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Looks relatively simple to me.? Knowing the area of the plates and the spacing between them, you can easily calculate the dielectric constant of the material between the plates.? You have a fairly good picture of the disassembled fixture, and more details can be found here:



That paper tells us that the electrodes are nickel plated cobalt (Fe 54%, Co 17%, Ni 29%), the insulator is Alumina (Al2O3), and the O-ring is
Viton (Fluro rubber).

There are a number of papers available, both on Keysight's website and other places, on measuring dielectric constant of water and other liquids, including papers from NBS (now NIST).? There are published tables of dielectric constants of dozens of materials.? I would probably start by reading all the available literature (there's a fair bit on the Keysight website, and likely more available in old HP literature and elsewhere), and then I would build a fixture and start making measurements.? That, of course, is where you would start finding the "unknown unknowns" (a term used by engineers long before anybody ever heard of Donald Rumsfeld).? As problems show up, you simply attack them, one by one.? An engineer I once worked with called this, appropriately, "peeling the onion."? Just poking around the web, I see that the dielectric constant of some liquids varies with temperature and pressure, so you'd want to factor that into your measurements.? (That would be one layer of the onion.)

I was once involved in the development of a soil moisture meter.? Most soils have a dielectric constant around 4, while water has a dielectric constant of 80 at 20° C, as I recall.? The dielectric constant of the soil/water matrix is therefore mildly dependent upon the type of soil, and highly dependent upon the concentration of water in the matrix.? Fascinating stuff.

Do let us know if you find anything interesting.