
12112007, 02:04 PM

#1

Dec 2007
Orange
Posts: 86

How do you figure FG from the OG assuming you expect 75% attenuation?



12112007, 02:30 PM

#2

May 2007
Nashua, NH
Posts: 1,598
Liked 10 Times on 8 Posts

take the decimal part of the OG (ie  (OG1)) and multiply by (1attenuation), and then add 1.
example:
1.080 and 75% attenuation would be (1.0801)*(10.75) + 1 = .080*.25 + 1 = 0.020 + 1 = 1.020



12112007, 03:02 PM

#3

Sep 2007
Apex, NC
Posts: 1,028
Liked 11 Times on 10 Posts

It's always easier when dealing with gravities to use "gravity points." That's the final two numbers of the hydrometer reading. A 1.052 OG would have 52 gravity points.
In simplest terms, subtract the attenuation percentage from the OG: 52 minus 75% = 13.
To find the attenuation percentage, just multiply the OG (in gravity points) times the expected percentage: 52 x .75 = 39, then just subtract the attenuated portion: 52 minus 39 = 13 or a final gravity of 1.013.
To find your apparent attenuation from your OG and FG numbers subtract the FG from the OG to get the amount attenuated, then divide that number by the OG. So if your OG was 1.052 and your Final Gravity was 1.013: 52 minus 13 = 39. 39 ÷ 52 = 75 or 75% apparent attenuation. A final gravity of 1.015 would be 71% attenuation (52  15 = 37; 37 ÷ 52 = 71)
Chad



12112007, 03:08 PM

#4

May 2007
Nashua, NH
Posts: 1,598
Liked 10 Times on 8 Posts

Quote:
Originally Posted by Chad
It's always easier when dealing with gravities to use "gravity points." That's the final two numbers of the hydrometer reading. A 1.052 OG would have 52 gravity points.

Technically in the more general case, you should be taking all 3 numbers after the decimal, for the cases where your OG is over 1.100  however, I would assume that anyone who dares brew a beer that big would likely already be experienced enough to realize that



12112007, 03:44 PM

#5

Dec 2007
Orange
Posts: 86

i guess i was not experienced enough to know that
114 * .75 = 85.5
114  85.5 = 28.5
1.0285 should be my expected final gravity?
.114 * .25 + 1 = 1.0285
kk  i get it ... thanks guys



12112007, 04:45 PM

#6

May 2007
Nashua, NH
Posts: 1,598
Liked 10 Times on 8 Posts

That'd be a rough guideline, and probably fine for your regular, runofthemill, medium or lowgravity beers. However, attenuation is not fixed  it is going to depend on your wort to a degree, not just your yeast. If your wort has an unusually high or low level of unfermentables, your attenuation is likely to change as a result. I think it is probably also more upintheair when you're talking about such a highgravity beer, where the yeast is under a lot more stress.



12112007, 05:15 PM

#7

Dec 2007
Orange
Posts: 86

i am doing my best to keep the stress levels down (rousing and letting the CO2 escape)
unfermentable sugars will probably be my drawback





