MT Field Strength Calculator
Magnetic particle inspection works only when the applied field is strong enough to magnetize the part to about 80% of saturation. ASTM E1444 §7.5 gives the rule of thumb: for a solenoid coil running through a long part, H = NI / L (amp-turns per coil length). For a yoke, field strength is verified by the 10 lb / 40 lb lift test. This tool runs the coil calculation and confirms yoke compliance.
How it works
Inside a long solenoid the H-field on axis is N × I / L where N is turns, I is current in amperes, and L is solenoid length in metres. The result is in A/m. Convert to oersteds (the unit MT procedures still use) via 1 Oe = 79.58 A/m. ASME V Art. 7 §T-753 requires 40 to 120 Oe (3,200 to 9,600 A/m) on the surface of a ferromagnetic part for indications to form reliably with wet fluorescent particles. The yoke alternative is empirical: 4.5 kg (10 lb) AC yoke lift, 18 kg (40 lb) DC yoke lift, verified daily per ASTM E1444 §7.4.4.
Formula
H = N × I / L (solenoid, A/m) ; 1 Oe = 79.58 A/m
H = N × I / L (solenoid, A/m) ; 1 Oe = 79.58 A/mWorked example
5-turn coil, 1,200 A through-current, 300 mm part length inside the coil. H = (5 × 1200) / 0.300 = 20,000 A/m = 251 Oe. That exceeds the 40–120 Oe specification of ASME V Art. 7, which means the coil setting needs to come down — typically by reducing current to 600 A, yielding 126 Oe. Most field setups run the calculation backward: target 100 Oe → I = (target × L) / N = (100 × 79.58 × 0.300) / 5 = 478 A.
| Variable | Value |
|---|---|
| input: mode | Solenoid coil |
| input: turns | 5 |
| input: current | 1200 |
| input: length | 300 |
| output: field_oe | 251 |
| output: field_AmpPerM | 20,000 |
When to use this tool
Use when setting up a coil-shot MT procedure, verifying that a recurring inspection setup still meets ASTM E1444 §7.5 amp-turn requirements, or auditing a vendor procedure against ASME V Art. 7 amp-turn limits.
Limitations
Where this calculator stops being accurate:
- H = NI/L holds inside an infinite solenoid. For real coils, the field at the centre is 60–80% of the infinite-solenoid value depending on length-to-diameter ratio.
- For parts shorter than the coil, multiply by an L/D shape factor per ASTM E1444 §A1.1.
- Yoke lift-test is empirical — the actual field at the test surface is not measured, only the pole-piece pull.
- Residual field measurement after demag requires a Hall-effect gauge; this tool does not size demagnetization current.
- AC and HWDC currents produce different penetration depths; this calculation gives the surface field magnitude only.
Frequently Asked Questions
What is the difference between amp-turns and field strength?
Amp-turns (N·I) is the magnetomotive force — the input driving the field. Field strength H is the resulting magnetic field intensity at the part surface, in A/m or oersteds. ASTM E1444 §7.5.1.1 specifies coil amp-turns: 30,000 to 50,000 AT for low fill-factor parts. ASME V Art. 7 §T-753 specifies field strength: 40–120 Oe measured with a Hall gauge. Both target the same outcome; the amp-turn rule lets you set up without a gauge on hand.
How do I verify field strength without a gauge?
Two field-accepted alternatives. (1) Magnetic particle field indicator (pie gauge, ASTM E1444 §A1.3) — a notched ferrous disk that shows particle build-up patterns when the field is adequate at the indicator location. (2) Yoke lift test — the yoke must lift 10 lb (4.5 kg) on AC, 40 lb (18 kg) on DC, verified at start of shift. Neither alternative replaces a calibrated Hall-effect gauge for procedure qualification, but both are accepted for routine production work.
What field strength is correct for crack detection?
ASME V Art. 7 §T-753 sets 30 to 60 G (3 to 6 mT) tangential at the surface. Below 30 G, sub-surface and tight cracks miss formation. Above 60 G the field saturates surface features and produces wide, hazy indications that obscure small cracks. Typical wet fluorescent particle work targets 40–50 G, which is the central two-thirds of the working window and gives consistent results across operators.
Why do procedures still use oersteds instead of A/m?
Historical inertia. The SI unit A/m replaced oersteds in 1948 (CGS to SI) but MT instrumentation, ASNT training materials, and most field-use Hall gauges (e.g. Magnaflux Type 1000) still display Oe. ASME V Art. 7 lists both. ASTM E1444 §3.2.9 defines the conversion: 1 Oe = 79.58 A/m. Site documentation should record whichever unit the gauge reads, with the conversion attached.
References & Standards Cited
- ASTM E1444/E1444M-22 Standard Practice for Magnetic Particle Testing
- ASME BPVC Section V (2023), Article 7 Magnetic Particle Examination
- ASTM E709-21 Standard Guide for Magnetic Particle Testing
- ASNT SNT-TC-1A (2020), Personnel Qualification — Magnetic Particle Testing
Founder of NDT Connect and Atlantis NDT. 15+ years in industrial inspection across oil & gas, petrochemical, and offshore. ASNT Level III certified across five methods. Drives platform standards for the NDT Connect marketplace.