Cuprajake
I had bought a used one many moons ago. I haven’t used it for a quite a while (its time consuming and can be a rabbit hole), but should.
I do recall that it had to be calibrated with distilled water. The temperature of the surface of the refractometer glass and the temperature of the water had to be within a narrow band; I used an accurate digital temperature meter and thermistor wire to do that.
And that exact filtration of the espresso sample is critical. I have a bunch of plastic syringes with thick needles and filter tips for that, designed for extracting lab samples. I have the Amazon links if you need them.
All to much faff for me. Too much to do and too little time.
But I haven’t tested what I’m doing for a long time and am curious.
