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Sorry for the simplicity of the question, but I asked Google and she didn't know. Stackexchange is my next stop.

I took a brix reading of my steam beer wort at 13 brix (~1.052 SG), but it was probably about 110 degrees.

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edited! It was meant to read brix – Pietro Aug 20 '12 at 14:14

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It's not usually necessary to cool - most refractometers have built in Auto Temperature Compensation (ATC) to correct for temperatures up to 30C/86F. However, for hot samples in a warm environment then some cooling may be needed.

When the ambient temp is cool (<20C/70F) the ATC will be sufficient and that the quantity of liquid is small and has a very small thermal mass compared to the body of the refratometer. So, it will cool quickly from mash temps, while only warming the refractometer by just a few degrees.

But if the ambient temp is close to the upper range of the ATC, then the additional sample may warm the body above what the ATC can handle. Then cooling is needed.

So, if in doubt, leave the refractometer for a few minutes. I've noticed a change of at least +0.5% brix after a couple of minutes when sampling hot mash samples, presumably because the body temperature rises above the level that ATC will correct.

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ATC is meant to compensate for the temp of the refractometer, not the sample. – Denny Conn♦ Aug 18 '12 at 20:25
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Sure, but in practice it amounts to the same thing - it doesn't take long for the sample and refractometer to equalize. And should the body temperature increase a few degrees because of the warm sample, the ATC will take care of that. – mdma♦ Aug 19 '12 at 6:52
I would second always letting the sample cool on the refractometer for a few minutes before reading, I've been burned by this before. – hartski Aug 19 '12 at 13:02
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mdma, I'm not so sure about that. Ever tried using a refractometer in a 35F garage? If you don't keep it warm before you use it, the sample will cool but I still get an incorrect reading due to a cold refractometer. – Denny Conn♦ Aug 19 '12 at 15:16
I have seen something similar, and I agree. You have to be sure the temp is within the range the ATC can handle. The refractometer usually includes a spec for the temperature range over which ATC will compensate - 10-30C/50-86F is typical, so 35F is too cold for the correction to work, which is why you need to warm it up to get an accurate reading. At room temperature, the ATC will work properly, and will also compensate for the slight raise in temperature created by the warm sample, but of course only if the equilibrium temp is below 30C/86F. – mdma♦ Aug 19 '12 at 17:52

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