Alc. by volume vs alc. by weight?

[Kmcogar]

Put a recipe into brew pal on my iPhone. It tells me the alcohol by volume and alcohol by weight.

Alcohol by volume is 6.1%
Alcohol by weight is 4.7%

So which one will my homebrew turn out to be?

What's the difference?

Thanks

 

[BigB]

Read this for the full explanation, but your beer will actually be both...it just depends on how you view it: BEER 101: THE DIFFERENCE BETWEEN ABV AND ABW | Eighteen Twenty Nine

 

[Kmcogar]

Soooo, when I bottle the beer...it should be the 6.1%?

 

[BigB]

Yes, it should be 6.1% if you are measuring by volume. AND it should be 4.7% if you are measuring by weight. Like that article states...in the U.S. most beers are measured by volume... so if you wanted to compare the alcohol content of your beer against say that of a BMC, then you would use the ABV. AND most recipes you will find here and from kits...are measuring ABV.

 

[Kmcogar]

Thanks BigB

 

[Zak]

To convert ABW to ABV, simply multiple the ABW by 1.25. That means the 5 percent ABW beer mentioned earlier is actually 6.25 percent ABV.

Is the article off in it's calculation? Cause to get the ABV and ABW the OP post it would have to be more like 1.29 or 1.3

 

[ajf]

Simply multiplying ABW by 1.25 to get ABV is an approximation (and not a very accurate one)
Let's assume that you have 1 liter of beer with a gravity of 1.010 at 20C, and that the beer has 5% ABW.
At 20C, that beer will have a weight of 1010 grams (by definition).
5% of 1010 grams is 50.5 grams, so the beer will contain 50.5g of ethanol.
Ethanol has a density of 0.78945 Kg / Liter at 20C so the volume of ethanol would be 0.064 liters (50.5 / 0.78945 / 1000)
This translates to 6.4% ABV rather than the 6.25% estimated by the formula ABW * 1.25 = ABV.
Now assume that you have 1 liter of beer with a gravity of 1.020 at 20C, and that the beer has 5% ABW.
At 20C, that beer will have a weight of 1020 grams (by definition).
5% of 1020 grams is 51 grams, so the beer will contain 51g of ethanol.
Ethanol has a density of 0.78945 Kg / Liter at 20C so the volume of ethanol would be 0.065 liters (50.5 / 0.78945 / 1000)
This translates to 6.5% ABV which deviates even more from the formula, and illustrates that the gravity of the wort must be taken into consideration when accurately converting ABW to ABV

-a.

 

[Zak]

so temp when getting the gravity readings will have an effect on the difference between ABV and ABW, which is more accurate?

or does the difference between og and fg more important in determining the difference?