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Old 03-23-2012, 12:36 AM   #1
william_shakes_beer
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Default beer math question

Got my refractometer today. Brewing this weekend, and I'd like to use it to adjust the wort at the start of the boil by adding light DME to make up for any effeciency issues I might have.

Given: My recipie calls for an OG of 1.050 in a 5 gallon (to the fermenter) batch.

Given: SG preboil is 3.5 gallons at 1.035

I ignore boiloff, since water lost will be replaced by topoff after cooling.

1. Calculate Sg after topoff

3.5/5=0.7*.035=1.0245


2. Calculate how much DME to add
DME provides 45 gravity points per pound per gallon.

Need to add .050-.025=.025 or 25 gravity points. 25 GP x 5 gallons = 125 GP needed in extract. 125/45 = 2.77 pounds DME needed

Is my math correct?

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Old 03-23-2012, 02:49 PM   #2
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Good math. Its alot easier with a brewing program like beer smith or pro brewer. If you put up the cash for a refractometer, the extra 20 for the program would be well worth it.

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Old 03-23-2012, 04:11 PM   #3
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Your math looks good. I got 2.8333lb DME, but I calculated 25.5gp from the DME. The extra ounce will not make a noticeable difference.

Regarding brewing software: simple calculations like this help keep your mind sharp - there isn't any need for software unit you get into more difficult calculations (ie. IBU). This is just my <unsolicited> $0.02.

Another unsolicited point, this one sort of semantic: you say 25GP x 5gal = 125GP. The units do not work out. To keep this clear in my brewing calculations I distinguish between gravity points (gp) and gravity units (GU), where 1gp x 1gal = 1GU. The gp is what is read off the hydrometer or refractometer, the GU represents the total amount of sugar in the wort.

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Old 03-23-2012, 04:19 PM   #4
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Another +1 on good math. Here's another way to calculate:

Target: 50GP x 5gal = 250 GU
Actual 35GP x 3.5g = 122.5 GU
So 127.5 GU needed / 45 PPG DME = 2.83 pounds

Good points mango. Understanding the math makes on-the-fly calculations & adjustments much easier. Plus people who brew with me think I'm a genius scientist when I'm madly scribbling calculations on a notepad. I don't tell them I'm just adding.

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Old 03-23-2012, 05:38 PM   #5
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Quote:
Originally Posted by TyTanium View Post
Another +1 on good math. Here's another way to calculate:

Target: 50GP x 5gal = 250 GU
Actual 35GP x 3.5g = 122.5 GU
So 127.5 GU needed / 45 PPG DME = 2.83 pounds
I much prefer this equation to the OP's.

I'm brewing a 12% barleywine (partial grain) this weekend and Chapter 6 in Ray Daniel's Designing Great Beers provides great examples of calculating the amount of DME needed to hit the target gravity. Great book!
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Old 03-23-2012, 05:46 PM   #6
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Fully agree with magno, do the math yourself to fully understand how the calcs work. It's certainly cheaper to make your own spreadsheet on the calculations.

I make a spreadsheet for batch sparge calculations and brewers notes for each batch.

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Old 03-23-2012, 05:47 PM   #7
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One issue is ignoring Boil Off. You going to cool the wort, top off, then add your DME and reheat, then re-cool? You need to figure out the SG (target and actual) of the Post Boil Volume so you can adjust there.

Edit. Or you could adjust with the PreBoil numbers

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Old 03-23-2012, 06:25 PM   #8
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Thanks. I wanted a reality check before I jump into the brew day. This is the first time I have had the ability to do a gravity check prior to the boil. Nanobru: I understand the software is an investment that can make the calcs easier. I usually use tastybrew to set my target OG. However, as a somewhat technically minded person I find it contributes to my understanding of the relationships if I can work the numbers the old fashion way. Magno; Understood the difference between GP and GU. I was using them interchangably, will correct in the future. Tytanium, Yes, that looks like a simpler method. It will also allow me to eliminate 3 intermediate calculation cells on my spreadsheet. JeepDiver; No, I usually top off with tap water. I understand the discussions about the water being not sterilized by the boil, but it hasn't made me sick yet, so I will continue to do so. That doesn't meay that your process is incorrect, it just means that I don't have a pot large enough to do a full boil.

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