Why does my Vbe multiplier voltage oscillate when wet?

You have inductance in the collector lead, from the probe wiring.

You have capacitance between collector and base: expect 10 or 20 pF of Cob. Being wet brings the dielectric constant of water into that wire_wire capacitor of base_collector leads, boosting that Cob

At high frequencies, the BETA is rolling off, adding an extra pole.

I suggest you insert 1uH in the collector lead. Add 10 or 20 pF from collector to base.

Resim.

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Nov 9

Given the variations, initially labeled as "oscillations" are extremely slow, I have to agree with Brian Drummond --- electrolysis.

Good experiment with detailed measurements!

We have no evidence that it's oscillating (in the sense of an L-C or R-C oscillator) though an oscilloscope may reveal something a DMM can't.

It was clearly still drying out. One source of periodicity may be the formation and release of bubbles of H2 and O2 under electrolysis, probably across the C-B junction thanks to its higher voltage.

If you made both 5:1 and 1:1 Vbe multipliers, and operated them both wet with the terminals visible, you may be able to see bubbles on the higher voltage one, and that would allow you to test this hypothesis. You may see a correlation between bubbles and voltage reading.