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Magnetic particle

Circular magnetization and prod current

The current range for a head shot or central conductor from the part diameter, and for prods from the spacing and thickness.

For reference only — verify against your procedure and the governing code.

The current rules are written per inch: millimetres are converted to inches before they run.

Head shot or central conductor

mm

The greatest distance across the part.

For an internal conductor; 1 for a head shot.

mm
mm

With the conductor against the wall, ASME V uses conductor diameter + 2 × wall.

Result

Minimum (300 A/in. = 11.8 A/mm)
590.6A
E709 normal ceiling (500 A/in. = 19.7 A/mm)
984.3A
Maximum (800 A/in. = 31.5 A/mm)
1,575A

Prods

Governing document
mm

The 3/4 in. break is read as 19 mm, as ASME V writes it.

mm

Result

Amperes per inch (per mm) of spacing
90 – 110 (3.54 – 4.33)A/in. (A/mm)
Prod current
531.5 – 649.6A

Formula

  • Direct contact: I = 300 to 800 A × diameter (in.)
  • Offset internal conductor (ASME V): diameter = conductor diameter + 2 × wall
  • Multiple passes: I = single-pass current ÷ passes
  • Prods: I = A per inch × prod spacing (in.)

Units. Both documents write the current rules per inch. Lengths typed in mm are converted to inches before they run, and the per-mm figures beside them are converted from the per-inch ones. The 3/4 in. prod-thickness break is read as 19 mm, as ASME V writes it.

Assumptions and limits

  • Diameter is the greatest distance across the part (the greatest cross-sectional diagonal for non-round parts).
  • E709: normally 500 A/in. or lower; up to 800 A/in. for inclusions or low-permeability alloys; up to 1,000 A/in. with an offset internal conductor for inclusions in precipitation-hardening steels.
  • Field adequacy is still demonstrated with a field indicator or shims (ASME V T-764).

Sources

  1. ASTM E709-21, Standard Guide for Magnetic Particle Testing — §14.3.1 prods; Appendix X3.2.1 circular magnetization; X3.2.2 encircling-coil formulas (Eqs X3.1–X3.3, Note X3.1, X3.2.2.4).
  2. ASME Boiler and Pressure Vessel Code, Section V (2025), Article 7 — T-752.2 prod current; T-753.2(f) hollow parts; T-754.1(b) direct-contact current; T-754.2 internal conductors.

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For reference only — verify against your procedure and the governing code.

Standards and documents are cited by their publishers' names to identify them. TRACE is not affiliated with, endorsed by or sponsored by ASTM International, ASME, the U.S. Nuclear Regulatory Commission, NIST or the Laboratoire National Henri Becquerel. Formulas are paraphrased and cited by clause; get the documents themselves from their publishers.