When to measure Final Gravity

[Chris]

Just a quick question regarding when to measure FG.

In the past I've been measuring FG after bottle conditioning has taken place (some months after fermentation).

However I was recently wondering if this is the correct way to do it? Should it not be done prior to priming? I was thinking that when I batch prime before bottling, I am adding sugar, so would that not have an effect on gravity? But then during bottle conditioning is more alcohol not produced?

Some advice on this would be greatly appreciated as I can't find specific information in any of my books.

[delMonte]

Good question. I've just always assumed FG is the measure before bottling. Never did compare the gravity at bottling vs weeks/months after conditioning to see if it differs.
I couldn't find an exact definition either, but J.Palmer writes alot of info on this page: http://www.howtobrew.com/appendices/appendixA.html

Is this so you can get a precise alc. percentage instead of an aprox.?

[Chris]

I've read Palmer's chapter, which as usual for him goes into a lot of depth and explains things clearly. BUT I'm still none the wiser as to the effect of bottle conditioning.

Hypothetically, I would expect FG to be the same before and after bottling (as sugar has been converted to alcohol). However, assuming that conditioning produces marginally more alcohol, I would expect alcohol content to be slightly higher.

The confusion is how to measure the increase in alcohol if FG is the same (assuming there is an increase at all).

Yes, the reason I want to find out is to get as precise a percentage as possible. Calculating this from OG/FG is another (and highly confusing) conundrum!

[delMonte]

Ah, found something else for you:
http://www.brewcraft.com.au/wa.asp?idWebPage=9616&idDetails=173

[Chris]

Thanks for that. The following paragraph from your article sums it up.

"The second reading is of the finished beer. This is taken when you are ready to bottle your beer. this readin is called the FINAL GRAVITY."

So it turns out I should have been taking it before, not after conditioning.

However, I have since found two articles that state that bottle conditioning DOES increase alcohol content:

Article One
Article Two

So the question remains, how the heck do you measure the extra alcohol???

[delMonte]

You are right, it does change. But this calculator on the Brewcraft Aus website takes that into account:

"NOTE: The calculated alcohol percentage includes an allowance for the alcohol made in the bottle during secondary fermentation"

So it adds a bit of % on their automatic formula.

[Chris]

Sorry delMonte, I didn't read the note!

Well that's good enough for me then, will use that calculator from now on.

Cheers for your help.

[delMonte]

Cool. Glad to help. Me learn stuff too.
What strength are you brewing anyway?

I think I might start testing gravity now when 'ready to drink' and convert using the proper formula; just so I can write the accurate % on the label and prepare my mates for what kick to expect.

Salud!

[Chris]

Ah I only brew around 3.5-4.5 %. I'm more into session beers than rocket fuel.

What's the formula you use? I use a spreadsheet I made based on Karl Balling's formula. No idea if it's correct :)

I'm going to compare the results of my formula to the calculator on Brewcraft's site. Will be interesting to see if I'm in the right ballpark.

Yeah I like to know what to expect from my beer; if it's less than 3.5% I know it needs a bit of tweaking, but if it's getting up there in the alcohol stakes I know to go easy on it.

[Edward Siddle]

that brewcraft calculator looks way too high to me. eg. an OG of 45 to an FG of 10 giving 5.2% beer? i doubt it, even allowing for bottle conditioning. the one on the HBD link of dave miller's for alcohol by volume where you divide by 0.75 (or multiply by 1.333...) looks more like it to me.

and remember, the other reason you might measure final gravity before bottle conditioning or kegging etc is to confirm that fermentation has actually finished.

[Chris]

Hmmm, I'll have to try it out against some old notes.

Yep I always measure the SG before bottling over the course of 3 days to make sure it's well and truly finished.

[Simon]

I think you should take the OG of your wort just before you add the yeast. Then take the FG at the end of the fermentation, before you add any more sugar for carbonation. This should give you the percentage alcohol of your beer to a reasonable degree of accuracy.

You could then add the sugars for carbonation and take another gravity reading (in effect a second OG) and then take another reading after bottle conditioning or keg conditioning (like a second FG) to obtain a second % alcohol value that is the result of the conditioning phase. I am guessing that simply adding the two percentage values together would give you the final % alcohol level of your beer.

One thing I noted from my last batch is that I added 85g of dried malt extract for carbonation. This is less than 3% of the weight of the liquid malt extract that I added at the start of the process. For a start, the change in gravity due to such a small addition of sugar would probably be very small, perhaps a change of 0.001 or less. Measurement of such a small change in gravity with a hydrometer would be difficult and somewhat inaccurate. Secondly, the extra alcohol from the dried extract would be small in comparison to that from the main liquid extract. It’s probably not worth bothering about unless you have the tax man breathing down your neck!

[Chris]

I think you're right Simon, the results are going to be so negligible it's really not going to make much difference. And anyway I only wanted an indication of ABV.

From a purely academic point of view though I will go ahead and see what the difference is due to bottle conditioning.