FG reading - alcohol mistake

[Miraculix]

Hi guys!

Quick question, do we account for the fact that alcohol has a lower gravity than water when taking the final gravity of our brew?

I recently saw a table that did show the gravity of a water/alcohol solution and roughly speaking, the gravity of the water with 5% alcohol was approximately 0.01 lower than pure water, which has 1.

Do we account for that alcohol mistake? Otherwise, we wouldn't be able to really see how much sugar is left in the brew, as we measured the Og in water, but after brewing the water has become a mix of alcohol and water with a different gravity.

Thanks for your input guys!

 

[RM-MN]

No, final gravity measures the difference directly. If you use a hydrometer in your whiskey it will show a lower specific gravity than water because of the alcohol.

 

[Miraculix]

No, final gravity measures the difference directly. If you use a hydrometer in your whiskey it will show a lower specific gravity than water because of the alcohol.

Thanks for the reply, but does not answer my question.

Before fermentation, the medium in which the sugars are dissolved has an gravity of 1 (water) .

After fermentation, the medium in which the leftover sugars are dissolved has agravity of approximately 0.99 the water /alcohol mix(gravity depending on the amount of alcohol, the more alc the lower the gravity).

This means that we, theoretically spoken, would need to add 0.01 to our fg reading, if we want to calculate the amount of residual sugars left over after fermentation has finished.

Do we do that? If not, doesn't that make the fg reading very unprecise?

 

[Dcpcooks]

Your correct. It's the difference between real attenuation and apparent attenuation. The FG is slightly off due to alcohol.

To find real attenuation you would remove the alcohol and replace it with water. That's beyond the capabilities of most breweries and home brewers.

 

[Miraculix]

But to smooth it out roughly would be quite easy. We know roughly how much alcohol we will end up with because of the Og reading.

This will around 5% alc most of the time. A 5%alc /95% water mix has a gravity of 0.99.

This means that we could simply add this 0.01 gravity point to the final gravity reading and we would be a heck closer to the actual sugar content than with only ignoring the alcohol gravity in the mix.

 

[Dcpcooks]

You can make those calculations if you so desire. Personally, I don't feel the need to take it that far. Apparent attenuation is commonly accepted in the brewing community. I'd have to look and see if that part of the calibration process for hydrometers.

Big breweries use to to manage and monitor yeast health/performance during multiple pitches among other things as it will indicate how the yeast is performing.

 

[dyqik]

The answer to how we compensate for the alcohol in the solution depends on what you want to use the SG for. The formula for calculating ABV/ABW from OG and FG already has that correction in it, so there's no need to correct there. The actual attenuation can also be calculated from the OG and FG, so there's little reason to correct the SGs themselves. It's only if you were trying to make some other kind of measurement that you'd need to think about how the composition of the mixture changes the SG in more detail.

Attenuation calculations.

SG is not a fundamental measurement of the fermentable/nonfermentable sugar or alcohol content etc. in the wort, it's just an easy measurement to make that can be used in calculations that take account of the typical mixtures in brewing.

 

[Miraculix]

Did not see that the formula of the alcohol amount calculation based on Og and fg did actually include the gravity correction.

I only saw something like (Og - fg) = amount of sugar digested by the yeast


Multiplied by a factor this results in the approximate percentage of alcohol. But of course, assuming that the solving medium has a lower gravity after fermentation, this factor could have been amended accordingly.

 

[dyqik]

Did not see that the formula of the alcohol amount calculation based on Og and fg did actually include the gravity correction.

I only saw something like (Og - fg) = amount of sugar digested by the yeast


Multiplied by a factor this results in the approximate percentage of alcohol. But of course, assuming that the solving medium has a lower gravity after fermentation, this factor could have been amended accordingly.

Look at the calculators and formulae here. The OG and FG will give accurate ABV and both real and apparent attenuation, without you correcting the values.

 

[Baabaad00]

does 1/100 of a gravity point make any real difference at all? I mean we don't sterilize our equipment for use because it isn't necessary. We merely sanitize. Why? Because the end goal is to make something that taste good and that means we can have a certain allowable amount of microorganisms present in order to do that.

I'm confused on the reason why this is relevant in a practical sense. In the end, isn't a 5.08% ABV beer vs 5.00% taste the same?

 

[stz]

No. As you said it is very inaccurate, but completely sufficient for our needs.

I was going to write a bunch of waffle, but as the density of alcohol is 0.789 of distilled water, this is what? 0.211 of a point per percent of abv? Real beer mat maths here, but each point of gravity is worth what? 0.13125%? A correction of 0.97230625:1?

This falls into the category of a read error in the real world. We've some really nice saccharometers at work and I'm not confident enough to get enough significant figures off of them to work that out with any degree of accuracy for a typical strength beer.

Big industrial brewers will have a machine for it and it would be reasonable to expect them to. Smaller breweries no as commercially we are allowed a variance of 0.5% abv between what is taxed/advertised and what is actually present. We have to be prepared to show our methodology and that we are trying to do our best to be accurate and be prepared to defend this if asked so that is how precise it needs to be.

 

[Miraculix]

It is not 1/100th of a gravity point, it is a whole gravity point, which would make quite a difference if not taken care of.

Thanks for the links, I will read the formulas later!

 

[Miraculix]

Look at the calculators and formulae here. The OG and FG will give accurate ABV and both real and apparent attenuation, without you correcting the values.

Thanks for the link!

He writes it down a bit more elaborate than me, but that was exactly what I meant. Good explanations and good formulas.

That helps, thanks!

 

[Baabaad00]

But if everyone is taking that difference into consideration when targeting a flavor profile, what's the difference?

It's like saying, I've remeasured the absorbance of red and it's actually a shade of blue. Okay...but everyone has already called and perceives it as blue so we call it blue.

Same thing.

So perceived/labeled 5% isn't true 5%, but it balances the flavor profile to our needs. What's the point?

 

[Miraculix]

The point is that without taking it into account, your calculated alc vol would be around 20% higher than it actually is. Bit too much for my liking.

 

[stz]

The point is that without taking it into account, your calculated alc vol would be around 20% higher than it actually is. Bit too much for my liking.

Honestly I'm happy to be corrected, but it isn't 20%. 0.02769375 for every percentage of abv. An additional 0.13846875% in a 5% beer.

 

[Miraculix]

Honestly I'm happy to be corrected, but it isn't 20%. 0.02769375 for every percentage of abv. An additional 0.13846875% in a 5% beer.

In your 5% example, it would mean that you would get aproximately 6% in your calcs without taking the above into account.

But anyway, as it was linked to before by dyqik, there are formulas around that take all of this into account, so no problem at the end.

 

[Baabaad00]

So all in all, it's practically speaking, 2.7% error is mostly irrelevant. If I had that margin of error in everything in life, I'd be happy.

 

[TasunkaWitko]

So all in all, it's practically speaking, 2.7% error is mostly irrelevant. If I had that margin of error in everything in life, I'd be happy.

You must not be German - we like to think about things, tinker around with them, and look for ways to improve on them or be more precise.

 

[Baabaad00]

Oh. I get it. Go for it. I'm just saying it's exactly as you describe, tinkering...

Tinker definition:
to busy oneself with a thing without useful result
Source

 

[TasunkaWitko]

 

[Miraculix]

So all in all, it's practically speaking, 2.7% error is mostly irrelevant. If I had that margin of error in everything in life, I'd be happy.

I don't know which type of math you are using, but if your calculations show 6% alc and reality is 5% alc, it looks like 20% error to me.

 

[Baabaad00]

Honestly I'm happy to be corrected, but it isn't 20%. 0.02769375 for every percentage of abv. An additional 0.13846875% in a 5% beer.

0.02769375 x 100 = 2.7%

5% x 1.027 = 5.135%

So a 5% beer is true 5.135% according to that calculation, as @stz cites.

 

[Miraculix]

5% alcohol in water solution = gravity of roughly 0.99, one whole gravity point lower than pure water

1 gravity point in the Wort roughly equals one % of alcohol in the final beer.

1% of alcohol alcohol in a 5% beer is one fifth of total percentage of alcohol meaning 20%.

But anyway, everything has been said about the topic, why do you guys keep coming back trying to prove something?

 

[pepindavid]

Your questioning made me research this...

Take a look at this article: https://byo.com/hops/item/408-calcu...ion-extract-and-calories-advanced-homebrewing

Your thinking is right, but the math is not the same. Apparently, Balling made the correct formula.

And choosing the "alternate" formula on https://www.brewersfriend.com/abv-calculator/ uses the corrected one.

The difference is less than what you thought, but still significant as you go into higher gravity.

 

[Kee]

5% alcohol in water solution = gravity of roughly 0.99, one whole gravity point lower than pure water

1 gravity point in the Wort roughly equals one % of alcohol in the final beer.

It takes a drop of roughly 40 gravity points to get a 5.25% beer. An extra drop of 1 gravity point would up your alcohol from a 5.25% beer to a 5.38% beer according to numbers I punch into a calculator.

https://www.brewersfriend.com/abv-calculator/

 

[Miraculix]

I do not care mate, we are all talking permanently in other directions.

I made my point, point was proven, math behind it is already there, so why bother more.

Maybe I got the gravity point wrong and I meant 10 gravity points. This whole gravity point system is a bit dubios to me, as we measure the gravity ... so if a beer has an og of 1.06, a gravity point i was refering to would be 0.01.
Might be 10 in reality? dunno...

 

[pepindavid]

In the end, though, you hunch was right, but it doesn't change the calculation by 10 gravity points (well, it changes the real gravity by something like that, but then you can't use that revised gravity in the usual ABV calculation - the factor doesn't work amymore).

You simply have to use the alternate calculation, and you'll be very close.

Examples:

OG 1.050, FG: 1.010 =
Usual calc: 5.25%
Adjustec calc: 5.34% (tha's a difference of 0.09% ABV...)

OG 1.065
FG 1.015
usual calc: 6.56%
adjusted calc: 6.85% (a difference of 0.29%)

OG 1.100
FG 1.020
usual calc: 11.81%
adjusted calc: 13.23% (a differnce of 1.42% ABV)

Bottom line:
For anything under 7%, the difference is minimal (existing, but minimal). Different story for high ABV beers.