Unexplainable Original Gravity Discrepancies

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The Punt

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Hello, fellow homebrewers.
I have been brewing all-grain for a couple of years now, and i've had a re-occuring problem with just about every batch. I use a refractometer to measure my mash and wort gravity. I have been calibrating it with distilled water at 20c at the start of each brew day for my last couple of brews, but this hasn't made a difference to the problem.
Basically, i'll measure my wort about half way through the boil to check that it's on track. I take a small sample out, cool it for 30 seconds in a dry stainless dish, then take a reading. It is usually exactly what i expect it to be.
I take another reading from the kettle at flame-out, again cooling the sample. This reading is almost always exactly what I WANT my Original Gravity to be. All good so far.
I chill the wort (with an immersion chiller, until my last batch when I changed over to a counterflow chiller) and pump it into the fermenter. I take another sample from the fermenter at pitch temp (20-25c), and it is ALWAYS at least 4 gravity points lower than the reading at flameout. For higher gravity brews, it has been up to 6 or so points lower.
My question is how can I lose 4 or more gravity points between the flameout and the fermenter?
I have tested the fermenter samples with a hydrometer, and they always match the refractometer reading.
I recirculate the wort through the pump and hoses throughout the boil, so its not water in the hoses.
I do cover the kettle after flameout, so I thought maybe it is condensation, but surely there wouldn't be enough condensation to THIN the wort by 4 points...?
Its not a huge problem, but it is frustrating as I can't hit my expected figures. For example, my last brew, a 4.5% blonde ale was 1.044 at flameout (exactly what I wanted) but was 1.040 by the time i chilled it and transferred it to the fermenter. It'll finish at about 4.2%.
My solution at the moment is to purposely overshoot my gravity targets. But, as a perfectionist, i need to know why this is happening.
Sorry for the long-winded post, but has anyone had this happen to them, or have any idea's what would cause it?
Cheers,
JW.
 
Taking a small sample of boiling wort and spreading it out into a very thin layer. You get a fair amount of evaporation from the high surface area (about 10%) from your small (say about 1mL) sample which is artificially increasing the gravity of your sample by the time it cools.

When I'm taking refrac samples I draw up a couple of mL into a plastic bulb pipet (usually one comes with the refract) and drop it upside down into a glass of cold water, leave it for a few minutes to cool (or at least to get well below the boiling point) then transfer to the refractometer.
I'm using a digital refractometer which have a small bowl shaped depression where you put the sample and early on I noticed that if you put in boiling wort and pressed the read button several times in a row, the gravity reading kept going up and up for a couple of minutes, until the sample cooled a little. So same problem you are having.
Mark
 
Taking a small sample of boiling wort and spreading it out into a very thin layer. You get a fair amount of evaporation from the high surface area (about 10%) from your small (say about 1mL) sample which is artificially increasing the gravity of your sample by the time it cools.

When I'm taking refrac samples I draw up a couple of mL into a plastic bulb pipet (usually one comes with the refract) and drop it upside down into a glass of cold water, leave it for a few minutes to cool (or at least to get well below the boiling point) then transfer to the refractometer.
I'm using a digital refractometer which have a small bowl shaped depression where you put the sample and early on I noticed that if you put in boiling wort and pressed the read button several times in a row, the gravity reading kept going up and up for a couple of minutes, until the sample cooled a little. So same problem you are having.
Mark
Cheers, Mark.
I never would have thought of that, but it makes sense. I'll cool the wort sample in the eyedropper or a small sealed container and see if it makes a difference.
Thanks.
 

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