Fluke PM6306 LCR Meter: can not compute impedance from R and C

[sjlukacs]

hello one and all,

so i'm connecting a raspberry pi to a Fluke PM6306 LCR meter via GPIB.  and i'm trying to do a very simple thing.  i put a capacitor into the test clips, hit AUTO and the meter measures and displays capacitance in uF as the primary display and either the impedance or phase as the secondary display. 

ok, so in python i'm connected to the meter and setting the frequency and then grabbing the resistance, R, the capacitance, C, and the impedance, Z.  i'm then using R and C to compute the impedance to see if the computed value matches the meter's impedance and it is not close at all.  the code is:

   Xc = 1/(2*pi*f*C)
   impedance = sqrt(R**2 + Xc**2)

where f is the frequency Hz, pi is 3.14159, and R and C are returned from the meter's interface.  i believe these are the correct equations.

i'm getting like:

f: 3000.0, Z: 12.637, Zcomputed: 124.54949566680497, R: 123.9, C: 4.1763e-06

where Z at 12.637 is no where near Zcomputed at 124.549.

what am i missing here?  thank you in advance, lucas

[2N3055]

You might look at the notions of serial and parallel equivalent circuits of impedance.

Some quickly found texts on the topic. Chroma white paper is very good.

https://assets.testequity.com/te1/Documents/pdf/series-parallel-impedance-parameters-an.pdf
https://www.allaboutcircuits.com/textbook/alternating-current/chpt-5/series-parallel-r-l-and-c/
http://www.freeclassnotesonline.com/Equivalent-Circuits-Lab.php

In short, if you connect only capacitor, you will have complex impedance from predominantly reactance of capacitor, in series with small ESR of capacitor.
You need to measure in serial mode.

At 3 kHz, 4.1763uF will have 12,7-j  Ohm and  ESR in series should be much less than 123,9 Ohm. Aluminum elco might have 10 Ohm ESR, ceramic or block capacitors fraction of the Ohm...

You can verify all this with an oscilloscope and signal generator and a single resistor if you have the equipment..
There are even applications for PC where you connect LCR to a simple circuit connected to audio card and can do measurement.
I mention that only for educational purpose. You have Fluke PM6306 so use that. Of course, that is if Fluke PM6306 is fully operational and in the spec.
If not, a sanity check might be in order, to verify whether it is operator or your instrument is not OK.

Fluke PM6306 in good shape is top of the line instrument. Take time to learn to use it properly. It will be worth it.

[sjlukacs]

yes, that was it.  putting the lcr meter in serial mode made all of the computations match and finally make sense.  thank you so much for your help.  lucas