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Home Brew Forums > Home Brewing Beer > All Grain & Partial Mash Brewing > Pre Boil Gravity vs Post Boil Gravity
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Old 08-06-2011, 03:55 PM   #1
mcurtis431
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Default Pre Boil Gravity vs Post Boil Gravity

I am a little confused. I am very new to AG brewing. My first batch was an American IPA with a target OG of 1.062. I used Beer Smith to help create the recipe. After I mashed and sparged I took a gravity reading of the pre-boil and it was around 1.04ish. Beer smith uses this number to caSculate the extraction efficiency. Using this number gave me a terrible efficiency result. However, when I boiled down to my target volume my gravity was much closer to the target 1.062, thus bumping my efficiency up.

My question is, which reading do you use to calculate your efficiency? It doesn't make sense to me to use the pre-boil reading since this is not the volume you will be fermenting at. When designing a recipe, which gravity reading should I use as my OG?

I have yet to read a clear answer.....


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Old 08-06-2011, 04:08 PM   #2
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I take a pre-boil gravity to determine my efficiency. You have X gallons at Y gravity and compare it to the max theoretical extraction you could get from your grain bill. I would guess you didn't adjust your volume for your pre-boil gravity so you were calculating your efficiency on your final recipe volume coming in at 1.040 vs 1.062.

The OG is what you have post boil in your final volume of wort that goes into your fermenter.


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Old 08-06-2011, 04:09 PM   #3
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Pre and post boil should yield the same efficiency, since there's no change in the amount of sugar in the wort, just in the amount of water.
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Old 08-06-2011, 04:14 PM   #4
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A mathematical example might help here.

You had a 5 gallon batch with a OG of 1.062. If you multiply the 62 times the number of gallons, you have 310 points in your wort. For a preboil reading, you likely had 7 gallons of liquid, which probably had a gravity reading of 1.044 (310/7).

So, if you know what your preboil liquid level is, you can get a gravity reading and estimate what your gravity will be postboil the same way.

If you had gotten a 1.044 reading preboil on 7 gallons, you would have measured 308 points in your wort (7 times 44). By dividing the 308 by 5, you'll get your postboil gravity, 1.062.
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Old 08-06-2011, 04:18 PM   #5
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Use the post boil with adjusted water volume to calculate efficiency and you starting OG. Why take two readings ( pre and post ) when all calcs can be done with 1 reading? I get lazy sometimes.

What was your OG supposed to be after the boil?
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Old 08-06-2011, 04:22 PM   #6
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Since the original poster is new to AG brewing, taking a preboil reading is valuable to understand whether or not efficiency was close enough to what the recipe called for.
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Old 08-06-2011, 04:54 PM   #7
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I had a similar case where the pre-boil gravity of my beer should have yielded a lower post-boil gravity based on the boil off. Somehow, instead of the calculated post-boil gravity, it ended up higher, giving me two different numbers for my overall efficiency. I'm thinking OP had a similar problem. I also used BeerSmith for my math, so it could be a flaw with that.....
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Old 08-06-2011, 05:37 PM   #8
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also, a pre boil reading is great when you don't have your process dialed in yet and need to know how much boil off you really need to hit close to your desired OG. When I started AG I had 1 batch that had a much higher effeciency than I anticipated and didn't figure that out until I had it boiled all the way down - and it was a long boil. I could have shortened my boil and had a more reasonable OG if I had taken a pre boil OG. Was still great beer but not percicely what I had envisioned.
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Old 08-07-2011, 01:50 PM   #9
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when you have a reference to OG that is the wort chilled down to room temperature in the fermenter having the exact batch size volume before pitching the yeast.

The preboil gravity is the SG (specific gravity) of the worts preboil volume collected before evaporation, boiler equipment loss, or shrinkage from chilling. This stage is where you make your adjustments to the gravity so once boiled, chilled, and transfered into the fermenter, you hit your recipes targeted volume and OG
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Old 04-19-2012, 05:37 AM   #10
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good stuff.


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