Cores with a higher path length to cross-sectional area should have higher turn counts to mitigate the effects of the wire. Five to ten turns are recommended through the aperture of the core or around the center post in the case of a mated geometry or multi-aperture type core. This will prevent the permeability measurement from being skewed high (especially in low permeability materials) by the inductance of the wire itself. Multiple turn windings should be used to mitigate the significance of the inductance added by the WIRE in the measured inductance of the core assembly. Once the L 0 for the core has been calculated, the inductance must be measured. Toroidal type cores are the easiest to calculate due to their uniform cross-sectional area, circular path length, and lack of air gap. The equation to calculate L 0 is as follows: In this equation N=the number of turns on the core. This parameter is needed to calculate air core inductance denoted as L 0 (H) for the core in question. The goal here is to calculate core parameter C 1 (cm -1) which is the effective path length (∑ l) divided by the cross-sectional area (A e). The first step in evaluating initial permeability in a closed magnetic structure is to measure the main physical dimensions of the core. “Open” magnetic structures can be evaluated for permeability as well though the ability to differentiate between different material grades depends heavily on the dimensions of the core and the winding applied. Cores with air gaps in the structure can be evaluated as well though the ability to differentiate between material grades is reduced as the length of the gap increases. By far the easiest geometry to evaluate would be a toroidal type shape though any core with a closed magnetic structure can be evaluated with a reasonably high degree of accuracy. With a bit of extra effort, a signal source along with an oscilloscope could be used.

In general, an LCR meter is the optimal piece of equipment to make this measurement. To evaluate the permeability, you will need insulated wire and some way to measure inductance with a 10kHz sinusoidal signal (in a pinch 1kHz-100kHz should be about the same for most materials). Calculating the initial permeability on the sample in question is the most reliable way to get an approximation of what material it is made of. This is the inductive portion of the complex permeability of a material measured specifically at 10kHz with a flux density of less than 10 gauss (sinusoidal excitation).

The main characteristic differentiating Fair-Rite’s material grades is initial permeability.

Ferrite toroid inductance calculator