Magnetic path length for a ferrite drum core.

[dr pepper]

As some will know I've been messing with drum cores.
How do I work out approximately the magnetic path length for a drum core, I need to work out an approximate saturation flux level for my particular core (I only have al factor and bsat parameters nothing else).
Do I assume that the core has enclosed sides or is the path length 2 x each 'C' section.
(the former looks like a cotton reel having 2 'C's back to back cross sectional view).
I dont know permeability so I'm not able just to stick on 10 turns make some electrical measurements and calculate path length.

 

[Mike odom]

2 unknowns needs two equations... you could do multiple turns and multiple measurements and then calculate both path length and perm.

 

[dr pepper]

I could go about this a different way, instead of working out how many turns brings me close to saturation at a given current I can work out how many turns brings me close to saturation at a given voltage by rearranging faradays law.

This isnt so difficult, but it wouldnt help much if I was designing a dc inductor (which I'm not I'm designing a transformer), and I'd like to know how to measure path length.