Technique Development, Procedures, and Process Control
Building a technique
A written technique follows a fixed sequence: pre-cleaning, magnetization, particle application, inspection (usually with the field on), demagnetization if required, and post-cleaning. Every variable must be specified: wet or dry, fluorescent or visible; magnetization type and direction; current or ampere-turns with tolerance; particle concentration; dwell time; and viewing conditions. In the continuous technique, particles are applied while current flows. The residual technique relies on the field remaining in the part and needs high retentivity and higher magnetization levels. Use at least two magnetizations roughly 90° apart unless geometry guarantees the flux direction.
Process control
Standard checks include bath concentration by settling test in a pear-shaped tube, contamination and particle agglomeration, black-light intensity and dark adaptation, yoke lifting power, ammeter accuracy and cable condition, and periodic verification of the technique with a field indicator, Ketos ring or flux strip. Wet baths have practical temperature limits (roughly 40–125 °F, 4–52 °C) and must be monitored for evaporation and carry-over. Demagnetize when residual fields would interfere with welding, machining or later magnetic inspection, and verify the result with a field indicator (commonly ≤ 3 gauss / 240 A/m, or whatever the contract demands).
Traps
Over-magnetization buries real defects in background; under-magnetization misses tight cracks. Applying particles before the field is established, or after it has decayed, is a frequent procedural error. Demagnetizing with AC and then verifying with a DC-sensitive instrument, or forgetting that handling can remagnetize a part, also appears. Safety items - ventilation, UV eye protection, flash point of the vehicle - are fair exam material.
Rule of thumb
A clean part tested at the wrong current proves nothing. Confirm the field with a QQI or field indicator at the start of each shift and whenever the setup or part geometry changes.