I've used mine in mash temp wort, was fine. That being said, just pull a sample and let it sit a few minutes. The small amount of wort cools off pretty quickly. Or even put it into a fridge or freezer for a few minutes to speed things up. I get inconsistent and inaccurate results with my refractometer, so I've gone back to the hydrometer for everything. Make sure you correct for temperature to get an accurate reading.