ASTM E709 Magnetic Particle Field Strength

Empirical magnetizing-current formulas for direct circular magnetization, internal conductor, prod spacing, and encircling-coil longitudinal magnetization

◉ ASTM E709-21 (CURRENT EDITION), APPENDIX X3 · NOT A REGULATORY SUBMISSION TOOL

⚠ Rules of thumb, not requirements. Source: ASTM E709-21 §X3, the current edition, read from its own text. E709 states these empirical formulas plainly as guidelines that "must be used with judgment" and can lead to over-magnetization or poor coverage — they are not mandatory values, and §X3.2.2 (encircling coils) says the formulas are included for historical continuity and it would be quicker and more accurate to measure the field with a Gauss (Tesla) meter than to calculate it. Field strengths from empirical formulas should always be verified with a Hall-effect gaussmeter or AS 5371 shims (§X3.2.1.1).

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1. Direct Circular Magnetization (§X3.2.1.1)

Part Diameter (greatest distance across the section) mm

Examining for inclusions or low-permeability alloys (allows higher current)

2. Internal Conductor Induced Magnetization (§X3.2.1.2)

Part Diameter (greatest distance across the section) mm

Internal Conductor Diameter (for effective examination length) mm

Precipitation-hardening steel, examining for inclusions (allows up to 1000 A/in.)

3. Prod Spacing Current (§14.3.1, general practice)

Prod Spacing mm

90–115 A/in. of prod spacing (3.5–4.5 A/mm), typical spacing up to 8 in. (200 mm).

4. Yoke Field Strength (§14.3.2)

5. Air-Core Encircling Coil, Longitudinal Magnetization (§X3.2.2)

Part Length, L mm

Part Diameter (or Effective Diameter for a Hollow Part), D mm

Number of Turns in the Coil, N

Coil Inside Diameter (for fill-factor) mm

Total Cross-Sectional Area of the Part, Aₜ mm²

Cross-Sectional Area of the Hollow Portion(s), Aₕ mm²

Part is hollow (compute effective diameter from areas above)

L/D is capped at 15 for all formulas below (§Note X3.1); for L/D below 3, a pole piece or an alternative method such as induced current should be used instead. Fill factor: low (coil area ≥ 10× part area, part positioned close to the coil wall or centered), high (coil area < 2× part area), intermediate (between).

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