Does vodka affect FG reading?

[Elrond]

Or do I need to do the math calculation? I used 2 cups of vodka in secondary to sanitize the toasted coconut and mesh bag.

4.5 gallons post-boil

9 lb 2-row
1.5 lb chocolate malt
2 lb crystal 15L
1.5 lb rolled oats
1 lb roasted barley

OG/FG estimate calculator shows that my OG should be 1.088 and FG should be 1.025 for 8.3% ABV.
My OG was 1.082 and my FG is 1.036 for only 6% ABV, very disappointing. If that 6% ABV includes the vodka addition I will be extremely disappointed lol.
Thanks.

 

[Elric]

If you added the vodka to the beer it would give you a lower FG. Sorry to be bearer of bad news.

 

[RM-MN]

How many days has it been since you pitched the yeast to this beer?

 

[BrewZer]

Depends on how much of it you wind up drinking... yes, that's a joke, but not really.

 

[Elrond]

How many days has it been since you pitched the yeast to this beer?

14 days primary, then added coconut to primary for 7 days, removed coconut and racked to secondary for 7 days , then 6 days cold crash.
So about 4 weeks between pitching yeast and FG/bottling.
Now I'm almost certain that I am using uncalibrated thermometers and my mash temp is way too high.

Depends on how much of it you wind up drinking... yes, that's a joke, but not really.

Another stout I did used the same grain bill but 1 lb more 2-row, no coconut or vodka. It had 1.1 OG and 1.048 FG.
It actually turned out somewhat drinkable, but I added way too much maltodextrin so it was syrupy.

 

[Qhrumphf]

How are you measuring your FG? Are you using a refractometer by chance? If so are you correcting for the presence of alcohol?

 

[Elrond]

How are you measuring your FG? Are you using a refractometer by chance? If so are you correcting for the presence of alcohol?

Hydrometer

 

[rburrelli]

Adding vodka will add to your ABV. I don’t know the exact calculation. As stated, it won’t help your gravity measurements, but that is really not what you are looking for. You want to know if you will make you higher alcohol.

 

[Calder]

Adding the vodka will mess-up your OG-FG calculation for abv. You should have taken a gravity reading before adding the vodka.

1.036 is a high FG. The vodka should have lowered the FG. Are you sure it is correct.

 

[Qhrumphf]

You can pretty easily adjust your ABV figure by treating the alcohol as a blend using weighted average. If you have 20L of 5% ABV beer and add 0.5L of 40% ABV spirit, you've got 1L+0.2L of ethanol in 20.5L total, for an ABV of 5.85%. You'd need to know the ABV before the addition of the spirits.

I don't think that's what the OP is asking for, though, at least not directly. As others have said, the addition of ethanol will lower the FG reading.

Spitballing here. If we assume ethanol is 0.789g/ml density (quick Google result, dunno the reference temp) and water is 0.997g/ml density (close enough at room temp as I'm assuming the ethanol figure is), the specific gravity of pure ethanol should be 0.791. 40% ABV spirit should then have an SG 0.916.

From there we could unweight average of the gravity. 0.125(0.916)+4.5(FG)=1.036*(4.5+0.125). This brings your FG pre-addition to 1.039.

(I'm sure there's a faster way to do that anyway, even if my reasoning and napkin math are correct).

 

[Qhrumphf]

From there, 1.082 to 1.039 gives you 6.18% ABV (high gravity- use the Brewers Friend long form equation). 4.5(6.18)+0.125(40)=(4.5+0.125)*ABV. That brings your actual ABV to 7.09%.