I received my refractometer today. (Justification: I wanted to check that my 'shake-test' honey was not too watery.)
Checked honey - 18.5% (Hurrah!)
Got some olive oil. It read 27%
Stored the olive oil in a little bottle and labelled it: Reference oil, 27%.
Now when I need to check something with my refractometer, I can use some of MY olive oil (yours may be different). If it reads 27%, I can go ahead and test the honey (or whatever) knowing that my refractometer is still calibrated.
If it reads something other than 27% for MY olive oil, I know that I need to calibrate it so it reads 27%.
Chances are, if I don't bang my refractometer around by playing shove ha'penny, it will remain calibrated without any further action.
It is that easy.
Why it has required three threads in the past fortnight, with people debating the merits of using prune juice, liquid kryptonite, the venom sac of the lesser spotted cobra, or anything else I don't know.
Hrumph!