Refractometer read 16.25 Plato, but when the sample cooled down, read 16.75.

[BrewVerymore]

Hello.

Today I was doing a brew and was reading the samples straight out of the boil kettle. I was taking samples out of the kettle with a cup and then from that cup, I was dripping it onto the refractometer. On my Final reading I read 16.25 on a hot sample (albeit a bit of a cloudy line), however, after letting that sample sit for about 15 minutes, I dripped it onto the refractometer again. This time it was 16.5 and the line was gradually becoming blurrier, but running towards the 16.75 mark.

My question is which reading is the more accurate one?

 

[govner1]

I'd say the cooled sample is more accurate. The ATC on a refractometer doesn't adjust for the temp of the sample but for the temp of the refractometer. I generally take a sample w/ a plastic pipette and cool the sample in the bulb of the pipette before taking a reading.

 

[shortyz]

always cool to 21c for accurate measurments with a refractometer.

 

[BrewVerymore]

Thanks a bunch guys!

 

[doug293cz]

If you let the sample cool in a open cup, you probably lost some water due to evaporation. This would cause the reading to be higher.

As @govner1 suggests, you should sample with a pipette, and let it cool in the pipette a little before putting a couple of drops on the refract window. You don't have to wait for the sample to cool much, as the few drops on the refract will cool quickly to equilibrium with the refract. Close the cover on the refract quickly with a warm sample to minimize any evaporation of water prior to reading.

Brew on