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Old 01-19-2012, 03:44 AM   #1
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BeerSmith says pre-boil volume should be 7.8 gals and pre-boil gravity 1.039.
My measured pre-boil volume is 7.8 gals and measured pre-boil gravity is 1.039.

BeerSMith says post-boil volume should be 6.7 gals and post-boil gravity 1.049.
My measured post-boil volume is 6.7 gals but my measured post-boil gravity is 1.042.

What happened here? All gravity measurements are corrected for temp. Isn't this basic science (basic, complex, genius level, it's all the same to me ()) and isn't my post-boil gravity 100% predictable given pre-boil volume, pre-boil gravity, and post-boil volume? If not why not? And if not, can someone explain what may have happened so I can prevent it from happening again?

Thanks.

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Old 01-19-2012, 03:45 AM   #2
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Something is measured wrong. Or, you took a gravity reading in hot wort.

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Old 01-19-2012, 03:52 AM   #3
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Something is measured wrong. Or, you took a gravity reading in hot wort.
nope. definitely not. I save each of the samples in color coded jars. I'm looking at the reading for the post-boil wort right now. It's 1.042 at 60 degrees F. Sounds like you're saying that my suspicions were right, though, and that post-boil gravity is a function of pre-boil volume and gravity and post-boil volume. Hmmmmm....
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Old 01-19-2012, 04:01 AM   #4
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I don't have the details in front of me but my last "BeerSmith" batch underestimated post-boil FG even though things were on the mark otherwise. I've gotten very lazy at taking notes and that made me ask myself why I don't have the info to figure out what went "wrong." Better notes next time... something tells me my method for measuring volume in the pot is the culprit along with a higher than anticipated boil-off rate. Regardless, it sounds like you are taking very good notes and I am curious as to your conclusions about the error.

BTW, are you using a hydrometer or a refractometer for you measurements?

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Old 01-19-2012, 06:19 AM   #5
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I don't have the details in front of me but my last "BeerSmith" batch underestimated post-boil FG even though things were on the mark otherwise. I've gotten very lazy at taking notes and that made me ask myself why I don't have the info to figure out what went "wrong." Better notes next time... something tells me my method for measuring volume in the pot is the culprit along with a higher than anticipated boil-off rate. Regardless, it sounds like you are taking very good notes and I am curious as to your conclusions about the error.

BTW, are you using a hydrometer or a refractometer for you measurements?
Hydrometer, temp adjusted from here: http://www.brewersfriend.com/hydrometer-temp/

This is only the 4th time I've used BeerSmith and I don't recall if the projected readings and actual readings were off on the other batches, but I'll keep an eye on it henceforth. I have noticed that the strike temperature it gives me for mash in is always high.

Intuitively, though, 1.042 is too low considering my pre-boil gravity. I'm bummed and wish I knew what went wrong. This beer will barely clear 4.0% ABV.
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Old 01-19-2012, 08:34 AM   #6
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Edit: Please ignore my stupidity.

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Old 01-19-2012, 04:15 PM   #7
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Without more (and more specific) info, it's hard to diagnose what may have gone "wrong" but the easiest culprit is probably the milling of your grain--to coarse generally means lower efficiency/lower extract yield, and that means lower Sg. That's one reason to measure your pre-boil SG: if it's lower than expected, you can boil off more liquid to hit your post-boil SG. You'll have less beer, but it will more closely match your expected final product!
Sometimes I feel like people don't even read the posts they are responding to. Why would you even post this, knowing that the OP DID measure his preboil gravity and volume and was on target?



I am going with measurement error, although that I will point out some math, here, which ought to be instructive as well as bring up a few questions.

In theory your preboil gravity points * volume should equal your post boil gravity points * volume. The case where this isn't true, is when you are leaving behind some assumed amount of trub when racking to the fermenter, so that your volume into the fermenter is off by, say .25-0.5 gallons of post-boil wort.


If Beersmith says 7.8 * 39 == 6.7 * 49 it is wrong.
Preboil:
7.8 * 39 = 304.2

Post-boil with same points, target volume:
304.2 / 6.7 = 45.4

On the other hand:

304.2 / 49 = 6.2 gallons (what beersmith ought to say, sans loss of volume to trub)

and

304.2 / 42 = 7.2 gallons (volume you should have given your gravity reading)




There HAS to be a measurement error, or multiple measurement errors, to account for this. What is your target batch size in Beersmith?


Also, how confident are you in your volume measurements? If you misread your preboil volume by, say, half a gallon, that could completely account for the discrepancy.
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Old 01-19-2012, 04:37 PM   #8
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Sometimes I feel like people don't even read the posts they are responding to. Why would you even post this, knowing that the OP DID measure his preboil gravity and volume and was on target?
Why? Because it was 3:30 am and I wasn't fully awake. That being said, "Guilty as charged."

To the OP--my bad! Sorry!
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Old 01-19-2012, 04:49 PM   #9
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I would be more willing to bet that your pre-boil gravity was incorrect due to the wort not being homogenous. When you sparge, that wort in your boil kettle is layered by gravity and is not consistent throughout. Did you stir VERY well prior to taking that preboil gravity measurement?

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Old 01-19-2012, 04:57 PM   #10
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Also, how confident are you in your volume measurements? If you misread your preboil volume by, say, half a gallon, that could completely account for the discrepancy.
I think this is the culprit. My boil pot isn't graduated so I have an odd way of measuring volume--it's basically a ruler and I bet I was holding it in the middle of the pot rather than against the side and that could be enough to account for at least one quart. Also this morning my hydrometer settled at 1.044 so I'm a bit closer than I thought but not by much.

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I would be more willing to bet that your pre-boil gravity was incorrect due to the wort not being homogenous. When you sparge, that wort in your boil kettle is layered by gravity and is not consistent throughout. Did you stir VERY well prior to taking that preboil gravity measurement?
This is also something to consider. A point here or there surely adds up.

Thanks to all for the helpful replies. I don't feel better about the beer by I feel better about knowing what went wrong so that I can avoid similar problems in the future.
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