Q: What are the main factors that control MPI system performance and sensitivity?
A: Operator skill, process control, equipment condition, light levels, magnetic field strength, field direction, strength of residual fields.
Q: Which three factors are most important for MPI sensitivity?
A: Field strength and direction, current level, and control/type of particle
Q: Why is field strength important in MPI?
A: Sufficient field strength creates clear indications; too low produces weak leakage fields, too high causes excess background and can damage parts.
Q: What is the best current type for detecting subsurface discontinuities in MPI?
A: Direct Current (DC).
Q: What is the best current type for detecting surface discontinuities in MPI?
A: Alternating Current (AC).
Q: What kind of particles provide the highest sensitivity for AC MPI?
A: Small, wet fluorescent particles
Q: How is a particle concentration check performed for MPI suspension baths?
A: A 100 mL sample is placed in a centrifuge tube, allowed to settle (30 min water-based, 1 hr oil-based), then measured and adjusted as needed.
Q: Typical concentration range for visible magnetic particle bath?
A: 1.2–2.4 mL/100 mL vehicle
Q: Typical concentration range for fluorescent magnetic particle bath?
A: 0.1–0.4 mL/100 mL vehicle
Q: What is the Ketos ring used for in MPI?
A: System performance checks of both wet/dry, fluorescent and non-fluorescent techniques—verifies system sensitivity and detectable defect depth.
Q: What is the minimum required lifting force for an AC yoke (ASTM E709)?
A: 10 lbs (4.5 kg) at 50–100 mm leg spacing.
Q: At what UV intensity should black light inspections be performed in MPI?
A: Minimum of 1000 µW/cm² at the inspection surface
Q: What is the recommended visible light intensity at the examination surface per ASTM E709?
A: 1076 lux (100 foot-candles).
Q: What do Hall Effect Meters measure during MPI?
A: Flux density produced at the surface of a magnetized article.
Q: What is the main advantage of direct contact (headstock/prods) magnetization technique?
A: Fast, enhances residual magnetism, circular field around part, effective for surface/near-surface defects.
Q: What limitation does the central conductor technique in MPI have?
A: Requires conductor size to match current; reduced field strength on heavy-walled cylinders may decrease sensitivity.
Q: When should Quick Break checks be performed on stationary MPI units?
A: Every 6 months or as recommended; ensures eddy currents for enhanced field detection.
Q: What safety consideration should be made for mercury vapor UV lamps in MPI?
A: Bulbs and filters get very hot—risk of burns and UV damage; wear safety glasses (avoid tinted/transition lenses).
