Impedance of capacitance probe buildups and coatings

[MakingAComeback]

According to the literature of one manufacturer their capacitance-based level instrument using RF admittance techniques is able to distinguish between the capacitive RF current through the medium's level as well as both the resistive and capacitive components of the RF current through the buildup on the probe. They claim that by "physical law" the resistance and capacitance of any coating are equal. Is that claim factual? If so, it's a new one on me.

 

[7anoter4]

If the capacitance sensor is of the type of a variable capacitance sensor then you would recalibrate it
for different liquids. But, if the capacitance sensor is of multielectrode capacitance sensor type there is no need for calibration. More advanced microprocessor-based units are self-calibrating; sensitivity and time delay adjustments are under pushbutton control. These units are often supplied with self-test capability and built-in temperature compensation.

If the sensor is SS [stainless steel] made no coating is necessary.
In case where the liquid is highly conductive or corrosive then capacitance probes typically are coated with PFA[Teflon PFA] or PTFE[Teflon], Kynar, or polyethylene.
But, even in case of PTFE [Teflon] coating, the thin film of this dielectric material does not produce an important error in level measurement.
Some point level sensors are available with build-up immunity, or coating rejection functionality. This is required when the process material is wet or sticky and likely to cause permanent coating. Build-up immunity is provided by the addition of a second active section of probe and a second insulator.
I don’t know why it is necessary to measure the liquid specific resistance, but as I know from test of HV electric machine insulation it is possible to measure tangent-delta= Xc/R and so one may appreciate the resistance.
See:

http://www.omega.com/literature/transactions/volume4/T9904-13-RF-CAP.html

http://www.china-sense.com/en/UploadFiles/2007121114923274.pdf

Regards

 

[MakingAComeback]

I did more research and found some interesting details that had been left out of the original material that led to my question.

Apparently a buildup of the measured medium on a capacitance probe can be modeled as a long string of infinitesimal series resistances with infinitesimal capacitance to ground at each junction between the resistances. It's said that this is very similar to a transmission line model. It's further stated that if the buildup is at least several inches long the transmission line math will show that the magnitudes of the resistive and reactive components of the characteristic impedance will be identical.

The level transmitter's circuitry can identify the resistive portion of the total current which includes the current through the medium and the current through the buildup. When the total capacitive reactance is calculated from the total current an amount of capacitive reactance equal in magnitude to the resistance is subtracted. This produces the value of the capacitance created solely by the level of the medium and effectively brushes off the effect of the buildup (pun intended).

If this is really how it's done I would love to see the math that shows the resistive and capacitive components of the equivalent impedance of the buildup model are equal.

Would anyone out there care to take a stab at it?