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:
- Inductive probe excitation frequency,
- target material electrical resistivity, and
- target material magnetic permeability.
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 | |||