A 5-digit Benchtop Multimeter.
These things normally run at $2000+, but I found a used electronics shop (
https://accusrc.com/) selling an old 1990s model (the Fluke 45) for only $120. Albeit with some parts missing, a few broken bits of plastic, and a dim screen... but hey, it actually works so I'm happy (and at a small fraction of the costs of a new 5-digit multimeter).
Seeing 500.00 mV measurements is pretty crazy when I think about how accurate/precise the parts are inside of this multimeter.
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I'm measuring odd effects in my circuits in the ~50-microvolt range, the ADCs of my chips seem to be messing up at this range and I'm not understanding why. Then again, this is an effect that I can only see because I have such a precise multimeter now. So maybe I should stop caring about such a minor difference and call my current project complete. I was aiming at 99.9% accuracy, but I'm closer to 99.7% accurate even after calibration, and I'm not really sure where this little bit of error is coming from.
99.7% is... still pretty good though and I'm proud of my results.