Low Refractometer Reading During Boil

I have a handheld refractometer (model # RSG-100ATC) that I use to take gravity readings pre-fermentation. Aside from wasting less beer for measurements, I especially like the ability to monitor gravity during the boil. The problem I have recently run into is that lately it seems the gravity readings during the boil come in much lower than after the wort has cooled and is sitting in the fermentor. For instance, yesterday I brewed a Helles that measured 1.048 at the end of the boil (my target) but once it was chilled down to 50 degrees and ready for yeast it measured 1.055. This has happened a few times now. Am I simply overestimating the "automatic temperature correction" feature of the refractometer and should chill wort down before taking readings from the boil kettle? Anybody else run into this problem?

I've seen that as well. I usually run the pipette under cold water for a few seconds, or rest it in a cup of star san that I usually have nearby for a few seconds. It is such small sample that it cools very fast. I think the ATC works in a fairly small range, so the same should at least be close the the calibrated temp. At least that's my experience.

I always let it sit for a few minutes, sometimes the brix changes as the temperature equalizes.

Even ATC doesn't work properly at extreme ends of the temp scale. I remember an old post from AJDelange on why, even with ATC you should still test ph at room temp anyway, I know that's not directly applicable here to a refractometer, but if you're taking a beer sample with a small plastic pipette just set it aside for 2 minutes and then drip the liquid onto the refractometer after it's cooled down. The small quantity of liquid cools very rapidly.


Adam