Magnetic Particle Yoke, Coil Amperage, and Field Orientation

A circular field from a central conductor or head shot runs at amperes per inch of diameter; a coil shot runs on amp-turns from the part's length-to-diameter ratio and the fill factor, and dividing by the turns gives the current. Both are conventions that get a field of roughly the right magnitude into a part of roughly the right shape. THE ORIENTATION TRAP IS WHAT MAKES THIS WORTH A CALCULATION AT ALL. A magnetic particle indication appears where a discontinuity interrupts the field, so a crack lying PARALLEL to the field lines produces no leakage and does not indicate -- at any amperage, with any particles, under any light. A circular field finds longitudinal cracks; a longitudinal field, from a coil or between the poles of a yoke, finds transverse ones. An examination performed with a single shot has examined the part for a single crack orientation, and a report describing it as an examination of the part is wrong. Two shots roughly at right angles are what an examination is, and that is a procedure requirement rather than a refinement. The L/D ratio is bounded in the standard formulae between 2 and 15 and is clamped here, because outside that range the coil relations do not apply and a part shorter than twice its diameter needs a different technique or an extension. Fill factor changes the constant substantially, so a part loosely centred in a large coil and one nearly filling it are different shots. The yoke lift check is the field tool most people actually use and the fastest verification in NDT: at maximum pole spacing a yoke must lift 10 lb on AC or 40 lb on DC, and one that will not is producing an inadequate field regardless of what its label says. FIELD ADEQUACY IS VERIFIED WITH A GAUSS METER OR A QUANTITATIVE QUALITY INDICATOR RATHER THAN BY THE AMPERAGE SETTING, because the actual field depends on the geometry, the permeability and the current path in ways no formula captures. This computes starting currents from entered figures. It does not write a technique or a procedure, select particles, wet or dry method, contrast or fluorescent, address lighting, viewing conditions, or demagnetisation, evaluate or classify indications, determine acceptance criteria, handle multidirectional or induced-current techniques, or address the arc-strike and part-damage risks of prod or head-shot contact. ASTM E1444, ASTM E709 or the applicable code and procedure, and the certified Level II or III examiner, govern.

circular field current = part diameter x amperes per inch; coil shot amp-turns = 35,000 / (L/D + 2) at high fill factor or 45,000 / (L/D + 2) at low, with L/D bounded between 2 and 15; and the yoke lift check of 10 lb on AC or 40 lb on DC at maximum pole spacing.

Magnetic particle starting currents as ASTM E1444 and E709 practice state them. The amperes per inch is entered (commonly 300 to 800) because it depends on the part and the technique.

Two published formulae and one lift-test figure.

Estimate. AHJ and licensed professional govern.

Field names used by the API: part_diameter_in, amps_per_inch, part_length_in, coil_turns, fill_factor, yoke_pole_spacing_in, yoke_current, circular_amps, ld_used, coil_amp_turns, coil_amps, yoke_lift_required_lb

• L/D bounded 2 to 15 outside that range the coil relations do not applyASTM E1444
• Amperes per inch is entered commonly 300 to 800, depending on part and techniquethe written technique
• Adequacy is verified, not calculated a gauss meter or quantitative quality indicatorASTM E709 / the procedure