[narvin]

I tried measuring orange juice on my refractometer after rinsing the prism under cold and hot water. Cold prism ---> 10.3 Brix; hot prism ----> 9 Brix. I never normally rinse with hot, but the swing away from 10 (average for OJ) is large enough to make me think that's part of the problem.

That's a good point.  If you leave the refractometer at room temperature for a bit, does it return to the same value?

[Phil_M]

I have a digital refractometer where the sampling area is open, so evaporation would likely bite me long before a standard refractometer user.

[charles1968]

I tried measuring orange juice on my refractometer after rinsing the prism under cold and hot water. Cold prism ---> 10.3 Brix; hot prism ----> 9 Brix. I never normally rinse with hot, but the swing away from 10 (average for OJ) is large enough to make me think that's part of the problem.

That's a good point.  If you leave the refractometer at room temperature for a bit, does it return to the same value?

Yes, or if dipped in a container of water at room temp it reverts quickly.

[charles1968]

I have a digital refractometer where the sampling area is open, so evaporation would likely bite me long before a standard refractometer user.

Probably best to read it immediately after sampling.

[narvin]

I tried measuring orange juice on my refractometer after rinsing the prism under cold and hot water. Cold prism ---> 10.3 Brix; hot prism ----> 9 Brix. I never normally rinse with hot, but the swing away from 10 (average for OJ) is large enough to make me think that's part of the problem.

That's a good point.  If you leave the refractometer at room temperature for a bit, does it return to the same value?

Yes, or if dipped in a container of water at room temp it reverts quickly.

So it sounds like the culprit for cheapo refractometer inconsistency is ATC.  If you keep the unit at room temperature, that shouldn't be an issue, since the mass of the prism is much greater than a few drops of wort would be able to change.

[charles1968]

I think it's a factor but not convinced it's the whole story until I do some more tests. Agree that sample drop temp doesn't affect the prism temperature much, but evaporation might be an issue for hot samples.

[HoosierBrew]

So it sounds like the culprit for cheapo refractometer inconsistency is ATC.  If you keep the unit at room temperature, that shouldn't be an issue, since the mass of the prism is much greater than a few drops of wort would be able to change.

Totally agree. And evaporation on hot samples definitely happens quickly, too - FWIW I cool my hot samples for a couple minutes in the freezer in an airtight plastic container, then take a reading. Just takes a couple minutes in the freezer to be @ room temp. My refracto stays @ room temp as well.

[charles1968]

From Braukaiser's blog:

"BTW, the ATC of this refractometer is also broken. For every brewing session I have to re-calibrate it with water. Since this is done very quickly, it doesn’t bother me too much"

Sounds like similar issue - temperature of the prism. Not "broken", just used at wrong temp.

[Phil_M]

Totally agree. And evaporation on hot samples definitely happens quickly, too - FWIW I cool my hot samples for a couple minutes in the freezer in an airtight plastic container, then take a reading. Just takes a couple minutes in the freezer to be @ room temp. My refracto stays @ room temp as well.

That's my plan next brew day. The nice thing about being set up for yeast culturing is an ample supply of borosilicate culture tubes. I plan to use a tube and a pair of tongs to collect a sample, cap, then cool till testable. A small amount of liquid like that won't take long to cool at all.

[erockrph]

Totally agree. And evaporation on hot samples definitely happens quickly, too - FWIW I cool my hot samples for a couple minutes in the freezer in an airtight plastic container, then take a reading. Just takes a couple minutes in the freezer to be @ room temp. My refracto stays @ room temp as well.

That's my plan next brew day. The nice thing about being set up for yeast culturing is an ample supply of borosilicate culture tubes. I plan to use a tube and a pair of tongs to collect a sample, cap, then cool till testable. A small amount of liquid like that won't take long to cool at all.

I have several of the old White Labs vials laying around for just this purpose.

[brewinhard]

+1 to refractometer being kept at room temps for best accuracy (even with ATC).

and don't forget to use distilled water when calibrating any equipment like refractometers/hydrometers.  At least that is what my anal mind does.

[charles1968]

How does ATC work on a refractometer? Is there an actual mechanism or does it just rely on thermal mass of the prism being so much greater than the small sample?

[narvin]

How does ATC work on a refractometer? Is there an actual mechanism or does it just rely on thermal mass of the prism being so much greater than the small sample?

The drops of wort should equalize in temp with the device pretty quickly.  But, the refractive index of a solution varies based on the temperature.

The models with ATC are supposed to have something that compensate for this, some piece of metal that expands or contracts based on temperature of the unit, so a sucrose solution in water measured at any (reasonable) temperature is corrected to a room temperature reading.  However, I don't think that really works very well.

[narvin]

http://www.refractometer.pl/hand-held-refractometer

Besides not being all that accurate, I bet you're going to throw the mechanism off if you change the temperature of the prism by running it under water.  The bimetallic strip is designed to compensate the reading for ambient temperature, so if the prism (and, therefore, your sample) is at a different temp it won't work.  This could also be why a very hot sample is not accurate, since it probably ends up above room temperature even after hitting the prism.

[charles1968]

http://www.refractometer.pl/hand-held-refractometer

Besides not being all that accurate, I bet you're going to throw the mechanism off if you change the temperature of the prism by running it under water.  The bimetallic strip is designed to compensate the reading for ambient temperature, so if the prism (and, therefore, your sample) is at a different temp it won't work.  This could also be why a very hot sample is not accurate, since it probably ends up above room temperature even after hitting the prism.

Yes the prism needs to be same temp as the bimetallic strip, otherwise the sample droplet will equalize with a prism that's hotter/colder than the compensator.

My ATC refractometer instructions say don't immerse in water, so to clean a sample off the prism I clearly have to rinse in a stream of water at ambient temperature and then dry. PITA as it means preparing a refractometer-cleaning bottle, but I don't see an alternative. If I can convince myself it's accurate on first reading I will just take one reading and leave it at that. I can't remember where I read that it's necessary to take multiple readings and average them but I'm thinking that's bad advice.