I just set up one of my soundcards to do LCR measurements. I took an old radial lead cap from bench marked 1100 uF, 35Vdc, and I mean old. I purposely measured using too large a series resistor, 10k, which became immediately apparent from the 'ratty' results. Toned it back down to 220 ohm, still too large, but the results were 'cleaner'.
Did not verify the methodology a lot, easier to ask this knowledgeable group about the results. The results were somewhat as expected.
Cap measured, approx 890uF, with Resr approx 90 milliohms. with age and never having voltage previously applied, the low value seemed appropriate (maybe) the Resr was disappointly high, but as expected. too
But what I saw regarding characteristics vs freq I don't know are true, or are these artifacts of what I did.
The plot of C vs log(f) produced a rather linear straight line descending down as freq went up. The plot of log(Resr-offset) vs log(f) also had a similar straight line. with offset slightly less that 84 milliohms
QUESTION: Are these relationships real, or artifacts of technqiue and/or accuracy?
For example, the Q of this cap at 20 Hz is around 25, so it didn't seem reasonable that the soundcard couldn't pick up the phase well enough to discern the Resr there, but Resr had increased at the lower frequency to around 100-200 milliohms [forgot which now!]
Also, C droppoed to around 830uF going up infreq and looked like it was asymptotically approaching 900+uF going down to '0' freq. But again, the Xc is very small compared to the Resr as you go up, so the values could be in question.
My point is, are the values about right or should I comb through the measurement technique?
PS: Earlier I said you needed a good VTVM to measure voltage and resistance. WRONG! Using soundcard for measuring C you only need a pure resistance of known value. You don't need to know ANYTHING else. assuming your soundcard's frequency is accurate :) Should be able to get better than 0.1% accuracy, too.