The Good, The Bad, and The Ugly
The Basics Materials that have been successful targets for eddy current sensors vary in resistance from 1.59 µOhm-cm for
silver (good), to 127 µOhm-cm for Inconel (can be bad). Depending on application requirements, even graphite at 1050 µOhm-cm
(usually ugly) has been used for a target. Magnetic / Non-Magnetic Distinction A small diameter sensor (<0.5") set up for a magnetic target will perform poorly (if at all) with a non-magnetic target, (and vice-versa). In most cases electronic bridge components must be changed in order to convert a system set up for magnetic targets to be used with non-magnetic targets. Large diameter sensors (>0.5") typically need only to be recalibrated to the new target material. It is best to check with Kaman's Applications Engineering Group, or Service Group before considering any target changes. Kaman can provide sensors calibrated to your specific target material. In some instances the exact target (shaft, disc, stage component) should be provided to ensure best possible system performance. When no target material is specified, non-magnetic systems will be set up and calibrated using a 6000 series aluminum target. For magnetic systems, 4130 steel is used. Various target materials and their resistivity values are listed below. Materials underlined are considered magnet targets. This is by no means a complete list of materials that can be successfully used as targets with eddy current sensors.
Inductive 'eddy current' displacement sensors require an electrically conductive target.
The coil is excited by a high frequency AC signal. The less resistive the target, the more eddy currents are
produced in the target. This improves sensitivity and signal to noise ratio, ultimately increasing resolution.
As they pertain to eddy current position sensors, all conductive material can be categorized into two groups, magnetic (permeability >1), and non-magnetic (permeability of 1). Note that for eddy current sensors, permeability of up to 1.1 is still considered non-magnetic.
| Material |
Resistivity µOhm-cm |
| Silver |
1.59 |
| Copper |
1.7 |
| Gold |
2.35 |
| Beryllium |
4.3 |
| Aluminum |
4.5 |
| Molybdenum |
5.17 |
| Iridium |
5.3 |
| Tungsten |
5.51 |
| Cobalt |
6.24 |
| Zinc (cast) |
6.5 |
| Brass |
7.4 |
| Cadmium |
7.54 |
| Nickel |
7.85 |
| Bronze |
9.8 |
| Platinum |
10.6 |
| Palladium |
10.8 |
| Tin (cast) |
10.8 |
| Magnesium |
13.6 |
| 1030 Steel |
14 |
| Carbon Steel |
17.5 |
| Lead |
22 |
| Cobalt Steel |
28 |
| Chrome Steel |
29 |
| Tungsten Steel |
30 |
| Uranium |
37.5 |
| Mu-metal |
60 |
| 400 Series SS |
60 |
| 416 Steel |
60 |
| Iron (cast) |
65 |
| 4130 Steel |
65 |
| 200 Series SS |
69 |
| 300 Series SS |
72 |
| 17-4PH SS |
100 |
| Titanium |
113 |
| Inconel 625 |
127 |
| Inconel 718 |
127 |
| Inconel 725 |
127 |
| Graphite |
1050 |
Kaman's Applications Engineering Group is available to evaluate your application.
(R) Kaman Measuring Systems - All rights reserved, 2008.