Refractometer question

I've noticed that when I use my refractometer to check my pre-boil and post-boil gravity that if I put a sample of hot wort on the refractometer the reading will be lower at that moment than it will be 5-10 min later. I had assumed this was due to the wrote cooling to room temperature, but I started wondering if it was from the sample evaporating and concentrating on the device. Any experts out there who can clue me in?

L

Not an expert. More than likely your refractometer, unless it is very old is ATC, automatic temperature corrected. Which is a little misleading because it doesn't work at 212 however the small sample taken will cool down very quickly to the refractometer's calibrated range which is somewhere around 60-80 degrees.

I usually check the reading after about 30-40 seconds, sometimes a bit longer. Shouldn't take 5-10 minutes. You could try testing it with boiling tap water and see how long it takes to read 1.000, or whatever your tap water reads at ground water temps.