Evaporation Losses

T.O. 33B-1-1 d. Dirt, soil and other insoluble solids can be carried into the magnetic particle bath as a result of inadequate pre-cleaning. e. Acidic and alkaline solutions can be a contaminant of magnetic particle baths. Acidic and alkaline solutions can be residues of previous plating, paint stripping and cleaning processes. 1.6.3.2 Evaporation Losses. Magnetic particle bath vehicle materials used in open tanks are continuously undergoing evaporation, resulting in an increase in particle concentration. The rate of evaporation increases with warmer tempera- tures and larger tank surfaces. Evaporation losses take place very gradually so performance change may become significant before it is noticed. 1.6.3.3 Drag-Out. Particle concentration is reduced when particles that adhere to parts being inspected are not returned to the suspension. Like evaporation, the resultant change occurs slowly and would probably go unnoticed until significant performance loss is experienced. 1.6.3.4 Heat Degradation. Fluorescent dye stuffs are sensitive to elevated temperatures. Temperatures of over 140^oF (60^oC), can reduce the f luorescence and temperatures over 250^oF (117^oC), may destroy it completely. High temperatures in magnetic particle inspection materials usually occur when materials are improperly stored. A dark colored container stored in direct sunlight can reach temperatures above 140^oF. 1.6.3.5 Process Degradation. In addition to materials degradation during use, the equipment and process can deteriorate. The magne- tizing equipment can loose power, black light bulbs age and become dirty and critical procedural steps may be performed incorrectly or omitted. Periodic checks SHALL be accomplished to assure satisfactory performance. 1.6.4 Frequency of Process Control. One of the factors inf luencing the degradation of a magnetic particle system (materials, equipment and procedures) is the volume of parts being processed. Bath and equipment deficiencies can be expected to occur more often with increased workload volume. Since there is no uniformity in workload between activities, a single calendar schedule cannot be established. Each inspection activity SHALL set inspection intervals based on their workloads. The inspection intervals SHALL be documented as shown in Chapter 1, page 1-15, paragraph 1.4.5. (For Navy: use local form.) Guidance on inspection intervals is provided in the following paragraphs. a. If your workload requires operation for eight or more hours each day, perform the concentration test every eight hours or each shift. b. If your workload requires occasional or less than eight hours of operation, perform the concentration test prior to processing parts on that shift. 1.6.5 Material Requirements. 1.6.5.1 Applicability. NOTE Prior to bath replacement in a magnetic particle inspection unit, the equip- ment must be cleaned thoroughly according to the equipment maintenance manual. This does not apply to the addition of materials (either vehicle or particles) to maintain concentration. 1-44 Change 3