How to take accurate hydrometer readings when you have to add priming sugar from kit

[iMarkSkinner]

I'm still beginner even after a few years doing homebrew. This question is about how to accurately measure the strength of your beer.
Using a kit, when I mix everything up for fermentation I take a hydrometer reading, and another when I am done with the primary fermentation, using the SG readings I work out the strength increase, this is all quite understandable to my tiny mind. Then I would add sugar to keg to prime it and the remaining yeast will carry on fermenting, surely this would increased the alcoholic strength of the beer.

How would I measure that? It is my understanding that adding sugar for priming would make the very first SG reading irrelevant to the final SG reading of the finished beer. Here's my theory on how to work out the final %, please tell me if I'm barking up the wrong tree here.

SG reading 1: Just before adding yeast and sealing for primary fermentation
SG reading 2: Once primary fermentation is finished but before priming
SG reading 3: After adding priming sugar, just before sealing for secondary fermentation
SG reading 4: Finished beer

(2-1)+(4-3)=X

Finally working out total % based on X change in SG.

I would appreciate advice as I'm unable to find the answer with my Google fu. Also if the increase in % is trivial during secondary fermentation, please tell me but for the sake of my interest still tell me about the theory at work here.

Regards
Mark

 

[max384]

The only two gravity readings you need to concern yourself with are the OG and FG. OG is taken right before adding yeast. FG is taken right before adding priming sugar.

Formula: OG - FG * 131 = ABV

If you're concerned about the amount of alcohol that bottle conditioning adds, just add 0.2% to your final ABV number.

 

[HollywoodTK]

Corn sugar has 46 gravity poitns, so 1lb sugar in 1 gallon water will create a mixture with an SG of 1.046. 5 oz of corn sugar in 5 gallons of beer will raise the SG by a little less than 0.003.

5 oz * (1/16)[lb/oz] * 46 [points/gal] * 1/5 (as you are adding it to 5 gallons) = 2.875 points. That's a change of 0.002875

This should theoretically all ferment out so you'll see an alcohol percentage rise of about 0.375%

Percent ABV = (1.05*(OG-FG)/FG)/0.79 * 100

In this case, say your final gravity before adding sugar is 1.014. You add 5 oz sugar to 5 gallons and you get 1.016875. Here you would use 1.016875 as the OG and 1.014 as the FG to get the final ABV.

Will it all ferment out? Probably; most likely. You can verify if you'd like. In that case, use your formula there but plug the values into the ABV formula I posted above.

OG1 = 1.05
FG1 = 1.014

(1.05*(1.050-1.014)/1.014)/0.79 * 100 = 4.72% after fermentation

OG2 = 1.016875
FG2 = 1.015 (say it didn't all ferment out; note, it will)

(1.05*(1.016875-1.015)/1.015)/0.79 * 100 = 0.25% additional alcohol by volume

So your total ABV is 4.72+0.25 which is 4.97% alchohol.

An easier formula for the ABV is (OG-FG)*131