percent IACS
Electrical conductivityexact by definition
Percent IACS expresses conductivity as a fraction of the International Annealed Copper Standard, which is defined as exactly 58 megasiemens per metre at 20 degrees Celsius. One percent IACS is therefore exactly 0.58 MS/m. Eddy-current conductivity meters read directly in these units, and aluminium alloys are sorted and heat-treatment verified by them: 6061-T6 sits near 43 percent IACS and 2024-T3 near 30.
Watch out: The reference is defined at 20 degrees Celsius, and metal conductivity falls about 0.4 percent per degree, so an eddy-current reading taken on warm stock and not corrected will read low. IACS is also a conductivity ratio, not a resistivity ratio: 50 percent IACS is twice the resistivity of copper, not half.
| 1 %IACS | 5,800,000,000 μS/cm |
The standard was fixed in 1913 on the best commercially annealed copper of the day. Modern refining beats it, so good electrolytic tough pitch copper measures 101 to 103 percent IACS, and oxygen-free grades a little higher still. A figure above 100 percent is not an error.