Refractometer Accuracy ??

[Alpha King]

I am having a problem getting an accurate S.G. reading when sampling pre-boil wort (7 bbl system). Pulling a sample out at the beginning of a rolling boil (so that the wort is not stratified) the readings are consitently high by 2 to 3 degrees plato. The refractometer is supposedly ATC (automatic temp. compensation) and has been calibrated. When the same sample is pulled off and chilled to 60 degrees the hydrometer reading is always much lower and accurate. The refractometer also agrees with the hydrometer with a chilled sample. Anyone else run into this problem ?? Thanks

[CaptainEBC]

We see the same thing. If we have to add a little water towards the end of the boil to adjust for target S.G, we always shoot for 2 points higher than the final to compensate for the Refractometer reading and it comes out exact.

[Pompeiisneaks]

I've used a refractometer in homebrew usage and found the same problems. I think, though, that ATC ones do need you to let it sit for a few seconds to a minute to let that correction work. Also they don't work once you start fermenting, as the alcohol throws it really off. So they're only good for checking the wort prior to pitching... There are a few tools to adjust a refractometer measurement post ferment but they also seem to always differ quite a bit from my hydrometer.

EDIT: I also did a bit of digging, ATC refractometers don't take any sample temperature just fine, they adjust for different ambient room temperatures. You'll still want to cool the sample to a similar temperature (which on a few drops is pretty quick on a refractometer) Read this, pdf from a manufacturer, there are two notes on ATC, one on page one,(4?) one on page 3(6?). http://www.nationalmicroscope.com/im...(pp%204-9).pdf

That alone helps me and I'll have to test again... just put a few drops on the plate, let it cool for about a minute, and then check again. If you're testing post HX, then it probably should be fine though.

[kai]

Yes, typically from memory the ATC on a refrac only covers up to about 30°C. But, by the time you put a few drops on the lens of the refrac, your temperature has just dropped significantly closer to the temperature of the refrac itself. That's where refrac ATC is more relevant... ambient temperature rather than sample temperature.

But, if you find your refrac is more accurate on a cooled sample, well, cool the sample