On refractometer readings...

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bushwilliams

bushwilliams

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Okay, so here is the question, likely answer is simple. I take a reading, say of my OG. The reading may be 1055 however if I set down the refractometer and forget about it for an hour it may have crept up to 1065. Is this normal? My gut (and expected numbers) tells me the initial 1055 is the correct choice, but then why would it float like that? Is the ATC (I have ATC) just getting wonky as the bead sits on the glass? Your thoughts are as always, appreciated.

Note (ATC Refractometer, calibrated with RO/DI to 0.00 on each brew day)samples in question are pulled from 150-170 degrees and read immediately. When allowed to cool they are increasing.
 
I'm just guessing here, but maybe evaporation of some liquid may leave a slightly denser wort sample. Again, just a guess - I may be WAY off.
 
Hmm that makes a little too much sense now that you lay it out there. This is good, my gut told me those initial numbers were right (especially since they are usually the ones I am trying so hard to hit) thanks for your input, its great to have faith in my gear.
 
Might also check the instructions. I believe most ATC's on refractometers have a range where the ATC works.
 
I just did a brew the other day. Took OG reading right away w/hot wort and got a readding of something like 14 brix. Waiting about 1 min I get a reading about 20 brix. Never thought about evaporation. I've been using the higher number because I figure that the hot wort has to cool a bit. I'll never have wort at 60* which I understand is what the meters are calibrated for.
 
C-Rider said:
I just did a brew the other day. Took OG reading right away w/hot wort and got a readding of something like 14 brix. Waiting about 1 min I get a reading about 20 brix. Never thought about evaporation. I've been using the higher number because I figure that the hot wort has to cool a bit. I'll never have wort at 60* which I understand is what the meters are calibrated for.
Click to expand...

What? Don't you have refrigeration in Hawaii? Take a small sample in a metal measuring cup and set it on a cube of ice or in a small container of ice water. You can cool it quite quickly that way. Be sure to have a thermometer so you can take it off the ice when it gets to 60.
 
Also, ATC compensates for the temperature of the refractometer, not the sample. You don't need to wait long for the few necessary drops to cool, but you will get a low reading if you try to measure a hot sample right away.
 
bpolivka said:
Also, ATC compensates for the temperature of the refractometer, not the sample. You don't need to wait long for the few necessary drops to cool, but you will get a low reading if you try to measure a hot sample right away.
Click to expand...

Very interesting! I thought ATC was for the sample, not the temp of the refractometer. How long would it take for a drop of wort to cool? I'm guessing not too long.
 
bpolivka said:
Also, ATC compensates for the temperature of the refractometer, not the sample. You don't need to wait long for the few necessary drops to cool, but you will get a low reading if you try to measure a hot sample right away.
Click to expand...

Really? I take a sample and look at it then write down the gravity, Then I look again and write down the brix. During this time nothing changes. I wipe the lens clean and take another sample. I have not seen any changes. I have even waited a few minutes and looked again and it is still the same. I don't know what I am doing wrong/right.
 
RM-MN said:
What? Don't you have refrigeration in Hawaii? Take a small sample in a metal measuring cup and set it on a cube of ice or in a small container of ice water. You can cool it quite quickly that way. Be sure to have a thermometer so you can take it off the ice when it gets to 60.
Click to expand...

Never thought of that. Gotta try next brew on Thursday.
 
just take your sample, and dip the bulb of the pipet into water, the sample size is so small that it will cool rather fast.

-=Jason=-
 
ATC is auto temp correction. (edit - it works because there is a bimetallic strip, like what's in a thermostat, that changes length/curvature depending on how warm/cold it is...this moves a lens that's focusing light from the prism, changing where the "blue line" falls on the scale)

bpolivka is exactly right. This corrects the reading for changes in temperature for the refractometer. See, the reading from a refractometer depends on the refractive index of both the sample and the thing the sample rests on, (the prism). ATC corrects for the fact that the RI of the prism changes with temperature.

We can put hot wort on a refractometer because the amount we put on is very small. It cools quickly, and everything is OK.

PROBLEMS:

If you dump a whole bunch of hot wort on the refractometer, like if you put 20 drops on it, you'll heat it up too much and get funny readings.

If you take multiple readings within a short period, you'll heat it up and get funny readings, (I try to rinse the prism with room temp water between samples to get it back to a good temp).

Likewise, washing it with hot water, then taking a reading, will screw up your reading.

(noticing a trend?)

Keep the prism and refractometer body at a decent temp, (within 10 or 20 deg of room temp is a good rule of thumb, but there are usually designated ranges that ATC can work in). Use the minimum amount of fluid required to take a sample, and wait a few seconds at least, (I wait 30 to 60 seconds at least) between placing the sample on the prism and taking a reading.

By the way, evaporation is REALLY minimal. When you have that coverplate over top of the prism, only the edges are exposed to air and able to evaporate. So you have minimal surface area exposed for evaporation. Add in the fact that you'd have to have the whole width of the sample change SG to get a different reading, and that you're now mass transfer limited for water to move to the edge of the coverplate to evaporate, and it's a non-starter.
 
shortyjacobs said:
ATC is auto temp correction. (edit - it works because there is a bimetallic strip, like what's in a thermostat, that changes length/curvature depending on how warm/cold it is...this moves a lens that's focusing light from the prism, changing where the "blue line" falls on the scale)

bpolivka is exactly right. This corrects the reading for changes in temperature for the refractometer. See, the reading from a refractometer depends on the refractive index of both the sample and the thing the sample rests on, (the prism). ATC corrects for the fact that the RI of the prism changes with temperature.

We can put hot wort on a refractometer because the amount we put on is very small. It cools quickly, and everything is OK.

PROBLEMS:

If you dump a whole bunch of hot wort on the refractometer, like if you put 20 drops on it, you'll heat it up too much and get funny readings.

If you take multiple readings within a short period, you'll heat it up and get funny readings, (I try to rinse the prism with room temp water between samples to get it back to a good temp).

Likewise, washing it with hot water, then taking a reading, will screw up your reading.

(noticing a trend?)

Keep the prism and refractometer body at a decent temp, (within 10 or 20 deg of room temp is a good rule of thumb, but there are usually designated ranges that ATC can work in). Use the minimum amount of fluid required to take a sample, and wait a few seconds at least, (I wait 30 to 60 seconds at least) between placing the sample on the prism and taking a reading.

By the way, evaporation is REALLY minimal. When you have that coverplate over top of the prism, only the edges are exposed to air and able to evaporate. So you have minimal surface area exposed for evaporation. Add in the fact that you'd have to have the whole width of the sample change SG to get a different reading, and that you're now mass transfer limited for water to move to the edge of the coverplate to evaporate, and it's a non-starter.
Click to expand...

Tks, I must have a cheap one. No ATC. :). I notice a difference if I look right after placing a drop or two on the lens and then again after about 1 min.
Sometimes up to 5 points BRIX higher on the second reading.
 

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