Technical Library

Minimum Recommended Eddy-Current Target Thickness

Eddy-Current TechNote LT02-0011

Copyright © 2008 Lion Precision. www.lionprecision.com

Summary

An eddy-current sensor’s field penetrates the target to a certain depth; therefore, there is a minimum target thickness for optimal results. This document lists the minimum thicknesses of many common materials and discusses the determining factors and calculation of minimum thickness.

Minimum Recommended Target Thickness


Field Penetration

Eddy-current sensing requires a minimum target thickness to allow sufficient penetration of the inductive probe’s electromagnetic field. The depth of penetration is determined by three factors:

Lion Precision inductive sensors typically use an excitation frequency of 1 MHz. 12mm and larger probes may use 500 kHz, depending on the calibration.

Calculating Minimum Thickness

Minimum target thickness is three times the target material’s “skin-depth.”

Skin-depth (δ):

δ=1.98[ ρ/(fµr)]^½ inches

δ=50.3[ ρ/(fµr)]^½ mm

minimum target thickness = 3δ

where:

ρ= electrical resistivity, μ-ohm - cm

f = oscillation frequency, hertz

μr = magnetic permeability

Field density decreases exponentially with depth (1/e). At three skin-depths eddy current density is about 5% of the surface density. Three skin-depths is the minimum target thickness suitable for optimum performance.

Exceptions

With proper system calibration and application, a target thickness less than three skin-depths might be measured effectively. Contact Lion Precision for details and testing.

Minimum Thickness (3 skin-depths) of some common materials



Nonferrous Materials

Material

ρ

μr

Probe (fMHz)

Minimum Thickness

mm

mils

Silver 1.59 1 U3 to U8 (1.0) 0.19 7.5
U12 to U50 (0.5) 0.27 10.6
Copper 1.71 1 U3 to U8 (1.0) 0.2 7.8
U12 to U50 (0.5) 0.28 11
Gold 2.21 1 U3 to U8 (1.0) 0.22 8.8
U12 to U50 (0.5) 0.32 12.5
Aluminum 2.65 1 U3 to U8 (1.0) 0.25 9.7
U12 to U50 (0.5) 0.35 13.7
Zinc 5.97 1 U3 to U8 (1.0) 0.37 14.5
U12 to U50 (0.5) 0.52 20.5
304 SST 72 1.01 U3 to U8 (1.0) 1.27 50.2
U12 to U50 (0.5) 1.8 70.9
Lead 20.8 1 U3 to U8 (1.0) 0.69 27.1
U12 to U50 (0.5) 0.97 38.3
Brass 6.4 1 U3 to U8 (1.0) 0.38 15
U12 to U50 (0.5) 0.54 21.3
Tin 11.5 1 U3 to U8 (1.0) 0.51 20.1
U12 to U50 (0.5) 0.72 28.5
Titanium 47 1 U3 to U8 (1.0) 1.03 40.7
U12 to U50 (0.5) 1.46 57.6

 

Ferrous & Magnetic Materials

Material

ρ

μr

Probe (fMHz)

Minimum Thickness

mm

mils

Nickel 6.4 1240 U3 (1.25) 0.01 0.4
U5 (1.5) 0.01 0.3
U8 (1.0) 0.01 0.4
U12 to U50 (0.5) 0.02 0.6
416 SST 57 700 U3 (1.25) 0.04 1.5
U5 (1.5) 0.04 1.4
U8 (1.0) 0.04 1.7
U12 to U50 (0.5) 0.06 2.4
Iron 11 100 U3 (1.25) 0.04 1.8
U5 (1.5) 0.04 1.6
U8 (1.0) 0.05 2
U12 to U50 (0.5) 0.07 2.8

 

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