Home Brew Forums > How would you know?

01-06-2011, 03:32 AM   #1
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 How would you know?

I plan on brewing my first all grain batch weekend after next. I have been reading How to Brew and coincidentally listening to a podcast from Brew Strong on sparging. In the podcast Jamil says that if you don't achieve the high enough efficiency (between 60% and 80%) you can just add some light DME to the boil to achieve the OG you should have gotten for your recipe.

My question is, how do you know you have the pre-boil OG you are supposed to have? I read Bobby M's all grain primer (www.suebob.com/brew) and know I can calculate the expected gravity and then calculate my efficiency for my mash/sparge. But all the recipes I see have the after boil OG listed not the pre-boil value. Do I Just calculate the 100% value of the grain bill and if my pre-boil gravity is in the 60 to 80% range range feel satisfied or is there something else to look at?

01-06-2011, 03:53 AM   #2
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Gravity Units.

Gravity Units = (# gallons) x (last two digits of gravity)

Example: If pre-boil, you have 6.5 gallons of wort that is 1.045, you would have 292.5 gravity units (6.5 x 45). Then after the boil when you have reduced to 5 gallons, you take the 292.5 gravity units and divide by 5 gallons to get a gravity of your final wort of 1.085.

Hopefully that makes sense.

So, if you take your recipe and calculate gravity units, you have a target. This will not change during the brewing process after the mash/sparge and any other adjustments (like the DME you speak of).

Example: If your recipe is 5 gallons of a 1.060 IPA, you would have 300 gravity units. If after your mash, you have collected 7 gallons of wort that has a gravity of 1.038, you know that you are on track to have a 1.053 beer (7 gallons x 38 = 266 gravity units. Then 266 gravity units / 5 gallons = 53). At that point, you are missing 34 total gravity units (your target of 300 minus the 266 you currently have).

Each pound of DME is going to give you approx 45 ppg or 9 points for a 5 gallon batch. So, take those 34 points (for the full batch; in this case 5 gallons) you needed and divide it by 9 points and you will realize you need 3-4 lbs. of DME to make up the extra points.

I'm kind of drunk right now, but hopefully that makes sense

BTW - Every brewer should read Designing Great Beers by Ray Daniels - It covers this concept as well as many others a lot better than I probably just did

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01-06-2011, 04:00 AM   #3
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I'm not drunk and that made perfect sense to me.

Thanks for the clear answer (and have another brew on me)!

By the way, I haven't read the book you mentioned but I have read Palmer's how to brew and Brewing Classic styles. Seems like the more I read the more I find I still need to learn.

01-06-2011, 04:21 AM   #4
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Designing Great Beers does a great job of explaining fairly simple "brewing math" and then gives guidelines for brewing to style. Palmer's and Jamil's are both great too.
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01-06-2011, 04:26 AM   #5
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Quote:
 Originally Posted by Grizzlybrew Gravity Units. Gravity Units = (# gallons) x (last two digits of gravity) Example: If pre-boil, you have 6.5 gallons of wort that is 1.045, you would have 292.5 gravity units (6.5 x 45). Then after the boil when you have reduced to 5 gallons, you take the 292.5 gravity units and divide by 5 gallons to get a gravity of your final wort of 1.085. Hopefully that makes sense. So, if you take your recipe and calculate gravity units, you have a target. This will not change during the brewing process after the mash/sparge and any other adjustments (like the DME you speak of). Example: If your recipe is 5 gallons of a 1.060 IPA, you would have 300 gravity units. If after your mash, you have collected 7 gallons of wort that has a gravity of 1.038, you know that you are on track to have a 1.053 beer (7 gallons x 38 = 266 gravity units. Then 266 gravity units / 5 gallons = 53). At that point, you are missing 34 total gravity units (your target of 300 minus the 266 you currently have). Each pound of DME is going to give you approx 45 ppg or 9 points for a 5 gallon batch. So, take those 34 points (for the full batch; in this case 5 gallons) you needed and divide it by 9 points and you will realize you need 3-4 lbs. of DME to make up the extra points. I'm kind of drunk right now, but hopefully that makes sense BTW - Every brewer should read Designing Great Beers by Ray Daniels - It covers this concept as well as many others a lot better than I probably just did
Thanks Grizzly, that made perfect scene to me also! Cut and pasted!

01-06-2011, 04:32 AM   #6
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Quote:
 Originally Posted by bad67z Thanks Grizzly, that made perfect scene to me also! Cut and pasted!
Glad to help. I use this equation every time I brew. Glad I can spread the good word!
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01-06-2011, 07:15 AM   #7
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For the guy who is about to brew his first batch ever, there's an even easier way to do things.

Your hands will be full getting a handle on the main, important bits. You'll get a decent beer. Don't sweat the math and gravity till next time (at earliest!)

01-06-2011, 02:47 PM   #8
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I disagree... I was pissed as hell at my first AG batch because I had just spent 6.5 hours trying to make an IPA, only to find out my OG was 1.044. If I had just taken a preboil reading, I would have known to add some DME.

01-06-2011, 03:05 PM   #9
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Do you guys just throw your preboil wort sample in the fridge for a little bit or is there some sort of temp correction for the specific gravity of a wort at post mash temps??

01-06-2011, 03:15 PM   #10
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I do both. I chill for a bit to get it down into the range where I can trust an adjusted reading.

Well, that was before I got a refractometer! Now I simply draw a tiny sample with a pipette, blow on it for few seconds, drop it on the refractometer and take a reading.

Having to chill a sample mess with drawing several ounces was what made me spend the \$40 on a refractometer.