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Old 06-20-2011, 03:11 AM   #1
StochasticBrew
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Default IBU calculations

Hello all,

I have a few questions regarding Tinseth's and Rager's equations. First of all, I tend to brew three gallon batches of partial mash brews. I usually mash in about 1 gallon of water and add a half gallon when I rinse the grains before I bring the water to a boil. Before I start my hops, I add in the malt extract, which is usually liquid. After the boil I add the wort to 1.5 gallons of cool water and top it off if I am below the three gallon mark.

Now, when I look at the Formulas mentioned above, I notice they both ask for the wort gravity and wort volume, but I am not sure if this means the wort before I bring it up to three gallons (i.e., the boiling wort) or the after I have brought it up to three gallons. Furthermore, I don't know how to estimate gravity given a particular grain bill (specialty grains and malt extract) and an assumed efficiency level. I have used beer tools in the past and it was able to estimate the O.G. and boiling wort gravity. So, does anyone know what formulas these programs use to do this?

Summary. I want to know if

1) I use the final wort volume of just the boiling wort volume in my calculations and

2) how to estimate the specific gravity of either the boiling wort or final wort given an arbitrary grain bill.

Thank you.

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Old 06-20-2011, 07:08 PM   #2
jmf143
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1 pound of Dried Malt Extract (DME) dissolved in 1 Gallon of water has a SG of 1.044. This is referred to as "44 points per pound per gallon (PPG)". If you're doing all grain each of your varieties of grain will have their own corresponding "theoretical maximum" PPG value. The amount you actually realize will depend on your mash efficiency. For instance, American 2 row is around 38 PPG; if your mash efficiency is 75% you'll realize .75 * 38 = 28.5 PPG. Figure out all of the PPG for all of your malt extract / grains, divide by your final volume into the fermentor and that is your approx. SG.

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Old 06-20-2011, 07:16 PM   #3
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use final wort volume, but boiling wort gravity for tinseth. probably the same with rager, but im not sure.

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Old 06-20-2011, 11:34 PM   #4
StochasticBrew
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Thanks for the help.

Do you know of any good sites that approximate values for different varieties of grains? Also, because I am doing a partial mash, do I just sum the PPG's of my malt extract and specialty grains?

Finally, is there a scalar I can multiply the DME PPG by to achieve an equivalent result for LME?

Thanks again

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Old 06-21-2011, 01:22 AM   #5
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you can use the wiki for the malts, it lists most varities: http://www.homebrewtalk.com/wiki/index.php/Malts_Chart

its not quite as easy with malts, theres an efficiency factor to account for. for a PM, 75% efficiency is probably a good factor to assume, i.e. assume you're only getting 75% of the listed ppg.

yup, 1lb LME = 0.8lbs DME.

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Old 06-21-2011, 07:34 AM   #6
StochasticBrew
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Thanks a lot guys.

I did a lot of reading and I got my spreadsheet working. The Tinseth formula produces IBU readings drastically lower than what beertools does with its standard formula. However, my spreadsheet gives me similar results for boiling wort PPG and OG. I also have a similar estimate for FG. So, I think I just have to brew and adjust to Tinseth's formula.

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