Calculating alcohol after the fact

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HappyWino

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This is a re-post of a question I asked in the Brew Science forum before I realized that it was a sub-forum of "Beer". Hopefully posting here will get some more wine\cider related feedback as that is what I am making! :)

I have long wished (as have many others I am sure) that there were an easy way to calculate actual ABV of a finished product but always believed it required more complex lab equipment than I am willing to get involved in.

Recently a friend came across a couple of different sites offering a calculation that should yield ABV based off of Hydrometer and Refractometer readings of the finished brew. VinoCalc has one here

It seems too good to be true, does anybody understand the science behind this enough to endorse it? Are there any caveats such as only working for liquids with little to no residual sugars?

Ultimately I would like to use on off-dry ciders as well as dry wines.

Thanks for any insight.

Cheers

HW
 
This is very frequently used. The basic gist is that a hydrometer measures dissolved sugars. So if you measure the change in dissolved sugars (initial reading vs final reading) during fermentation, you know the amount of sugars the yeast converted to alcohol, and can estimate ABV.

EDIT: Perhaps I misread your question - are you asking if there's a way to tell solely from the finished brew...i.e., without an initial reading, by comparing the hydro to refract readings? If so, 1) please don't take my first answer as condescending, and 2) yes, this method seems reliable to me, though I've never used it. I'd be concerned that the margin of error would be too wide for my liking without precise measurements.
 
With a Hydrometer you don't even need to calculate anything. Take an Original Gravity, take a Final Gravity subtract the FG from the OG, and then look at a chart to see what the ABV is. It is a piece of cake and the way most of us do it. With what you are wanting, it is even easier because you want dry wines and cider. So all you will need is the OG no FG (taking an FG is still a good idea to insure the brew is truly dry) the OG will give you the Potential Alcohol, and as long as it is dry the PA will become ABV. nothing to it. I love using my Refractometer for taking OGs due to the small sample size and the fact that solids in the liquid do not effect it, but it doesn't work as well once alcohol has been produced because the alcohol throws off the reading.
 
The short answer is (OG-FG) x 131= ABV%

For example, if your OG is 1.100, and your FG is .990, the ABV is roughly 14.4%

(1.100-.990) x 131
(.11) x 131= 14.4
 
I'm not sure about the OP, but last time I made a wine (plum wine) - I wondered something similar. I crushed many pounds of plums and added sugar on top of that, but I have no idea how much sugar I was getting from the plums because a hydrometer reading would only show how much sugar was in solution - not the sugars that were still within the plum pulp and juice contained therein.

For the OP, you can take a refractometer reading and a hydrometer reading at the end and you should be able to calculate the original gravity/abv. I'm no expert, but my understanding is that the reason for this is that the hydrometer reading isn't affected by alcohol in solution, but the refractometer's reading is. Having both numbers allows us to calculate how much alcohol is in solution.
 
A hydrometer doesn't only measure "dissolved sugars", it measures the specific gravity of a solution. That can be affected by sugars, pulps, yeast, shoe dirt, cat hair...anything that makes the liquid thicker. We (the brewing community) use the generic ("sugar") version to give us an estimate of potential alcohol.

If it ONLY measured sugar, the SG would never go below 1.000. It is also measuring how much of a thinner liquid is mixed in to the water (in our case, alcohol). Pure alcohol measures .787, by the way.
 
DoctorCAD said:
A hydrometer doesn't only measure "dissolved sugars", it measures the specific gravity of a solution. That can be affected by sugars, pulps, yeast, shoe dirt, cat hair...anything that makes the liquid thicker. We (the brewing community) use the generic ("sugar") version to give us an estimate of potential alcohol.

If it ONLY measured sugar, the SG would never go below 1.000. It is also measuring how much of a thinner liquid is mixed in to the water (in our case, alcohol). Pure alcohol measures .787, by the way.
Click to expand...


Fantastic point!!!!!!!! That is why a refractometer is also a nice tool to have. Before any alcohol has been created a refractometer is still slightly vulnerable to things throwing off a hydrometer but not by much, so it will give you a much closer OG. After the alcohol has been produced a refractometer's readings are no longer accurate due to the alcohol so a hydrometer is better for FG.
 
Daze said:
After the alcohol has been produced a refractometer's readings are no longer accurate due to the alcohol so a hydrometer is better for FG.
Click to expand...

Totally disagree here. The refractometer can't be read directly (can't simply multiple brix*4 as you can pre-ferment) and you need to know the OG, but I almost exclusively use refractometer for post fermentation readings and FG. I periodically double check my readings against a hydrometer and it has yet to be anything but spot on accurate.
 
bruin_ale said:
The refractometer can't be read directly (can't simply multiple brix*4 as you can pre-ferment) and you need to know the OG, but I almost exclusively use refractometer for post fermentation readings and FG.
Click to expand...

Agreed. Which is exactly why, as you said, you can infer the ABV using the difference between the refract & hydro readings.
 
bruin_ale said:
Totally disagree here. The refractometer can't be read directly (can't simply multiple brix*4 as you can pre-ferment) and you need to know the OG, but I almost exclusively use refractometer for post fermentation readings and FG. I periodically double check my readings against a hydrometer and it has yet to be anything but spot on accurate.
Click to expand...


What I said wasn't totally wrong :) I knew you could correct for the refractometer reading but for the purpose of this thread it seamed like to much info. That said I should have mentioned it so as not to give the impression that a refractometer doesn't work when there is alcohol in the brew, my bad. Thanks for setting the thread straight!!!

BTW what formula do you use to alcohol correct your reading???
 
Any decent brew software has the calculator built in, it's not really a straight forward as the hydrometer ABV formula. Too be honest, I don't know the formula because I just use those calculators.
I've been using the Brewzor Android app on my phone lately as the calculator.
 
HppyWino,

It sounds like you're asking, if the Original Gravity can be calculated based on Hydrometer and Refractometer readings? And secondly, what are the limits of that calculation? So here goes...

The short answer is yes, the limits are hard to know without examining the exact formula used but my attempt to create a derivation causes me to believe that it is long, complicated, and makes many assumptions; empirical evidence suggests most of the available software or spreadsheets keep the answer well inside the measurement error of a home brewer's equipment. The good news is this shouldn't be impacted in any way by residual sugars in the measured sample; however, since brewing Refractometers are calibrated for a solution of sucrose and water, super dry wines may be a problem. Based on the spreadsheet posted at MoreWine!, with 1 Degree Brix minimum reading refractometer, you would be unable to use this method for an Actual Gravity below 0.946. Gadgets to the rescue, super dry wines may be measured in %ABV with a Vinometer to reasonable precision; again, online calculators could be used to obtain the OG with %ABV and Actual Gravity.

You can take advantage of the refractometer correction calculators by using a hydrometer to measure the Actual Gravity and the Refractometer to measure the Apparent Gravity; input the Apparent Gravity into the calculator and adjust the Original Gravity value until you get the Actual Gravity as the result.

If you want to know why this works:
A Hydrometer measures Specific Gravity of a fluid as a function of its density. The printed scale simply reads-out the results of the equation to convert the density of the sample fluid as a function of the floatation height divided by the density of pure water.​
A Refractometer works by using the Index of Refraction for the sample fluid compared against a scale calibrated for the desired readout unit. Refraction is the apparent bending of a beam of light caused by the change in the speed of light as it travels through different mediums. The Refractive Index of a material is dependent on the substance. For reference, the Index of Refraction for light at a wavelength of 600 nm is 1.33 for water at 20 degrees C and 1.36 for Ethyl Alcohol.​

The introduction of alcohol (or, technically, anything other than water or sucrose) causes the index of refraction for the solution to change and thus the Refractometer must have a correction applied to account for the change of the Refractive Index. This changes means that a Refractometer reading cannot be used to directly calculate %ABV because one or both of the readings will be affected by measuring a solution outside of what the scale is calibrated to read.
 
WhtHawk said:
HppyWino,

It sounds like you're asking, if the Original Gravity can be calculated based on Hydrometer and Refractometer readings? And secondly, what are the limits of that calculation? So here goes...

The short answer is yes, the limits are hard to know without examining the exact formula used but my attempt to create a derivation causes me to believe that it is long, complicated, and makes many assumptions; empirical evidence suggests most of the available software or spreadsheets keep the answer well inside the measurement error of a home brewer's equipment. The good news is this shouldn't be impacted in any way by residual sugars in the measured sample; however, since brewing Refractometers are calibrated for a solution of sucrose and water, super dry wines may be a problem. Based on the spreadsheet posted at MoreWine!, with 1 Degree Brix minimum reading refractometer, you would be unable to use this method for an Actual Gravity below 0.946. Gadgets to the rescue, super dry wines may be measured in %ABV with a Vinometer to reasonable precision; again, online calculators could be used to obtain the OG with %ABV and Actual Gravity.

You can take advantage of the refractometer correction calculators by using a hydrometer to measure the Actual Gravity and the Refractometer to measure the Apparent Gravity; input the Apparent Gravity into the calculator and adjust the Original Gravity value until you get the Actual Gravity as the result.

If you want to know why this works:
A Hydrometer measures Specific Gravity of a fluid as a function of its density. The printed scale simply reads-out the results of the equation to convert the density of the sample fluid as a function of the floatation height divided by the density of pure water.
A Refractometer works by using the Index of Refraction for the sample fluid compared against a scale calibrated for the desired readout unit. Refraction is the apparent bending of a beam of light caused by the change in the speed of light as it travels through different mediums. The Refractive Index of a material is dependent on the substance. For reference, the Index of Refraction for light at a wavelength of 600 nm is 1.33 for water at 20 degrees C and 1.36 for Ethyl Alcohol.

The introduction of alcohol (or, technically, anything other than water or sucrose) causes the index of refraction for the solution to change and thus the Refractometer must have a correction applied to account for the change of the Refractive Index. This changes means that a Refractometer reading cannot be used to directly calculate %ABV because one or both of the readings will be affected by measuring a solution outside of what the scale is calibrated to read.
Click to expand...

Head hurts now... Even for a math and science nerd like me. Think ill drink a little more now.
 
TyTanium said:
The real question is, where is the OP?
Click to expand...

Sorry about that, for some reason I don't understand I wasn't getting notifications from the thread, I thought it was sitting silent, boy was I wrong :)

Please allow me to clarify;

For standard, run of the mill ferments I use OG and FG to calculate potential alcohol, just as several people noted is the easiest way.

What I am looking for is a way to determine the ABV on a non-standard ferment, like the cider I have been fermenting for 3 months with several top ups of concentrate and different apple juices along the way. I knew the OG, and I know the current SG, but I am not quite detail oriented enough to have measured exactly how much sugar was in each addition or exactly how much I topped off with each time.

What I have now is effectively a solution that I no longer know the OG of, so the easy method won't work.

As I understand the method to work, it takes advantage of the fact that if you take a Brix reading of the fermented solution (with a refractometer) it will not show zero, even when there is zero residual sugar, because alcohol throws off that reading. They take the current gravity (by hydrometer) and apparent Brix (by refractometer) as inputs and use them to somehow calculate ABV without any additional information.

All that said, it seems like that could all be thrown off if the final product didn't actually have zero residual sugar, I was hoping somebody understood the algorithm being used and could weigh in on that, and how accurate the method was in general.

Does that help?

Cheers

HW
 
Haha, no apology necessary. And wow - your post is extremely well-written. Unfortunately I have no idea how to answer your question :)
 
My method.

Drink exactly 6 oz tequila, drive somewhere real fast, then note elapsed time and free breathalyzer result.

Wait a day.

Drink exactly 6 oz homebrew, drive somewhere real fast. Pull over only when elapsed time equals the ET of tequila incident. Note result of free breathalyzer test.

Using ratio and proportion math, calculate ABV of homebrew based on known ABV of tequila and breathalyzer results.

Pay thousands of dollars for free breathalyzer tests, and take cabs for a year.

Make sure to read up on prison hooch before beginning this method.
 
I get what you are saying. The inaccuracy from topping off is minimal. If you are really concerned about it top off with something the same SG as the original and then the calculations will still be dead on.


I also understand from a mathematical point where you are coming from. I knew there was a calculator that accounted for the alcohol in solution when taking a refractometer reading and correct it, based on OG and the refractometer FG, and thought maybe it could be used in reverse to use a FG refractometer reading compared to a hydrometer FG reading to determine ABV with out an OG or Just like you to account for topping up. I found lots of formulas but could not get them to work out they all gave me different answers and had different accuracy ranges. I finally concluded that 1% point either way was not that big of a deal in ABV. An OG and FG can be used to get you close and then estimate depending on what you toped up with. if you top up with water the ABV will go down if you top up with something that has more sugar than the original solution the ABV will go up. Either way toping up is minimal enough that it should not make that much of a difference.

Take a look at this thread I started asking similar questions to yours.
https://www.homebrewtalk.com/f25/using-refractometer-determine-abv-dry-wine-306289/
 
Any residual sugar will not affect either they Hydrometer or the Refractometer readings just because it has fermented. Simply, sugar is sugar. The Refractometer is calibrated to read the sugar and water solution. It won't mater that the sugar has not been fermented.

The Refractometer may have a limit if the fermentation is extremely dry because they are generally calibrated in Degrees Brix, which cannot go lower than 0 Degrees Brix or an SG of 1.000. In that case, a Vinometer will allow you to read %ABV directly.
 
HappyWino said:
Sorry about that, for some reason I don't understand I wasn't getting notifications from the thread, I thought it was sitting silent, boy was I wrong :)

Please allow me to clarify;

For standard, run of the mill ferments I use OG and FG to calculate potential alcohol, just as several people noted is the easiest way.

What I am looking for is a way to determine the ABV on a non-standard ferment, like the cider I have been fermenting for 3 months with several top ups of concentrate and different apple juices along the way. I knew the OG, and I know the current SG, but I am not quite detail oriented enough to have measured exactly how much sugar was in each addition or exactly how much I topped off with each time.

What I have now is effectively a solution that I no longer know the OG of, so the easy method won't work.

As I understand the method to work, it takes advantage of the fact that if you take a Brix reading of the fermented solution (with a refractometer) it will not show zero, even when there is zero residual sugar, because alcohol throws off that reading. They take the current gravity (by hydrometer) and apparent Brix (by refractometer) as inputs and use them to somehow calculate ABV without any additional information.

All that said, it seems like that could all be thrown off if the final product didn't actually have zero residual sugar, I was hoping somebody understood the algorithm being used and could weigh in on that, and how accurate the method was in general.

Does that help?

Cheers

HW
Click to expand...
That helps a lot. Just to clarify were you adding undiluted apple juice concentrate or apple juice made from concentrate? In either case can you estimate the volumes added? If so then we can determine the gravity of your additions and should be able to work them with your OG to get the true(ish) OG and your FG can be used as is. Ill have to look up how to adjust the og with the sugar additions. I can't remember off the top of my head and I'm posting from my phone.
 
Atek said:
That helps a lot. Just to clarify were you adding undiluted apple juice concentrate or apple juice made from concentrate? In either case can you estimate the volumes added? If so then we can determine the gravity of your additions and should be able to work them with your OG to get the true(ish) OG and your FG can be used as is. Ill have to look up how to adjust the og with the sugar additions. I can't remember off the top of my head and I'm posting from my phone.
Click to expand...

I added both :)

I am not really concerned with figuring out the ABV of this actual brew, really I am just interested in this method/capability in general.

Daze said:
If you are really concerned about it top off with something the same SG as the original and then the calculations will still be dead on.
Click to expand...

Hmm, that doesn't seem quite right, but I may be way off...here is how I (mis?) understand it;

The starting gravity represents the amount of sugar that is in the must before fermentation, and the final gravity the amount after fermentation. These figures allow us to estimate how much alcohol the yeast produced based on how much sugar it consumed. If more sugar is added during the process, (regardless of whether it contains the same relative volume of sugar as the original must) that calculation alone wouldn't be accurate as the yeast will have consumed more sugar, and therefore produced more happy juice (alcohol). Do I have a fundamental flaw in my understanding, 'cause if I do then you might have just made my life much easier :)

Cheers

HW
 
I know with out a doubt that it is correct. If you are topping up it is because some volume has been removed. Either by racking or taking a hydromiter reading. What was removed had either alcohol, sugar or both in it so that will need to be replaced to maintain the alcohol percent.

Look at it this way if you started out with a PA of 12% and you remove a cup of it 1/2 way through the process, you will have 6% alcohol and 6% PA still in sugar form. Because of the alcohol that was lost you need to add enough sugar to match what the current PA is (6%) as well as make up for the lost ABV (6% PA). In other words if you top off with a solution that has the same SG as the original must your reading will truly be accurate.
 
Daze said:
I know with out a doubt that it is correct. If you are topping up it is because some volume has been removed. Either by racking or taking a hydromiter reading. What was removed had either alcohol, sugar or both in it so that will need to be replaced to maintain the alcohol percent.

Look at it this way if you started out with a PA of 12% and you remove a cup of it 1/2 way through the process, you will have 6% alcohol and 6% PA still in sugar form. Because of the alcohol that was lost you need to add enough sugar to match what the current PA is (6%) as well as make up for the lost ABV (6% PA). In other words if you top off with a solution that has the same SG as the original must your reading will truly be accurate.
Click to expand...

Still struggling to get my head around this, thanks for being patient :)

If I pull out a cup of liquid halfway through the process, surely the remaining liquid is still 6% because I didn't remove pure alcohol, I removed equal parts alcohol and whatever else is in there. The absolute amount of alcohol goes down, but the relative (ABV) stays the same, doesn't it? :confused:

Cheers

HW
 
HappyWino said:
To have zero effect on final ABV wouldn't I have to replace both the sugar and the alcohol, or, add enough additional sugar to compensate for the lost alcohol? :confused:
Click to expand...

exactly!!! That is my point. if the OG was 1.100 and and then drops down to 1.050 if you take out a cup it will have half alcohol and half sugar. you need to replace it with a solution that is the same SG as the OG so that you maintain the ABV. So your replacement sugar solution needs to be 1.100.

Daze said:
If you are really concerned about it top off with something the same SG as the original and then the calculations will still be dead on.
Click to expand...

by original I mean OG
 

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