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Magnetic particle

Coil magnetization (ampere-turns)

Ampere-turns and coil current for longitudinal magnetization from the part's L/D ratio and the coil's fill factor.

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

The E709 constants are in inch units: millimetres are converted to inches before the formulas run.

mm
mm

Non-round parts: the greatest cross-sectional dimension.

mm
mm
Part position in a low- or intermediate-fill coil

Result

L/D
5
Fill ratio Y
36 — Low fill (Y ≥ 10)
Ampere-turns NI
9,000A·turns
±10 % band
8,100 – 9,900A·turns
Coil current I = NI / N
1,800A

Formula

  • Low fill, part against coil wall: NI = 45,000 / (L/D)
  • Low fill, part centred: NI = 43,000 · R / (6 · (L/D) − 5)
  • High fill (L/D ≥ 3): NI = 35,000 / ((L/D) + 2)
  • Intermediate: NI = [NI(hf) · (10 − Y) + NI(lf) · (Y − 2)] / 8
  • I = NI / N; Y = coil area / part area; hollow parts: D(eff) = √(OD² − ID²)

Units. The E709 constants are in inch units. Lengths typed in mm are converted to inches before the formulas run; L/D and the fill factor are ratios and do not change.

Assumptions and limits

  • These are ASTM E709's empirical formulas, which E709 itself keeps "for historical continuity only": verify the field with a gaussmeter or shims. Lengths go into the formulas in inches; results ±10 %.
  • L/D below 3 needs pole pieces or another technique; over 15, use 15 (E709 Note X3.1). ASTM E1444 requires an L/D of at least 2 for coil magnetization.
  • ASME Section V Article 7 (T-753.2) sets its own coil rules; when it governs, use them. They are not reproduced here.

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. ASTM E1444/E1444M-26, Standard Practice for Magnetic Particle Testing — §6.3.4 coil shots in ampere-turns; §6.3.5 minimum L/D of 2.
  3. 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.