Resistance measurements of particle layers in a resistivity cell, using a 6514

Hello, 

I am new to the use of electrometers and although I am familiar with the measurements concepts, I am struggling in getting resistance measurements.  

I have a resistivity cell for measuring the volume resistance of powders and liquids (built by us, photo attached, but very similar to this one: https://www.electrotechsystems.com/products/electrostatic-instumentation/resistance-probes-clamps/volume-resistivity-test-cells-for-liquids-and-powders/). The concept is to use known volumes of ash of different compositions, and measure their resistivity, 

I have for the measures a 6514 electrometer, that I understand uses a constant current for calculating a voltage drop depending on an unknown resistance, and using the Ohm law calculates the resistance, that I can then use for the volume resistivity knowing the dimension of my cell. 

I connect the HI and LO to my cell, and the ground to the metal enclosure that isolates the main cell. However, when in Resistance mode, my value oscillates constantly and not in a negligible way (e.g., from 0.38 to 0.8 GOhms or similar, oscillating up and down all the time but constantly going up). It goes up all the time before going down, and then up again. This using a compacted layer of particles for which I need to measure the resistivity. Is the issue in using a constant voltage here, or is there something I am missing? Should I instead connect a power supply to the chamber, and provide a voltage to the cell, and measure the resistivity based on the provided voltage? But then I should I do the connection?
Any help would be much appreciated,

All the best,
Antonio

For a powder I would recommend measuring resistivity by sourcing voltage and measuring current, yes. Since the 6514 doesn't have a built-in voltage source, this would mean using an external supply as you noted. For connections, place the 6514 as an ammeter in series with your voltage source and sample. Place the 6514 on the LO side of the system, so your circuit goes voltage source HI > sample > 6514 > voltage source LO.

Ideally the 6514 LO, not chassis/ground, would be connected to the metal enclosure and you would have another metal enclosure around that connected to chassis/ground. But I wouldn't worry too much about that until you're getting stable readings.

If you haven't seen them already, you might find sections 2.4 and 4.4.2 of the Low Level Measurements Handbook helpful: https://www.tek.com/en/documents/product-article/keithley-low-level-measurements-handbook---7th-edition#C4section3

Based on your description, you might be seeing capacitive behavior of the sample charging up then discharging, though it could easily be some other effect. Sourcing voltage and allowing plenty of settling time should return a more accurate result.