Geomagnetic fabric detection

Geomagnetic Fabric Detection – Shielding Effectiveness Assessment for Magnetic Field Protective Textiles

In Australia’s sensitive electronic industries – including medical imaging (MRI), particle accelerators (ANSTO, Australian Synchrotron), defence electronics, and high‑precision laboratories – geomagnetic fabric detection is essential to verify that shielding textiles (e.g., Mu‑metal, nickel‑iron alloys, amorphous ribbons) effectively attenuate the Earth’s magnetic field and other low‑frequency magnetic interferences. Geomagnetic fabric refers to flexible, conductive or ferromagnetic materials woven or laminated into sheets, foils or meshes used to create magnetic shields around rooms, enclosures, and cable ducts. Our ISO/IEC 17025 accredited laboratory provides comprehensive detection and characterisation services – including magnetic permeability (μᵣ), shielding attenuation (dB), thickness uniformity, saturation flux density, field‑induced noise assessment, and environmental durability – to ensure compliance with Australian standard AS 1053 for magnetic shielding.

Types of Geomagnetic Fabric Samples We Test

Our laboratory handles a wide range of geomagnetic shielding materials used across Australian industries:

• Nickel‑iron alloy (Mu‑metal, 80% Ni, 15% Fe) woven and laminated fabrics
• Amorphous ribbon (Metglas, Vitrovac) based shielding foils
• Permalloy and supermalloy sheets and flexible fabrics
• Composite magnetic fabrics with polymer reinforcement
• Multi‑layer shielding materials (alternating high‑μ and conductive layers)
• Shielding meshes for EM windows and ventilation panels
• Magnetic field shielding for CRT rooms, transformer vaults and battery rooms
• New production batches (incoming quality assurance)
• Shielding fabrics after installation (field verification)
• Competitor shielding materials (benchmarking attenuation)

Key Detection Parameters and Test Methods for Geomagnetic Fabric

We evaluate multiple critical aspects to guarantee the shielding performance of geomagnetic fabrics in Australian applications – from urban power frequency fields to biomedical MRI screening.

1. Relative Magnetic Permeability (μᵣ) – ASTM A342 / IEC 60404‑15

For geomagnetic fabric detection, the most critical property is relative permeability. We cut a toroidal sample (or stack of fabric layers) and wind primary and secondary coils. Using a fluxmeter, we measure the initial permeability at low field (0.5 A/m, 50 Hz). For high‑performance shielding fabrics, μᵣ should exceed 50,000. Materials with μᵣ < 5000 provide inadequate shielding against geomagnetic fields (≈ 50 μT). We also measure maximum permeability (μₘₐₓ) and the permeability vs. frequency response (up to 100 kHz).

2. Shielding Attenuation (dB) – ASTM D4935 (modified for near‑field and low frequency)

We place a 300 mm square fabric sample between two Helmholtz coils or a magnetic field source and a precision fluxgate magnetometer. A known low‑frequency field (50 Hz DC to 10 kHz) is applied, and the field strength on the shielded side is measured. The shielding effectiveness (SE) is calculated as SE (dB) = 20 log (H₀ / H₁). For geomagnetic shielding (Earth’s static field), SE ≥ 30 dB is required for sensitive electron microscopes; for MRI rooms, SE ≥ 60 dB is often specified.

3. Thickness Uniformity (mm) – Micrometer Measurement (ASTM D1777)

Using a calibrated thickness gauge with a foot area of 100 mm² at 0.5 N pressure, we measure the fabric thickness at 10 points per square metre. Variation should be less than ±5% of the nominal thickness. Non‑uniform thickness causes local magnetic field leakage (“hot spots”). For multi‑layer shielding, we measure each layer separately.

4. Saturation Flux Density (Bs) – Vibrating Sample Magnetometer (VSM) or BH Looper

We cut a small sample (3×3 mm) and place it in a VSM with a maximum applied field of 1000 kA/m. The saturation flux density (Bs, tesla) is recorded. For Mu‑metal, typical Bs is 0.8–1.2 T. Low Bs (< 0.6 T) means the fabric will saturate in moderate magnetic fields, losing shielding effectiveness.

5. Coercivity (Hc) – BH Loop Trace

From the same VSM measurement, we determine coercivity (A/m). Low coercivity (Hc < 10 A/m) is desired for demagnetisation after handling. High coercivity leads to remnant magnetism, which can distort nearby fields.

6. Electrical Conductivity (for eddy current shielding) – ASTM D4935

For fabrics that rely on eddy currents for high‑frequency shielding, we measure surface resistivity (Ω/□) using a four‑point probe. Conductivity of copper or aluminium layers should be > 5×10⁷ S/m. Low conductivity (< 1×10⁶ S/m) reduces effectiveness above 1 kHz.

7. Magnetic Field Attenuation Map (Spatial Uniformity)

We scan a 500×500 mm fabric sample using a 2D automated stage with a Hall probe (resolution 0.01 µT, step 10 mm). The variation in transmitted field is plotted. Acceptable variation: < 5% of average attenuation. Localised thin spots (pinholes) appear as bright zones on the map and are cause for rejection.

8. Hysteresis and Remanence After Demagnetisation

We magnetise the fabric to saturation (2000 A/m, 50 Hz), then demagnetise with an alternating decaying field. The residual magnetic moment (A·m²) is measured with a fluxgate magnetometer. For high‑quality shielding, the remanence should be < 5% of saturation. High remanence indicates that the fabric will introduce its own magnetic noise.

9. Frequency Response of Permeability – AC Permeameter

We measure effective permeability from 10 Hz to 100 kHz using a coaxial test fixture and impedance analyser. For geomagnetic field shielding (DC to 10 Hz), high μᵣ is critical; above 1 kHz, eddy currents reduce μᵣ. The cut‑off frequency (where μᵣ drops to 50% of its DC value) is reported.

10. Annealing Effect (Heat Treatment Verification) – Optional

High‑μ alloys require hydrogen annealing to achieve peak permeability. We take a sample before and after the annealing process, measure μᵣ and coercivity. An increase of at least 500% in μᵣ after annealing confirms proper heat treatment.

11. Environmental Durability – Humidity and Salt Spray – ASTM B117

We expose fabric samples to 240 hours of neutral salt spray (5% NaCl, 35°C) or 95% RH at 40°C. After exposure, we re‑measure μᵣ and coating adhesion. Acceptable: μᵣ loss < 10%, no visible rust on ferromagnetic layers. Corrosion of the shielding layer permanently degrades performance.

Quality Grading and Acceptance Criteria

Based on our geomagnetic fabric detection, we classify shielding textiles into three grades (clients provide specific acceptance criteria for their application):

• Grade A (Premium – MRI/Electron Microscope Rooms) – μᵣ ≥ 80,000, SE ≥ 60 dB (DC), thickness variation < ±3%, Bs ≥ 1.0 T, remanence < 2% of saturation, passes salt spray.
• Grade B (Standard – Shielded Rooms for Electronics) – μᵣ 30,000–80,000, SE 40–60 dB, thickness variation < ±5%, Bs 0.8–1.0 T, remanence 2–5%.
• Grade C (Reject – Not Suitable) – μᵣ < 15,000, SE < 25 dB, visible pinholes, corrosion after humidity test – immediate replacement.

Reporting and Deliverables

Our geomagnetic fabric detection report includes: sample identification (material type, nominal thickness, batch number), permeability values (initial, max, vs. frequency), shielding attenuation (dB at specified frequencies), thickness uniformity map, saturation flux density, coercivity, surface resistivity, 2D field map (shielding hotspots), remanence after demagnetisation, annealing verification (if performed), environmental test results, and a clear pass/fail conclusion based on client‑supplied criteria. Raw data (BH loops, field maps, test logs) are archived for 10 years.

In summary, rigorous geomagnetic fabric detection ensures that magnetic shielding textiles used in Australian scientific, medical and defence facilities provide the required attenuation of Earth’s magnetic field and other low‑frequency disturbances, protecting sensitive instruments and enabling precise measurements. Contact our laboratory to schedule batch testing for your next shielding project or to verify fabric performance after field installation.

Applications in the Australian Industry

• Medical imaging facilities (Sydney, Melbourne, Brisbane, Perth): Shielding for MRI and fMRI rooms.
• Scientific research (ANSTO, Australian Synchrotron, universities): Geomagnetic shielding for electron microscopes and quantum sensors.
• Defence electronics (naval vessels, command centres): Low‑frequency magnetic signature reduction.
• Industrial power electronics (transformer vaults, substations): Shielding against 50 Hz magnetic fields.
• High‑voltage laboratories and calibration centres: Controlled environment for magnetic measurements.