ABV Calculation after adding more honey

[Robintun]

Hello all, been a long time since I posted and have 27 batches under my belt but math still confuses me. I searched and found a number of posts as well as the Mead Calculator to figure this out and was hoping someone would help me see if I am calculating my Alcohol content by volume correctly. The issue is I was adding honey as time went on. I am hoping my calculations are right as I am currently making a braggot that I was hoping to add additional honey and water to over the next few weeks as I was able to extract more from out hives.

The calculation I am wondering about to see if I understand this right is:
1) So I started with a blueberry mead with 12 pounds of honey in a total of 6 gallons with a O.G. of 1.088. Yeast was Lalvin 71B-1122 though as far as I can tell that should not matter for the calculation (?). Temperatures of SGs only varied by about 2-3 degrees over time so I am not considering that as a significant change either.
2) later the SG 1.011 and added 1 pound 13.5 ounces more of honey stirred and new S.G. at 1.023
3) later the SG 1.004 and added 1 pounds 6 ounces of honey and stirred with new S.G. at 1.010
4) FG was 1.003
So as best I can tell the calculation would go:
ABV = (OG-FG)*131.25
13.5=(1.088-[1.003 -{1.023-1.011}-{1.010-1.004}]*131.25
13.5=(1.088-0.985)*131.25

If I use the PAC on my hygrometer: it goes from 12% down to about 1.5% and then up to close to 3.5% then to 0.5% then to 1.5% and finally 0.5%. Therefore around 14.5%.
Averaging the two ways of doing it I am assuming it is around 14% ABV, but I would really appreciate input from someone who is familiar with these calculations.

 

[Medsen Fey]

When factoring in the dilution created by adding another quart of honey, I get 13% ABV. You can measure the alcohol directly using spirit indication but it helps to have a fine scale hydrometer (the usual triple scale ones really aren't precise enough for doing it accurately).

 

[Robintun]

Ok so that is one of the main things I was wondering about - the volume added does it change the calculation? But I assumed it did not since it is a measure of specific gravity which already is a measure of volume in that it is measuring density which is mass per volume.

So If I want to do it accurately to about the 0.5% level with just my triple scale hygrometer what would you suggest is the best way. I currently have a braggot going, brewed the beer from a kit first and went from OG of 1.048 in a volume of 4.75 gallons. This fermented to 1.015 and I racked it off (not sure of final volume but given what was left I would say 4.25G) and added 5 pounds 2 ounces honey which I diluted with 2 quarts of water to mix to the must. The total volume of that mixed I added looked to be about 1.5G (even though I know it should be less than a gallon). After added the SG was 1.050. So I need to keep track of volume changes as well as SG changes to get a reasonably accurate ABV? Should I have measured the SG of the mixture I added before adding it to the must?

 

[Medsen Fey]

An example may help clarify this. I you take 1 gallon of finished mead with a gravity of 1.000 and 12% ABV and mix in 1 gallon of honey-water mixture with a gravity of 1.100. You are going to end up 2 gallons of mead with a gravity of about 1.050, and and ABV of 6%. The change in ABV does have some impact on the gravity but for simplicity you can ignore it. If that mead now continues to ferment and goes back to dry at 1.000 you've fermented another 50 gravity points in the total volume, and 131*0.050 = 6.5% ABV. If you say, hey, I had 12% ABV (based on starting and final gravity of the original gallon) and now I've fermented another 50 gravity points so I must have 18.5% ABV, then you've ignored the fact that when you mixed the 2 gallons together the ABV dropped to 6% and what you have has about 12.5% ABV.

You can see how this easily lends itself to many step-fed meads claiming to be 20+% ABV. The hydrometer does not effectively take into account the effect of dilution when you make determinations of ABV by subtracting differences in gravity.

In the case of your mead, the volume added was probably about a 1 quart, so your ABV will have been reduced by about 1/25, or 0.5% ABV.

In order to be close with your ABV, the key is to track the volume additions along with the gravity additions. As long as you know how much volume you've added, and what the change in gravity is, you can make estimates that should be pretty close.

Of course you can always use a refractometer and hydrometer reading together to get a good estimate of ABV, or as I mentioned before, you can measure it directly.

 

[Robintun]

Thanks for the example. While it clarifies it some and definitely seems logical, I admit to some stubbornness in understanding. They way I would want to explain it is I have 1 gallon that started with 1.050 and went down to 1.000 then add 1 gallon of honey-water mixture with a gravity of 1.100. It makes sense that at that point I would to end up 2 gallons of mead with a gravity of about 1.050, and and ABV of 6%. But then once that goes to 1.000 I can not get my head around thinking that that would not have been the same as saying 1.05 plus 1.05 (since you are taking in account the change in volume with the second reading) for a total of 1.100 and pretending this is what you started with - though you did not do this in the first place maybe because your yeast could not handle that high of and OG. But I will keep closer track of volumes.

A branching question though on the refractometer you mention though, is that what you meant my measuring it directly? I have only seen 2 sold retail and was under the impression they were really only effective on very dry wines (meads)? Is there one that can be used in semi-sweets? Something preferably under $500?

 

[Medsen Fey]

Thanks for the example. While it clarifies it some and definitely seems logical, I admit to some stubbornness in understanding. They way I would want to explain it is I have 1 gallon that started with 1.050 and went down to 1.000 then add 1 gallon of honey-water mixture with a gravity of 1.100. It makes sense that at that point I would to end up 2 gallons of mead with a gravity of about 1.050, and and ABV of 6%. But then once that goes to 1.000 I can not get my head around thinking that that would not have been the same as saying 1.05 plus 1.05 (since you are taking in account the change in volume with the second reading) for a total of 1.100....

Don't fret. It takes a little to get used to the idea, which is why most people don't do it. So, lets repeat the process with your example.

You start with a must of 1.050 (without fermenting). Then you add another gallon of must at 1.100, and what do you get? Not a must of 1.100, but a must of 1.075 (potential ABV 9.8%). So effectively your starting gravity in the 2-gallons volume was 1.025. If you can get your arms around adjusting you starting gravity that is an approach that works

Looking at the same thing the way I do, if you start with 1.050 in a gallon and ferment it dry to 1.000 you have 1 gallon with 6.5% ABV. Now add your gallon of must at 1.100 and you have 2 gallons of liquid with 3.25% ABV, and a gravity of 1.050. You ferment that dry to 1.000 raising the ABV by 6.5% to about 9.75% (with a little rounding error).

A branching question though on the refractometer you mention though, is that what you meant my measuring it directly? I have only seen 2 sold retail and was under the impression they were really only effective on very dry wines (meads)? Is there one that can be used in semi-sweets? Something preferably under $500?

Homebrew heaven sells refractometers for about $90. With a bit of searching you can probably find one for 1/2 that; not terribly expensive. If you combine a refractometer reading and hydrometer reading together you can use a calculator like vinocalc to provide an estimate of the ABV.


I hope that helps.

Medsen

 

[Robintun]

Thanks I follow you better now especially with the first example, except for 1 part.

Don't fret. It takes a little to get used to the idea, which is why most people don't do it. So, lets repeat the process with your example.

Not a must of 1.100, but a must of 1.075 (potential ABV 9.8%). So effectively your starting gravity in the 2-gallons volume was 1.025.

Medsen

Why do you say OG would be 1.025 when just the sentence before you said the average would be 1.075. Please tell me that was a typo or I did not finally get what you have been trying to explain. Even still I get your point.

And on the refractometers, they are still for measuring the potential. There is no instrument to figure out the direct alcohol content of a solution of something you did not make and know the before and after? You know, something you put a drop in and it says, "this has 5% EtOH contnet", short of a mass spectrometer that is.

 

[Medsen Fey]

Why do you say OG would be 1.025 when just the sentence before you said the average would be 1.075. Please tell me that was a typo or I did not finally get what you have been trying to explain.

What I was trying to convey (and poorly admittedly) was that if you started with 1 gallon and a gravity of 1.050, and turn it into a 2 gallon volume (by just adding water for example), then the effective staring gravity for the 2 gallon total would be 1.025. I hope that make it more clear.

And on the refractometers, they are still for measuring the potential. There is no instrument to figure out the direct alcohol content of a solution of something you did not make and know the before and after? You know, something you put a drop in and it says, "this has 5% EtOH content", short of a mass spectrometer that is.

The refractometer measure the refraction of light and is calibrated to read a sugar/water solution. When you mix a must, the refractometer and a hydrometer reading will be essentially identical. As alcohol is produced, it throws the refractometer reading off, because ethanol has a higher refractive index than the sugar/water solution, and it will cause you to get a higher reading on the refractometer than what you get with a hydrometer. With a typical dry batch, you may have hydrometer reading of 0 Brix (1.000) while the refractometer will have a reading of 8-12 Brix due to the amount of alcohol.

Knowing the fact that the more alcohol that is in solution, the greater will be the difference between the refractometer reading and the hydrometer reading, they have developed a formula that can give you a pretty good estimate of the ABV based on the difference between the two readings. Vinocalc is one example of a calculator that you can use. So while it is not a direct measure of the alcohol, it is a very good estimation of actual ABV based on that refractive change.

What I meant by measuring it directly was using the spirit indication method (which is technically still an indirect measurement, based on the change in gravity after boiling off the alcohol, but it is a measurement of actual alcohol present).

 

[Robintun]

Great, nice link and thanks for your time.