How to determine ABV?

[Seghes]

I had lots of help recently from this forum with a near fail first venture in BIAB. The recipe predicted OG 1050 and I got 1040. Most likely causes were not having a fine crush and underestimating attenuation (by following the recipe) and not mashing long enough. Best offered practical solution for now was add more sugar, so I added 350g of brewing sugar which brought the whole FV to life again. Tilt hydrometer readings are now showing a consistent average of 1011 after 12 days so I’m looking at kegging in a day or so. Recipe FG target was 1009, ABV 5.3% My revision estimates show 1040 to 1011 to be 3.8%.
My question is whether this revision is accurate if I changed the whole thing by fermenting with more sugar. SG at the time rose a couple of points to 1016. How can I now calc ABV?

 

[Hazelwood Brewery]

You can’t use your original gravity and the new final gravity because you’ve moved the goal-posts.

Assuming you have a 23 litre batch, this much sugar will have added around 0.6% so add this to the ABV you calculated before adding sugar.

1 pound of sugar per US gallon gives 46 points

350g = 0.77 pounds so that becomes 35 points

23 litres = 6 US gallons so divide by 6. Gives you 6 points

Assume yeast attenuates 80%. 4.8 points will be converted to alcohol

Multiply those points by 0.13125=0.63%

 

[Seghes]

You can’t use your original gravity and the new final gravity because you’ve moved the goal-posts.

Assuming you have a 23 litre batch, this much sugar will have added around 0.6% so add this to the ABV you calculated before adding sugar.

1 pound of sugar per US gallon gives 46 points

350g = 0.77 pounds so that becomes 35 points

23 litres = 6 US gallons so divide by 6. Gives you 6 points

Assume yeast attenuates 80%. 4.8 points will be converted to alcohol

Multiply those points by 0.13125=0.63%

I will be so happy with this result I was expecting a “weaker” session beer for my Hefe Weiss (wheat beer). But how would I know yeast attenuation % ?

 

[Hazelwood Brewery]

It’s usually written on the pack (see below top-left corner) or on the manufacturers website. It will be a range too so the figure is not absolute. Also, the figure will be affected by the fermentability of your wort.

 

[foxbat]

You can’t use your original gravity and the new final gravity because you’ve moved the goal-posts.

Assuming you have a 23 litre batch, this much sugar will have added around 0.6% so add this to the ABV you calculated before adding sugar.

1 pound of sugar per US gallon gives 46 points

350g = 0.77 pounds so that becomes 35 points

23 litres = 6 US gallons so divide by 6. Gives you 6 points

Assume yeast attenuates 80%. 4.8 points will be converted to alcohol

Multiply those points by 0.13125=0.63%

My calculations are a little different because dextrose is 100% fermentable. Therefore I'm making the assumption that the FG is what it would be without the dextrose addition, because none of it is left behind and there's 6 fully converted points to add to the numbers.

Therefore I'd calculate it as:

1.046 - 1.011 = 4.6%

 

[Seghes]

1.046 - 1.011 = 4.6%

This just gets better and better. Thank you.

 

[Seghes]

Thanks Hazelwood, Aha, yeast packet said 70-75% attenuation. Can I ask which calc tool, if any, are you using?

 

[Hazelwood Brewery]

My calculations are a little different because dextrose is 100% fermentable. Therefore I'm making the assumption that the FG is what it would be without the dextrose addition, because none of it is left behind and there's 6 fully converted points to add to the numbers.

Therefore I'd calculate it as:

1.046 - 1.011 = 4.6%

I may be wrong but I’m not sure you can treat this as one fermentation and use the upper and lower gravity values. There were two fermentations in which each achieved different attenuation and had different levels of fermentability.

I may be just over complicating things but trying a worked example...

Beer A: OG=1100, FG=1020, ABV=10.5%
Beer B: OG=1050, FG=1010, ABV=5.25%

50:50 blend of beer A and beer B = 7.875%

Adding the 50 points from beer B to beer A creating an OG=1150 and then using a blended FG, say 1015 if we’re normalising for the same volume of both beers would give an ABV of

1150-1015x0.13125=17.7%

Have I got this right?

 

[Hazelwood Brewery]

Thanks Hazelwood, Aha, yeast packet said 70-75% attenuation. Can I ask which calc tool, if any, are you using?

No tool - except me of course

 

[Seghes]

No tool - except me of course

 

[foxbat]

I may be wrong but I’m not sure you can treat this as one fermentation and use the upper and lower gravity values. There were two fermentations in which each achieved different attenuation and had different levels of fermentability.

I may be just over complicating things but trying a worked example...

Beer A: OG=1100, FG=1020, ABV=10.5%
Beer B: OG=1050, FG=1010, ABV=5.25%

50:50 blend of beer A and beer B = 7.875%

Adding the 50 points from beer B to beer A creating an OG=1150 and then using a blended FG, say 1015 if we’re normalising for the same volume of both beers would give an ABV of

1150-1015x0.13125=17.7%

Have I got this right?

I think it's simpler than a two fermentations view. I look at it as no different to the commonly used technique when doing Belgians of adding the simple sugars (dextrose) into the primary after the yeast has got through the more complex malt sugars to fend off a stalled fermentation.

Beersmith has a setting for this. You can select "Primary" as the point when you add a fermentable. If I do this with a quantity of dextrose then it simply adds the expected number of points from the sugar to the OG.

 

[Manners]

I may be wrong but I’m not sure you can treat this as one fermentation and use the upper and lower gravity values. There were two fermentations in which each achieved different attenuation and had different levels of fermentability.

I may be just over complicating things but trying a worked example...

Beer A: OG=1100, FG=1020, ABV=10.5%
Beer B: OG=1050, FG=1010, ABV=5.25%

50:50 blend of beer A and beer B = 7.875%

Adding the 50 points from beer B to beer A creating an OG=1150 and then using a blended FG, say 1015 if we’re normalising for the same volume of both beers would give an ABV of

1150-1015x0.13125=17.7%

Have I got this right?

Are you not double counting gravity points in this example?

I would be inclined to add the additional gravity points from the sugar addition to the OG to get OG+ and then do the usual calc using the FG to work out abv.

It makes sense in my head but I stand to be corrected!