Skin Depth Frequency Table

SKIN DEPTH FREQUENCY TABLE

This table gives you the frequency (in kHz) at which a given alloy and material thickness will have a skin depth of 1.

It is recommend that this frequency and 2 times this frequency be mixed for a support plate cancellation when performing eddy current inspections of heat exchanger tubes with the given alloy and wall thickness.
For example, a 90/10 CuNi tube with a wall thickness of 0.025in. would have skin depth frequency of 111kHz.

Inch 0.01 0.018 0.02 0.022 0.025 0.028 0.032 0.035 0.042 0.049
mm 0.254 0.457 0.508 0.559 0.635 0.711 0.813 0.889 1.067 1.245
BWG 31 26 25 24 23 22 21 20 19 18
Alloy r ohm-meter ———————— Frequency in kHz ——————————-
Copper 1.72E-08 68 21 17 14 11 8.6 6.6 5.5 3.8 2.8
AL2024T4 5.77E-08 226 70 57 47 36 29 22 18 13 9.4
Adm Brass 7.18E-08 282 87 71 58 45 36 28 23 16 12
Al Brass 7.50E-08 294 91 74 61 47 38 29 24 17 12
90/10 1.77E-07 695 215 174 144 111 89 68 57 39 29
Chromium 1.96E-07 769 237 192 159 123 98 75 63 44 32
70/30 3.83E-07 1504 464 376 311 241 192 147 123 85 63
Zirc 5.07E-07 1991 614 498 411 318 254 194 162 113 83
Titanium 5.56E-07 2184 674 546 451 349 279 213 178 124 91
SS 304 6.90E-07 2708 836 677 559 433 345 264 221 154 113
SS 347 7.18E-07 2821 871 705 583 451 360 275 230 160 117
Inconel 1.01E-06 3981 1229 995 823 637 508 389 325 226 166
Tit6A1-4V 1.72E-06 6769 2089 1692 1399 1083 863 661 553 384 282
Graphite 7.84E-06 NA 9497 7692 6357 4923 3925 3005 2512 1744 1282

Inch 0.058 0.065 0.072 0.083 0.095 0.109 0.12 0.134 0.148 0.165
mm 1.473 1.651 1.829 2.108 2.413 2.769 3.048 3.404 3.759 4.191
BWG 17 16 15 14 13 12 11 10 9 8
Alloy r ohm-meter ———————— Frequency in kHz ——————————-
Copper 1.724E-08 2 1.6 1.3 0.98 0.75 0.57 0.47 0.38 0.31 0.25
AL2024T4 5.766E-08 6.7 5.4 4.4 3.3 2.5 1.9 1.6 1.3 1 0.83
Adm Brass 7.184E-08 8.4 6.7 5.4 4.1 3.1 2.4 2 1.6 1.3 1
Al Brass 7.50E-08 8.7 7 5.7 4.3 3.3 2.5 2 1.6 1.3 1.1
90/10 1.77E-07 21 16 13 10 7.7 5.9 4.8 3.9 3.2 2.6
Chromium 1.96E-07 23 18 15 11 8.5 6.5 5.3 4.3 3.5 2.8
70/30 3.83E-07 45 36 29 22 17 13 10 8.4 6.9 5.5
Zirc 5.07E-07 59 47 38 29 22 17 14 11 9.1 7.3
Titanium 5.56E-07 65 52 42 32 24 18 15 12 10 8
SS 304 6.90E-07 80 64 52 39 30 23 19 15 12 10
SS 347 7.18E-07 84 67 54 41 31 24 20 16 13 10
Inconel 1.01E-06 118 94 77 58 44 34 28 22 18 15
Tit6A1-4V 1.72E-06 201 160 131 98 75 57 47 38 31 25
Graphite 7.84E-06 915 728 594 447 341 259 214 171 140 113