Brake fluid moisture tester (with schematic)

Your car's braking system uses hydraulic pressure to activate the pistons that push the brake pads against the rotors. It's important that the fluid is routinely checked for progressive moisture absorption, as the fluid is usually glycol based and very hygroscopic.
Water contamination of the glycol can result in a lower boiling point and liberation of bubbles of gas in use. That gas is more compressible than liquid, so it can make the brakes less effective.
This tester can be dipped into the brake fluid reservoir to give an instant indication of the fluid's condition, by measuring its electrical conductivity.
This unit used very different circuitry than I was expecting. It took a surprising amount of time to reverse engineer it. The unit claims to be suitable for DOT3, 4 and 5 brake fluid, although DOT 5 is actually oil based. I wonder if they mean DOT5.1 which is glycol based.
On closer examination, the markings on the side are a measurement scale in mm. Maybe to indicate depth in the liquid.
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