teach me about gravity...

[bzwyatt]

I want to know more about what OG and FG numbers mean and can tell me about a beer.

What I know, basically:

- Original gravity tells you how much sugar is dissolved in the beer, which equals potential alcohol content. Final gravity tells you how much sugar is dissolved in the beer, which you can compare to OG to find out with algebra how much alcohol is theoretically in your beer.

- I've read that a stable FG # is a good indicator that a beer is ready to bottle/package.

That's pretty much it...

I want to learn more because I brewed an IPA a couple weeks back, OG was 1.066. After 15 days, the gravity was 1.020. Two days later it was 1.020 and I bottled it. But I'm not sure if two days is long enough to consider the reading to be 'stable'. It seems like 1.020 is a high FG #... but I'm not sure.

It seems like there is more to know than the two basic facts I already do know about it. I've read quite a bit, and I'm sure this discussion is on here, but I'm not finding it. Any references? Or am I missing something obvious in Palmer's book?

Thanks in advance.

 

[evrose]

You've got the basics down. There's always more to learn on different nuances and implications of gravity measurements. One particular related topic is attenuation. For an insanely detailed discussion on attenuation, see here: http://braukaiser.com/wiki/index.php?title=Understanding_Attenuation

That site has a ton of great detail on the entire brewing process. Very technical detail.

 

[bzwyatt]

Thanks for the link.

I've also started doing all grain, so I guess that is getting me to pay more attention to it. First batch I didn't measure anything, just did the brewing. Second one I measured gravity after the mash, it was 1.042. I know that doesn't mean anything specific without giving more info, but the 5.5 gal recipe called OG 1.046, and I measured it from 8 gallons of wort before I started the boil, so I figured I did ok.

But I need to learn more to really know what it all means and how to use it to find the info I want.

 

[WDT]

Wanted to chime in.

Most hydrometers are calibrated at 60F. Mine is off by +.001 @ 70F, and off +.005 at 100F. This could skew your reading.

I know that doesn't mean anything specific without giving more info, but the 5.5 gal recipe called OG 1.046, and I measured it from 8 gallons of wort before I started the boil, so I figured I did ok.

You want to measure O.G. after the boil, when you boil the wort down you are losing water and concentrating the sugar content. You are probably up a bit from 1.042.

My routine is mash, boil, cool, then take OG reading (at 60F). Pitch the yeast at desired temp and ferment out until complete. You can tell this by waiting for the gravity to stabilize over the course of 2 or 3 different readings in about a 5 day span.

The original gravity isn't important if you don't want it to be, a guy I know doesn't even take an OG reading. The FG number is much more important because you will know that the beer is done.

Cheers!

 

[dstranger99]

^ but taking the OG will give you the ABV, with just the FG, you won't know it.........

 

[WDT]

^ but taking the OG will give you the ABV, with just the FG, you won't know it.........

Correct, I was just stating he could skip the OG reading if he didn't care to know the ABV.

 

[woozy]

Second one I measured gravity after the mash, it was 1.042. I know that doesn't mean anything specific without giving more info, but the 5.5 gal recipe called OG 1.046, and I measured it from 8 gallons of wort before I started the boil, so I figured I did ok.

You need volume. A gravity reading of 1.042 at 8 gallons is completely different from a gravity reading of 1.042 at 5 gallons, of course.

If you take the gravity points (subtract the 1 and multiply by 100) of 42, which represents how many units of sugar you have in an amount of water, and multiply by the volume of 8 gallons you get 42 * 8 = 336 which represents a total amount of sugar in some form of units. If you boil this down the sugar won't go away so when you boil this to 5.5 you'll still have the 336 units of sugar but the sugar will be thicker. 336 units/5.5 gallons is 61 gravity points so your post-boil gravity will be 1.061.

You did better than okay!

Gallons? But gravity has nothing to do with units of measurement?

This would come out the same if we had used metric. 42 points * 32 liters is 1344 units. 1344 units/22 liters is 61 => 1.061. So the unit of volume doesn't matter because the unit of sugar will be different but proportional for liters.​

In math, the formula is

Gravity Points1 * Volume1 = Gravity Points2 * Volume2​

which is useful.

But what you probably want to know is your mashing efficiency. Grains have a certain percentage by weight of maximum (never actually achieved) sugar. (Roughly 80% but different grains will vary). We measure (in the USA) this yielding potential in PPG (points-per-pound-per-gallon) which is the ability of a pound of grain to raise the gravity points of a gallon of water. Your grain bill will have its own potential yield but if it were, say, a potential yield of 37 PPG that would mean that if you were to extract **every** bit of sugar from 1 pound of grain, dissolve it into 1 gallon of water, then the specific gravity would be 1.037. (Sugar, by the way, has a PPG of 46; 1 lb of sugar in 1 gallon of water yields a solution with a s.g. of 1.046.) (In metric, this would be a unity to measure the gravity points of 1 kg of grain in 1 liter of water; different units altogether).

If we presume your grain bill had 37 PPG and you had 11 pound, we'd calculate: 37 PPG * 11 pounds = 407 maximum potential units of sugar. What you actually got was 42 points * 8 gallons = 336 units of sugar. Thus you extracted 336 of a possible 407 or 82.5%. Thus your efficiency was 82.5%

[Yes, PPG * lbs = points * gallons. It makes sense if you think of it as total unitis of sugar. Technically the math is:

37 points/(lb/gallon) * 11 lbs = 407 point-gallons.
and
42 points * 8 gallons = 336 point-gallons.

Or you can impediately divide to get the gravity points.

407 max units/8 gallons = 51 max points = 1.052 max
42 points/52 points = 82.5%]​

Which would be pretty damned good.

But of course I don't know the potential of your grain bill or the size of your grain bill.

which you can compare to OG to find out with algebra how much alcohol is theoretically in your beer.

Technically, that's arithmetic; not algebra.

 

[dstranger99]

Correct, I was just stating he could skip the OG reading if he didn't care to know the ABV.

It's one of my favorite things to do, I like to know my ABV right down to the decimal.....I'm a real nerd..........lol

 

[bzwyatt]

Right, I got you, dstranger99 and WDT.

And I know my OG wasn't very precise. I wasn't too worried about it, just wanted to get an idea. It was only my second all grain batch and I am focused on making beer first, getting the physical process down second, nothing else third, haha. Taking gravity readings is a 'detail' I'm not overly concerned about right now. But next batch, I might have enough focus and time to spare during the process to pay more attention to it.

Thanks for all the responses.

Edit, cause the last couple posts happened while I typed this before -

Yes woozy that is what I want to know! There are lots of variables I know make a big difference and my reading was not precise at all. But I just wanted an idea, like I said. But that is definitely what I'm looking for and in future batches, the info I want to get. Thanks!

 

[bzwyatt]

Doesn't a variable make it algebra? I remember my first algebra classes having stuff like -

x + 10 = 15

Isn't that algebra?

Arithmetic is 2 + 2 = ? I always thought...

I don't actually remember the formulas for gravity, but we're finding an unknown, right?

 

[woozy]

Doesn't a variable make it algebra? I remember my first algebra classes having stuff like -

x + 10 = 15

Isn't that algebra?

Arithmetic is 2 + 2 = ? I always thought...

Off topic but... Solving for a variable makes it algebra. Having a formula (with variables) and plugging in values is arithmetic.

ABV = (OG - FG)*131; OG = 1.061, FG = 1.017

is arithmetic because you are just plugging in numbers. This is like Area of Triangle = 1/2 b*h. No algebra, just a formula. The second we manipulate the formula or "work backwards" we are in algebra territory.

But it's all semantics.

I don't actually remember the formulas for gravity, but we're finding an unknown, right?

Well, technically when we do

2 + 2 = ?

we are solving an unknown; we don't know what 2 + 2 is and we are solving it!

Likewise saying "What is the area of a triangle 6 inches along the base and 8 inches tall?" or "If your oG is 1.061 and your fg is 1.017, what is your alcohol rate?" is just plugging in numbers and and doing arithmetic.

Algebra occurs when you have to manipulate the unknown and treat it as a known to solve.

A = 1/2 b * h; b = 6 and h = 8
=> A = 1/3 * 6 * 8 = 24 square inches => arithmetic!

A = 1/2 b * h; b = 7 and A = 21
=> 21 = 1/2 * 7 * h => h = 6 inches => algebra!

 

[bzwyatt]

Right. I don't know why, but without looking at the formulas much, I previously figured you must have to manipulate the numbers and use algebra to get the info you want. But I see now that it is a formula and there isn't an 'unknown' or 'variable' on the 'wrong' side of the =, like in an algebra equation.

 

[bzwyatt]

So alcohol is less dense than water (and whatever other compounds are in beer, I assume... ?) and when I take a sample for gravity readings with my turkey baster, which is only like 6" (half the length of my... you know) I get a sample of the beer with the lowest gravity, no?

How do I account for that? Is it a significant difference? I can verify that the gravity is stable this way, but can I calculate the overall gravity, including the lower strata of beer? ... does it matter with 16-20" of beer ( a fermenting bucket)?

 

[KepowOb]

So alcohol is less dense than water (and whatever other compounds are in beer, I assume... ?) and when I take a sample for gravity readings with my turkey baster, which is only like 6" (half the length of my... you know) I get a sample of the beer with the lowest gravity, no?

How do I account for that? Is it a significant difference? I can verify that the gravity is stable this way, but can I calculate the overall gravity, including the lower strata of beer? ... does it matter with 16-20" of beer ( a fermenting bucket)?

If you did a full volume batch, it'll all be mixed evenly and you won't have an issue. If you top up with water after your boil, it can be rather hard to get a proper mix... In that case, your OG sample can be off because, as you assumed, your getting more water than wort, so it's not as dense. As far as I know, the only solution is to really mix it as well as you can.

 

[aiptasia]

32.2 feet per second per second.....

Oh wait, this isn't beer physics.

 

[bzwyatt]

Nooooo! I quit school to brew so I didn't have to think like that! Aaghh!

 

[Steelhorses]

from now on I'm PM'ing all my math/algebra formula questions to woozy....

 

[dgr]

My assumption: A beer at 1.062 should be hopped more than the "same" beer at 1.052 to maintain balance for that style.

Preboil SG is nice to have. Once OP gets beyond brewing beer to brewing beer He can use his preboil SG and his boil off rate to pretty accurately estimate his post boil SG. Knowing that will allow for consistency from batch to batch and to adjust hop additions to balance against malt/alcohol. You can dilute a high OG batch to hit your numbers. You can dump some and dilute to hit your numbers. You can add DME to a low OG batch, etc.

See woozy's great post on total sugar for the math to determine post boil gravity.