Og Lower then Pre Boil gravity.

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355Newell

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This is the second time this has happened to me. The first time a couple weeks ago I assumed I didn't get my 1st and 2nd running's stirred up well enough in my brew pot, so I let it go. Today I made sure to stir well, and i checked my gravity about every 15 minutes in the 60 minute boil. My pre boil today was 14.25 Brix, while my post came out to 12.75 brix. About every 15 minutes the reading dropped about .25 Brix. I use a refractometer so I'm fairly confident that the temperature of the wort isn't playing to much of a factor here. I measured around a gallon of boil off today, so I don't understand where these misreadings are coming from. The final OG is very close to what I expected it to be, so I guess all in all thats good.

The only thing I can come up with is that I'm brewing outside, and its about 25 degrees out today, would this mess with my refractometer? I do however come back inside the 65 degree house to double check these readings, and they don't change. I'm totally at a loss here.
 
It reads zero with our city water supply, I'm sure it may be off a fraction when using RO water. I checked it today before I started, and usually before every brew session since its incredibly easy to do.
 
Yeah just wanted to double check, still puzzling why the sugar content would be reading less. I would double check with a hydrometer for both the OG and pre-boil readings next time; something doesn't sound quite right with those refractometer readings.
 
Yea that's probably the best place to start. I was all excited when I entered the pre boil gravity into beersmith and it told me i had 96% efficiency! I should of known right then something was askew.
 
Something has to be screwy with the refractometer because scientifically it is impossible to boil off water, concentrate the sugars and get a lower gravity than pre-boil.
 
Yes I am pretty sure its cooling enough before I take a reading. I typically use my stirring spoon and gather a bit of wort on it and let it sit out in the air for a few minutes to let it cool off to the ambient air temp. I know its impossible for this to be happening, so I know something in my process is flawed. The only thing I can think of is the outside air temp yesterday was around 25 or so, and the last time this happened to me it was roughly the same. Last week I brewed inside on the stove top, and I didn't have this problem. So the only thing I can think is that the colder outside air is screwing with my refractometer.
 
Consider the evaporation rate of your wort sample at cold temps. A small sample in cold and dry air will evaporate at a rapid rate, giving you an innacurate reading. I collect a small sample in a small jar or glass, cover with a piece of plastic wrap to control evaporation, and bring in the house for a few minutes before I take a refractometer reading. Try it next time
 
WVbrewer, I think you nailed it. I brewed today, inside on the stove, and got typical readings. From now on while brewing outside in the cold Ill try your method of collecting a sample, and checking it inside. Thanks again.:mug:
 
I usually take a small sample with a pipette and put it on the refractometer and close the plastic lid. That allows it to cool to room temperature and not evaporate any. Then I just put it down for a few minutes to cool before reading it.
 
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