Why do I need to know the FG?

[raysmithtx]

My first batch (extract) has been fermenting for about a week now. As I read it the general consensus is to leave it in the primary for a few more weeks but I also read that I need to check the FG over a few days in a row (after the bubble stop) to be sure the fermentation is done.

If I read the FG and it's off can I do anything about it?

So my question is why do I need to take the FG? OK I know that I can use the FG to calculate the alcohol content but other than that why? With the batch in the primary for a couple of more weeks the yeast will surely be done by then.

Can't I just wait until I'm ready to bottle it or move it to a secondary to read the FG?

It's my first batch and I'm trying to learn here as I make beer. I was taught to always ask "Why?" Yeah I know it can be irritating but it's the best way to learn.

Great forum with lots of helpful people. Thanks.

 

[Soup4you2]

Mostly to ensure that fermentation has stopped, and that you can calculate your final ABV.

Those would be the two main reasons to take a FG reading. Some other reasons would be for a nice taste test once you are done taking the reading and to look like a mad scientist in front of your wife or friends.

 

[unionrdr]

If you aren't sure you have a stable FG,& you prime & bottle,you can get bottle bombs. Flying glass is not fun. Besides being able to calculate the ABV%,it also helps with repeatability of the recipe. It doesn't require much time or effort,& serves a couple of purposes.

 

[OG2620]

It could help prevent bottle bombs.

 

[frazier]

You are basically correct that if you wait three weeks, you can pretty much count on fermentation being done. I have never taken the recommended two readings three days apart - I just bottle it up. But I usually wait four weeks.

Even so, I always do check the FG at bottling time. I want to keep tabs on my yeast selection, fermentation process, mash temps, etcetera - and knowing the FG adds a useful piece of data.

 

[billl]

People brewed for a thousand years before anyone took a gravity reading, so obviously you can make beer without it. However, those brewers weren't putting their beer in glass containers.

An no, the beer won't "surely" be done in 3 weeks or even 4 weeks. There are plenty of threads on here about fermentation stalling and then picking back up later.

 

[TopherM]

I haven't taken a FG reading in about 2 years and make plenty of beers that rate in the mid to high 30s in BJCP competitions. That said, here's why NEW brewers should:

1) The biggie is that new brewers tend to be impatient when it comes to waiting on a beer to reach FG. They want their beer in their belly!!! Carbing a beer before it reaches FG can lead to bottle bombs and a beer that is sweeter than the intended recipe. One of the reasons I don't really need to take FG is that most of my low to medium gravity beers are in primary fermentation for 3-6 weeks, and I have the experience to know from a wort sample if I've had a stuck fermentation that didn't reach FG.

2) Repeating recipes. If you want to repeat your favorite house recipe, and want it to be EXACTLY like the other one, you HAVE to control OG and FG. I know many brewers who are an*l about repeatability, and for pro brewers, it's a must. Well, in order to have close to 100% consistency, FG is one of the variables you have to control. There are lots of reasons a yeast might attenuate a bit more than they did on your last batch, and if you want that beer to taste exactly like the last batch, you need to stop yeast activity at the correct FG. I personally have 3-4 house brews that I make over and over again, but I'm fine with them having slight variations, so I don't worry too much about strict repeatability, thus the FG isn't that big of a deal.

3) Like someone else said, you need FG to calculate your exact ABV. Again, I don't care that much. You can get a rough idea of ABV by figuring that your yeast will attenuate around 75% of your OG. This is the calculation brewing software uses to figure out expected ABV. You will get to within about +/- 0.4 or so margin of error using expected ABV as opposed to the actual ABV if you take an accurate FG reading.

So that's my take. FG readings are kinda like telling a new driver that they need to keep their hands at 10 and 2 - of course you don't HAVE TO, but it's important to know WHY that's the best way, so when you run into an issue you can remember your basic training and possibly save your life (beer ). FG readings are a basic building block skill that you really need to UNDERSTAND to be a good brewer, so it's imperitive to have some hands on taking readings and interpreting that data when you are a new brewer, but once you understand why it's important and have that skill mastered, you can let it slide if you want. You just need to be knowledgable in the issues that the FG can possibly be a variable in in case you need to troubleshoot down the road.

So there you go!

 

[Yooper]

I don't think three FG readings are necessary. Instead, the readings should be at least 3 days apart (so, two readings).

Here's why- if the beer is at 1.018 today and 1.015 in three days, it's not done. And you could have bottle bombs if you bottle when it's not done.

That is the only reason, really. I mean, exact ABV doesn't matter. But bottle bombs are dangerous.

 

[DrVertebrae]

I'm with you, who really needs it. I've now brewed 16 or 17 batches over the past year and all have been great, except for the Whit beer, and all have fermented to completion. In most cases I leave the beer in the fermenter for 2 to 3 or even more weeks; have never used a secondary except for when I carb in the keg (I now also dry hop in it as well). I observe the air lock and can see the initiation and decline of CO2 production. I was looking at OG and FG originally but gradually got away from it. I guess that if you are rushing and want it out of the fermentation tank faster then you could follow the three readings in a row rule, but I don't so there.

Its interesting but not necessary to brew good drinkable beer. Probably the best advice I've read here thus far is about reproducibility. If you get a great one that you want to reproduce, then it is an absolute necessity. Otherwise, well, if you don't want to don't worry about it, don't. Personally, I think one of the best things about home brewing is the variability of the brews. Each one is different and a nice little surprise.

 

[flipfloptan]

When I first started brewing I checked FG. I don't check it anymore. If you have bubbles in the fermenter you know it is going to be beer one day. For taste and fermentation, I have found 4 weeks in the fermenter is the magic number for 90% of my beers. I'll keg and/or bottle after 4 weeks.

Big imperial stouts I will let sit another couple of weeks.

 

[Yooper]

I'm with you, who really needs it. I've now brewed 16 or 17 batches over the past year and all have been great, except for the Whit beer, and all have fermented to completion. In most cases I leave the beer in the fermenter for 2 to 3 or even more weeks; have never used a secondary except for when I carb in the keg (I now also dry hop in it as well). I observe the air lock and can see the initiation and decline of CO2 production. I was looking at OG and FG originally but gradually got away from it. I guess that if you are rushing and want it out of the fermentation tank faster then you could follow the three readings in a row rule, but I don't so there.

Its interesting but not necessary to brew good drinkable beer. Probably the best advice I've read here thus far is about reproducibility. If you get a great one that you want to reproduce, then it is an absolute necessity. Otherwise, well, if you don't want to don't worry about it, don't. Personally, I think one of the best things about home brewing is the variability of the brews. Each one is different and a nice little surprise.

Yes, but............... if you're kegging, you don't have to worry about bottle bombs. So, it's not really the same at all.

If you don't check the FG, and it's high, no biggie. If someone who is bottling doesn't, bottle bombs will result. That's a HUGE difference.

 

[beerloaf]

I haven't taken FG or even OG readings in some cases due to brewing the same beer over and over enough times that, due to having my process down, it turns out just about the same every time. I bottled a stout a few weeks ago where I didn't bother taking FG readings. The reason for that is it sat in a fermenter for 4 weeks. It was an average gravity brew (1.046)without needing that much time, so I knew it was well done. Guess what, no bottle bombs! Once you get your process down it's just repetition. I trust my process and equipment and yeast and know what to expect. To each his own. Do whatever works for you.

beerloaf

 

[DrVertebrae]

Same here, no bottle bombs ever. You see fermentation go for a few days and then gradually decline and you know its finished. The only time I wondered was when I first used a starter. the bubbling was done in 48 hours. That one I checked to make sure and sure enough, it was finished.

 

[jperry]

I think it helps to take an FG, so that you can try to replicate the recipe as close as possible...