Different OG - BK and Fermentor

I've noticed an odd post boil SG difference depending on when I take my sample. My practice has been to dip a small glass in the BK after the whirl pool, cool it to 70* and take a reading. The last few brew days I've also taken a sample from the fermentor after it has gone through the CFC. I'm getting anywhere from 3 -5 points difference in the readings with them being lower when taken from the fermentor. Is there an explanation for this? Seems odd that the sample from the BK cooled to 70* or immediately run through the CFC to 70* should be different.

One thing I can think of right off the bat is if the CFC had a leak and water was getting into your beer. This would water it down a bit and make the SG lower. Probably not the cause to your issue, but a possibility nonetheless.

A possibility but I couldnt imagine more then a pin hole leak which probably would add very little water. Now there would most likely be some water and or starsan left in the CFC from sanitizing though I do roll it over the sink to get as much out as I can prior to cooling. I'll have to figure out how much water it would take to lower SG that much.

Any trub/suspended solids in the post boil sample?

It is more likely that despite the whirlpool the wort in the kettle wasn't thoroughly mixed. This can lead to inconsistent sampling. I notice here that, even after the whirlpool, I have to stir the kettle or wait until it is in the fermenter to attain consistent readings.

chordwizard said:

A possibility but I couldnt imagine more then a pin hole leak which probably would add very little water. Now there would most likely be some water and or starsan left in the CFC from sanitizing though I do roll it over the sink to get as much out as I can prior to cooling. I'll have to figure out how much water it would take to lower SG that much.

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Indeed it would take a fair amount. I'm not sure your OG but let's say it was 1.060, it would only take about 0.3 gallons (or ~38oz) of water to drop it 3-4 points.

Personally I keep the starsan in the CFC until I open the kettle. I keep a crimp on the "wort out" tubing and uncrimp when I open the kettle, let the starsan drain out (the beer pushes it), and then crimp when I see beer in the tube, place into the carboy and uncrimp again to let the beer flow into the carboy.

But yeah at this point I'm going to have to go with the wort you sampled in the kettle wasn't as homogeneous as you thought.

Might your small sample in a glass experience some evaporation thus increasing your gravity reading?

Are you using a hydrometer or refractometer to measure?

Sorry for the delay, been a busy weekend.

Weezy - certainly possible. I do let it settle while it is cooling and try not to agitate it before I sample. There may be some in the sample from the fermentor as well as I don't use any form of filtering though I'm hoping I've left a lot behind in the kettle from the whirlpool.

jrcrilly - entirely possible. I'm not sure how much agitation it would take to fully mix it and if what I do is sufficient.

MagicMatt - I agree that it would take a significant amount of water to dilute it that much but I think I will check and be sure one of my fittings isn't leaking and having some affect on it. I am using a refractometer!

wilserbrewer - I have 3-4 oz in the glass and place it in the fridge to cool so I'm sure there is some evaporation and I hadn't considered that as a possibility.

So it may be possible that if you put all of the factors mentioned together (particulate, not well mixed, evaporation, dilution) that the sum of there parts may account for the difference I'm seeing?

chordwizard said:

MagicMatt - I agree that it would take a significant amount of water to dilute it that much but I think I will check and be sure one of my fittings isn't leaking and having some affect on it. I am using a refractometer!

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I think this solves it. Refractometers (the cheap ones we use for homebrewing) have a margin of error approaching your disparity. They are generally regarded as "ballpark" readings, meant only to gather a quick reading rather than an extremely precise one. Just take 4 or 5 readings of liquid from the same sample (as in, collect say 4oz, then use those 4oz for multiple readings, rinsing between each) and you'll see the effects - the readings will differ wildly. In fact, if I take a refractometer reading this is exactly what I do, and then average the results.

Because of this I generally only use a refractometer to get my pre-boil gravity and make sure I'm more or less in line with what is expected. I can always adjust the boil length slightly to correct for a point or two difference. But for OG and FG readings, I use a hydrometer 100% of the time as the refractometer just isn't precise enough.

One big problem is that the default units on them is Brix, and most have a SG scale as well so that's what people are reading, however the relationship betwen these two are not linear. Meaning that for a lower gravity sample, say 5 brix, the SG will be pretty close to correct. But if you take a 13 brix sample, the SG will be off by as much as 7-10 points. The correct way to use it in this case is to take the Brix reading and convert it with an online calculator that correctly handles the conversion factor. This will yield more accurate results but I still rely on my hydrometer for recorded readings.

MagicMatt - Though I agree about the refractometer readings being ballpark and I follow a similar routine to you, my experience (purely anecdotal) is that my reading don't vary that much. I did do multiple readings of both samples from my last brew, which prompted this question and they were within 1 point. I sampled BK, then Fer, noticed the difference so re-sampled and sampled again. Got basically the same readings again. Now I may have just been lucky and you may have identified the main culprit but it made me wonder if there was other factors influencing it.