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Old 03-25-2009, 10:28 PM   #1
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Default Check my math?

Howdy,

Got my first batch in the fermenter. Can't wait 'til it's ready to drink! Got a quick question for everybody. I've seen this explained several ways, so I want to make sure I understand what I'm shooting for.

I measured an OG of 1.047 before I pitched my yeasties (WLP005). The wort was at 68F, so the OG shouldn't need to be adjusted (or only by .001, if it does). The yeast, according to whitelabs.com, has an attenuation of 67-74%.

So, here's my "scratch pad" work:

47 * 0.67 = 31.49
47 * 0.74 = 34.78

47 - 31.49 = 15.51
47 - 34.78 = 12.22

Adding the thousand "points" back in gives me 1.01551 and 1.01222. Rounding to 3 places gives 1.016 and 1.012.

So, that means I should expect (assuming all goes well) to have a FG between 1.012 and 1.016 when fermentation is finished? Is that correct?

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Old 03-26-2009, 03:34 AM   #2
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you are correct in your math, except you don't need that many decimal places!

BUT -- yeast attenuation on the package is just a range and not canon. It's not done until it's done (aka when the G readings are the same for 3+ days).

Depending on what your process and ingredients where, I wouldn't be surprised to see this get down to 10 or less.

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Old 03-26-2009, 03:46 AM   #3
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You forgot to carry the 1.

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Old 03-26-2009, 05:02 PM   #4
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You don't have to do any subtraction.

If you have 74% attenuation, then you are left with 26% of the original gravity.

So,

FG_low= 47 pts * 0.26 = 12 pts
FG_high = 47 pts * 0.33 = 16 pts

And notice that my answers only have 2 significant figures because you are only giving 2 significant figures in each of the multiplicands (47 and 0.26, 0.33).

This is similar to when people figure out sales tax...usually, they do 5% times the price and then add the price back. Why don't you just multiply the price by 1.05 and get the answer in one shot?

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Old 03-26-2009, 05:06 PM   #5
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Quote:
Originally Posted by rocketman768 View Post
You don't have to do any subtraction.

If you have 74% attenuation, then you are left with 26% of the original gravity.

So,

FG_low= 47 pts * 0.26 = 12 pts
FG_high = 47 pts * 0.33 = 16 pts

And notice that my answers only have 2 significant figures because you are only giving 2 significant figures in each of the multiplicands (47 and 0.26, 0.33).

This is similar to when people figure out sales tax...usually, they do 5% times the price and then add the price back. Why don't you just multiply the price by 1.05 and get the answer in one shot?
Cool, that's easier. I know I had a lot of decimal places... was just carrying them through 'til the end, to account for rounding.
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Old 03-31-2009, 10:00 PM   #6
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Quote:
Originally Posted by rocketman768 View Post
You don't have to do any subtraction.

If you have 74% attenuation, then you are left with 26% of the original gravity.

So,

FG_low= 47 pts * 0.26 = 12 pts
FG_high = 47 pts * 0.33 = 16 pts

And notice that my answers only have 2 significant figures because you are only giving 2 significant figures in each of the multiplicands (47 and 0.26, 0.33).
Okay, new question:

First brew (LBHS recipe brown ale) is 10 days into primary now. I have had steady hydrometer readings of 1.021 the last two days, so I was planning on bottling today. However, looking at numbers, I'm wondering...

Based on the listed yeast (WLP005) attenuation (67-74%), I was expecting a final gravity of between 1.012 - 1.016. I'm only at 1.021. Also, that should give me an ABV of about 3.4%, correct? (1.047 OG - 1.021 FG = 0.026, multiplied by 131 = 3.4)

That seems a little low to me... did I not reach the "target zone" because maybe some of my yeasties kicked the bucket? (Pun intended.)

Thanks,

El Jefe
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Old 03-31-2009, 10:10 PM   #7
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Remember that attenuation also depends upon the fermentability of the wort. For extracts, you may be able to find this information somewhere since it should be pretty constant, but when mashing the temperature will drastically change fermentability, and therefore attenuation.

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Old 03-31-2009, 11:33 PM   #8
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Quote:
Originally Posted by mkade View Post
Remember that attenuation also depends upon the fermentability of the wort. For extracts, you may be able to find this information somewhere since it should be pretty constant, but when mashing the temperature will drastically change fermentability, and therefore attenuation.
This was an extract + steeping grains recipe. The page from LHBS didn't include the expected gravities, so I was basing my calculations of expected FG on my measured OG and the listed attenuation of the yeast from whitelabs.com.

I'm not really worried about it, since I've got steady gravity readings... just thought that the estimated 3.4% ABV was kinda weak for a brown ale.
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Old 04-01-2009, 01:44 AM   #9
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Some varieties of malt extract achieve higher attenuation than others. Do you know what brand of extract it was?
If you use liquid yeast, it is advisable to make a starter, especially if the yeast is a bit old. Did you make a starter?
The brew will attenuate better if it is well aerated before pitching. Did you aerate?
If you didn't aerate and/or make a starter, then fermentation can be a bit slow, especially if fermenting at lower temperatures.
As it is a highly flocculant yeast, I'd give it a rousing by rocking the fermenter to re-suspend the yeast for about 5 minutes, then check the gravity in another 3 or 4 days. I think you will find that it is still fermenting albeit slowly.

-a.

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Old 04-01-2009, 11:34 PM   #10
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The LME was from a big drum at the LHBS. No idea what brand. I did use liquid yeast, didn't see a date, but the shop owner got it from the fridge for me himself. I didn't make a starter... I thought you just shook the vial and dumped it in. I had taken it out from the fridge in the morning before I started brewing, so it was at room temp. Wort was 68. Before pitching, I put the lid on the bucket, sans rubber stopper, so air could get in, and rocked it back and forth for many minutes. I also poured from the brewkettle through a strainer to get it into the bucket, so I should have had plenty of aeration.

I hope it's not still fermenting... I bottled it all last night before seeing your message!

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