ABV calculation

[figgnewton]

So I just recently brewed a beer and my starting gravity was 1.075. After fermentation was complete I added 14 oz of 30 percent vodka to a 4 gallon batch. I should have made another reading before this but I waited til after... anyways the reading is now just under 1.040. I am wondering how I would figure out how much abv each beer actually is. Would non fermentable adjucts mess with the reading like flaked oats and lactose?

 

[billl]

You've got a measurement issue. 1.040 is not a reasonable FG. Alcohol is slightly less dense than water, so your FG of your "finished" beer would have been slightly over 1.040. That's still sugar water territory, not beer.

 

[Redlantern]

Is the 1.040 reading prior to or after fermentation?

If this is after fermentation, something is wrong. Did not try to do the math, but if you went from 1.075 to 1.040 before fermentation, I would believe the numbers.

I can almost buy those numbers as being the drop from adding vodka. That quantity would drop the OG fairly well.

If it is after fermentation, it sounds like you stunned or killed the yeast.

Let it sit and take a reading after a few days.

 

[figgnewton]

I checked it over a a week period after adding the vodka. I thought the fg should have been lower as well but was thinking the vodka may have had to do with it. I am wondering if it is my refractometer not reading correctly. I had a hydrometer that I broke so ended up buying this. All my readings with my hydrometer made sense.

 

[worlddivides]

Sounds like you might have a stalled fermentation. No beer should have a final gravity as high as 1.040. Also, since alcohol is lighter than water, adding vodka would lower the gravity. So, maybe before you added the vodka, you had a final gravity of 1.044 and afterwards you had a final gravity of 1.038. Either way, those numbers are way too high and if you try to taste it, I'm betting you'll find it way too sweet.

The ABV calculation, though, is:

(OG - FG) * 131 = ABV

So, in that case, at this moment you probably have an ABV of around 4.5% alcohol or so, but that you are shooting for an ABV of around 8.2% ABV or so (depending on the style and how low you want the FG).

 

[beergolf]

I checked it over a a week period after adding the vodka. I thought the fg should have been lower as well but was thinking the vodka may have had to do with it. I am wondering if it is my refractometer not reading correctly. I had a hydrometer that I broke so ended up buying this. All my readings with my hydrometer made sense.

There is your problem right there. Using a refractometer after fermentation is not accurate. There are claculators that will give you closer results, but best to take the reading with a hydrometer and report back.

 

[Yooper]

There is your problem right there. Using a refractometer after fermentation is not accurate. There are claculators that will give you closer results, but best to take the reading with a hydrometer and report back.

Yep.

Refractometers read the refraction of light in a sugar solution. Alcohol skews the reading.

The thing is, even a hydrometer won't read correctly when that much alcohol gets added, since alcohol is lighter density than water.

I have no idea the SG of pure vodka, but would guesstimate it to be something like .950 or so. Dry wine is .990, water is 1.000, so you can see that adding vodka wouldn't give an accurate reading once it's added with a refractometer OR hydrometer.

 

[figgnewton]

so basically I will have no idea what my alcohol content actually is lol. at least it tastes really good

 

[figgnewton]

also I thought the whole point of a refractometer is to take a reading before fermentation and then take one after the beer is done? That is what I always do with a hydrometer. I think I will just have to pick up a hydrometer again and test that.

 

[Yooper]

also I thought the whole point of a refractometer is to take a reading before fermentation and then take one after the beer is done? That is what I always do with a hydrometer. I think I will just have to pick up a hydrometer again and test that.

No, a refractometer is awesome before fermentation. You can take preboil readings, readings during sparging, etc, and it takes one drop of wort.

I'm a winemaker, and I can check the brix of grapes with a drop of the juice, or any other fruit I have.

But once fermentation starts, a refractometer can be pretty useless. You can tell when beer is done, because the reading won't change- but it's also not even close to accurate to give you an actual FG.

The only way it can work is if you use a decent online calculator, and even then they are 100% accurate, but it might be close enough for many people. Here's the only one I know of worth anything: http://seanterrill.com/2012/01/06/refractometer-calculator/

A refractometer is a great tool, but it won't replace a hydrometer. It's like a cordless drill- great tool, but it won't ever replace a hammer.

 

[55x11]

No, a refractometer is awesome before fermentation. You can take preboil readings, readings during sparging, etc, and it takes one drop of wort.

I'm a winemaker, and I can check the brix of grapes with a drop of the juice, or any other fruit I have.

But once fermentation starts, a refractometer can be pretty useless. You can tell when beer is done, because the reading won't change- but it's also not even close to accurate to give you an actual FG.

The only way it can work is if you use a decent online calculator, and even then they are 100% accurate, but it might be close enough for many people. Here's the only one I know of worth anything: http://seanterrill.com/2012/01/06/refractometer-calculator/

A refractometer is a great tool, but it won't replace a hydrometer. It's like a cordless drill- great tool, but it won't ever replace a hammer.

and you can still back out your ABV. You just need to understand how to use conversions and do some math.
pure ethanol has lower specific gravity than water, 0.789, which allows you to convert from OG and FG into ABV.
Similarly, pure ethanol has higher index of refraction than water (1.36 vs. water 1.33) so when sugar wort which has even higher index of refraction gets converted to ethanol, the brix drop but not all the way down to pure water (whereas in density measurements it could go below that of pure water).

Bottom line - your 1.080 drop to 1.040 in refractometer readings (or from 18.2 to 10 brix) means your FG is about 1.018.
(use BeerSmith Refractometer tool, or this calculator: http://www.northernbrewer.com/learn/resources/refractometer-calculator/)

if you did your measurements AFTER adding vodka, which is about .950 as Yooper said, and you added about 2.2% (14 oz into 5G), then your actual beer-only gravity is about 1.5 points higher or 1.0195 (so that weighted sum of 2.2% of .950 and 97.8% of 1.0195 gives you 1.018) - and addition of vodka brings it down a bit.

So beer by itself going from 1.075 to 1.0195 gives you about 7.3% ABV.

Then vodka at 2.2% of 40 ABV, brings it to 8.0% ABV.

 

[Calder]

You can still figure out abv.

As noted, your refractometer reading means nothing on it's own.

14 ozs of vodka (30%) added to 4 gallons increases the abv by .08%. That is equivalent to .006 on the OG.

You started with 1.075. Add the .006, and you started with an effective OG of 1.081.

Using 1.081 as the OG and the 1.040 refractometer reading, you can use a refractometer calculator to get your actual FG (estimate) and abv).

I plugged it into BeerSmith:

OG: 1.081, FG: 10.0 (refractometer reading) = FG 1.015 and 8.71 % abv.

 

[figgnewton]

makes more sense now. the projected started gravity was 1.078 and fg 1.018. It seems too smooth to be that strong. Also the reason for the vodka, it is a marshmallow flavored one so I soaked it in some chocolate nibs to try to create a s'more beer.

 

[oldjetdriver]

There is an app that will do the math to get ABV after fermentation. Search for Refracto

 

[55x11]

You can still figure out abv.



As noted, your refractometer reading means nothing on it's own.



14 ozs of vodka (30%) added to 4 gallons increases the abv by .08%. That is equivalent to .006 on the OG.



You started with 1.075. Add the .006, and you started with an effective OG of 1.081.



Using 1.081 as the OG and the 1.040 refractometer reading, you can use a refractometer calculator to get your actual FG (estimate) and abv).



I plugged it into BeerSmith:



OG: 1.081, FG: 10.0 (refractometer reading) = FG 1.015 and 8.71 % abv.

It's 0.8 abv increase from vodka, not 0.08. 14/128/4*30=0.8

 

[JayDog]

I just want to know why you added 14 oz. of vodka to 4 gallons of beer.

 

[okiedog]

Is there a calculator that will give you real attenuation based on hydrometer readings?

 

[figgnewton]

It was a marshmallow vodka, I added it to get the marshmallow flavor and you can not taste the vodka at all but the marshmallow flavor came through a little. This was for a s'mores beer

 

[eadavis80]

3 beers + smile = you did good. That's my ABV formula.

 

[figgnewton]

I basically needed to know if my abv was just wayyyyy off. I didn't realize that 1.04 using a refractometer is actually 1.018 post fermentation.

 

[m1k3]

I didn't realize that 1.04 using a refractometer is actually 1.018 post fermentation.

1.040 refractometer reading is only 1.018 post fermentation for a specific OG wort.

You have to know the OG of the wort to calculate the FG with a refractometer.

That is why this question cannot be solved with the information provided.

 

[figgnewton]

My original information was that my og was 1.075 and my fg was 1.040 but I also added the vodka before checking the fg

 

[Calder]

It's 0.8 abv increase from vodka, not 0.08. 14/128/4*30=0.8

Correct, but the .006 still applies - fat fingers.

1.040 refractometer reading is only 1.018 post fermentation for a specific OG wort.

You have to know the OG of the wort to calculate the FG with a refractometer.

That is why this question cannot be solved with the information provided.

He gave us everything. You just need to convert the alcohol to a gravity amount, and add that to the OG. See my previous post.

My original information was that my og was 1.075 and my fg was 1.040 but I also added the vodka before checking the fg

See my previous post, I think you went from an effective OG of 1.081 to an FG of 1.015.

 

[figgnewton]

Correct, but the .006 still applies - fat fingers.



He gave us everything. You just need to convert the alcohol to a gravity amount, and add that to the OG. See my previous post.



See my previous post, I think you went from an effective OG of 1.081 to an FG of 1.015.

I appreciate the input. This beer has to be the smoothest above 8 percent beer I have had, no burn of alcohol but sure can feel it after drinking a tall one.