What is the AC signal from a DC capacitance instrument?

[Ken F]

I am using a capacitance probe to measure the density of granular media.  During periodic density changes, the AC coupling signal correlates to the change.  Is the AC coupling signal an impedance or just the change in the DC signal?  Because the AC signal is a sine wave, is the signal inverted with respect to the DC signal?  Any thoughts or insight would be appreciated.  Thank you and have a great day.

[EadieU]

Do you have a specific brand or model of probe that you're referring to?  I can try to help you out, but it might be more appropriate to contact the probe's manufacturer.  

Not knowing anything about the specific probe in question, I can say that if the instrument returns an AC-coupled signal, it is likely just removing the DC offset to center the AC signal around 0.  

If you can post a link to the probe's product page, we can try to look at that to be of more help.  

Regards,

Mark E.

Applications Engineer

National Instruments

[Ken F]

Hi Mark E., Thank you for the reply.  Unfortunately the probe was custom built for a previous graduate student.  The controller for the probe is a Model AS-1521-RH, Measurement Technology Inc., Latham, NY.  Any thoughts?  Ken F.

[DavidMN]

Hi Ken F,

As this is a custom built probe, I do not know how it takes the measurement.  It is possible that the AC coupled signal is simply part of the probe's active response while forming the DC measurement.  I would get in touch with the party responsible for building the custom probe.

David

Applications Engineer

National Instruments