Conductivity and resistivity conversion

The International Annealed Copper Standard fixes 100 %IACS as the conductivity of commercially pure annealed copper at 20 C: 58.00 MS/m, equivalently a resistivity of 1.7241 uohm.cm (17.241 nohm.m, or 1/58 ohm.mm2/m). Every other conductivity is quoted as a percentage of that fixed reference, so the conversions are exact arithmetic and not empirical fits: sigma[MS/m] = 0.58 x %IACS and rho[uohm.cm] = 172.41 / %IACS.

Which unit you meet depends on the equipment in front of you. Aerospace conductivity meters and North American procedures read %IACS; European instruments and most material data sheets give MS/m; and every skin-depth, limit-frequency and phase calculation wants resistivity. Getting the unit wrong by the factor 0.58 puts the standard depth of penetration out by 31 %, which is the difference between a wall being interrogated and being missed.

Conductivity is strongly temperature dependent – roughly 0.4 % per degree C for aluminium and copper – and the IACS reference is defined at 20 C. ASTM E1004 therefore requires readings to be corrected back to 20 C before they are compared with acceptance limits. Correct first, then convert.

In aluminium airframe work conductivity is used as much for material verification as for flaw detection. Heat treatment condition, artificial ageing, exfoliation corrosion and fire or overheat damage all shift conductivity by several %IACS, so the conductivity meter is often the accept/reject instrument in its own right and the conversion has to be exact.