age, health and recent exposure to bright light. The controlled adaptation period reduces variation, but it does not make every observer identical.

6.8 Media and viewing checks

Before use, confirm as applicable:

• product identity, batch, expiry and temperature range;
• certificate or declaration required by the procedure;
• correct mixing ratio and agitation;
• colour, fluorescence and background;
• contamination, separation, foaming and unusual odour;
• carrier wetting, corrosion and flash-point controls;
• system performance on the specified reference piece;
• white-light or UV-A meter status;
• lamp/filter condition, output and visible-light background; and
• examination area cleanliness.

6.9 Common traps

• Believing more particles always improve sensitivity.
• Spraying wet media after the field has been removed during a continuous technique.
• Using thick white coating to conceal a poor surface.
• Measuring lamp output at a convenient distance instead of at the component.
• Wearing contaminated gloves in a fluorescent booth.
• Assuming every LED UV-A source is acceptable because it looks purple.

Chapter summary

Particles, carrier, surface and viewing conditions operate as one detection system. Dry media provide rugged near-surface capability and wet media excel at fine surface response. Visible systems depend on white-light contrast; fluorescent systems depend on controlled UV-A excitation, darkness and observer adaptation.