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Old 02-21-2012, 02:20 AM   #1
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Default using a refractometer to determine ABV on a dry wine

First of all I know how a refractometer normally works, take a OG before you pitch the yeast, write it down. Assuming the wine ferments out dry the OG can be used to know the final ABV, but if you take an FG with the refractometer rather than a hydrometer the reading will be off because of the alcohol. I know calculations can be made to correct the reading and get it to match the hydrometer. I am wondering if the same calculations could be made to determine the ABV of a dry wine that you did not take an OG of. obviously it would not work on a sweet wine but I am thinking on a dry wine it could work. See where I am going with this??? Any on have the calculation info??

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Old 02-21-2012, 02:22 AM   #2
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You'd have to know the OG to guestimate/calculate the ABV with a refractometer (or a hydrometer, too, I guess).

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Old 02-21-2012, 02:54 PM   #3
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Originally Posted by Yooper View Post
You'd have to know the OG to guestimate/calculate the ABV with a refractometer (or a hydrometer, too, I guess).
When I made this post I knew someone one was going to say just that. I appreciate the attempt at help but I understand how a refractometer and hydrometer traditionally work. You misunderstand what I am after. Let me see if I can explain it better.

if you start a wine and take a reading with both a refractometer and a hydrometer you will get the same reading. BUT if you take another reading after the fermentation has started the refractometer and hydrometer will have different readings because the alcohol will bend the light like sugar and make the refractometer read higher than the solution really is. so on the same solution you may get a hydrometer reading of 1.010 but refractometer reading of 1.032. There are calculators on the net that will correct for this. You put in the OG and then you put in the SG you just took with the refractometer and it will give you the correct SG reading accounting for the alcohol in solution. These calculations use the OG and current SG to factor out the amount of alcohol in the solution to give the correct SG. So basically

refractometer OG (+-* /(not sure what the calculation is)) refractometer SG = a corrected SG

As long as there is only one variable to solve for you can always solve it You know the OG, you know the current SG and can solve for the corrected SG


What I want to do is solve for the OG on a dry wine where you know the FG. You can test it with a hydrometer. You know the alcohol enhanced refractometer reading as you can take it and compare it to the FG. That reading as I said will be different than the refractometer reading and using the same calculation in reverse you can calculate the OG. which will result in giving you the ABV.

My question is not "can it be done" basic algebra says it can. What I want to know is how?? what is the conversion calculation normally?? From there I can do the math to work it back wards. These calculations would allow you to use a refractometer to measure the actual alcohol in something.
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Old 02-21-2012, 03:12 PM   #4
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There is a formula to calculate OG using the measured FG from a refractometer and a hydrometer. I don't have it and have never used it, but a friend of mine has been playing with it. We've been using it on batches that we knew the OG. We got some numbers that didn't match. I don't know if he figured out why. Last I heard, he wasn't convinced yet of it's accuracy. From what I understand, it is not dependent on having a dry wine.

There are situations where this could come in handy, but I would never use it in place of taking an OG. If you misread your hydrometer or refractometer by just a little, it normally doesn't matter much, but the error could be magnified if you plug the wrong numbers into the formula.

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Old 02-21-2012, 03:27 PM   #5
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From what I understand, it is not dependent on having a dry wine.
agreed the only reason I mentioned that originally is if it was dry you wouldn't need to take a hydrometer reading to get the FG you would know that it was around .995 or so.
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Old 02-21-2012, 03:43 PM   #6
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agreed the only reason I mentioned that originally is if it was dry you wouldn't need to take a hydrometer reading to get the FG you would know that it was around .995 or so.
Seriously? If you are trying to calculate OG from the hydrometer and refractometer FG, you need accurate numbers. Assuming the FG is "0.995 or so" would make your calculations worthless. Garbage in-Garbage out. That's my issue with this formula. You are calculating the OG by using the relationship between your hydrometer and refractometer readings. If either reading is off slightly, your error is magnified when the OG is calculated. Being off 1 point on one of your FG measurements might translate into a 5 or 10 point error in your OG calcs.
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Old 02-21-2012, 04:11 PM   #7
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I agree with what you are saying about the accuracy on one end effecting the accuracy on the other end but I really don't think it matters much. I look at it like this using a hydrometer requires two reading, one at the beginning and one at the end. If both of those are off a few points, especially in the opposite direction than your final ABV calculation will be off BUT it will be close enough. The same is true with using the refractometer readings and hydrometer reading at the end you are still having to use two reading to calculate the ABV.

I see what you are saying that the calculation could amplify an incorrect reading but that is not an absolute. if the refractometer correction formula was (I am totally making this up to illustrate a point) (OG - SG)1.25 -.45 than by multiplying by a number greater than one you would be increasing the inaccuracy, BUT if the formula was (OG - SG).85 +.75 by multiplying the inaccurate measurements by a number smaller than 1 you are decreasing the inaccuracy.

Either way I am not after lab accuracy. If I use my hydrometer and determine that my wine is 12.2% ABV The label is going to read 12% because the .2% is not that important. If my calculation is off by 5 points that just barley over .5% which is no big deal. The reason I want to know the calculation to use the hydrometer to determine ABV with out using an OG is not to achieve a higher level of accuracy but rather to be able to find out an ABV if I forgot to take an OG.

I am normally really good about taking hydrometer readings but I have a wine finishing up right now that I made on a whim one evening in kind of a hurry and neglected to take a hydrometer reading. I know what went in it and can probably estimate the ABV, BUT since fresh squeezed orange juice was one of the ingredients I would also like to use my refractometer to calculate ABV because it will most likely be more accurate.

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Old 02-21-2012, 09:32 PM   #8
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Quote:
Originally Posted by GinKings View Post
There is a formula to calculate OG using the measured FG from a refractometer and a hydrometer. I don't have it and have never used it, but a friend of mine has been playing with it. We've been using it on batches that we knew the OG. We got some numbers that didn't match. I don't know if he figured out why. Last I heard, he wasn't convinced yet of it's accuracy. From what I understand, it is not dependent on having a dry wine.

There are situations where this could come in handy, but I would never use it in place of taking an OG. If you misread your hydrometer or refractometer by just a little, it normally doesn't matter much, but the error could be magnified if you plug the wrong numbers into the formula.
Hey GinKings, what if the refractometer technique for determining ABV is more accurate not less?? Not trying to be argumentative here just thinking out loud and you brought up accuracy so let me bounce this logic off of you.

We all know how the hydrometer method works take an OG take a FG subtract one from another and then convert to alcohol. there is an inherent problem with this, topping up. When you rack the wine or cider off of the lees in most cases you loose at least 8 ounces of volume. Unless you top up with something the exact same SG as the OG reading you will trow off the results. Of course if you correct for this than not a problem but who calculates the change every time they top up due to racking or a hydrometer reading??? I don't think it is common practice. Example:

Lets say your OG was 1.085 and when you racked it you topped up with 1 cup of water. Diluting it with water would be the same as starting with an OG of 1.080. The refractometer however (especially on a wine that is completely dry) is going to measure the alcohol % in the solution (after some calculations of course) so it will automatically account for all topping off. Maybe that is why your numbers aren't agreeing "We got some numbers that didn't match." Just a thought. I would still love to know the calculations so that I can do this myself.
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Old 02-22-2012, 10:57 PM   #9
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I recall the refraction of alcohol is so close to water as to be inconsequential.

I just plug the refractometer readings into a sheet removed from one of many broken hydrometers. Or, merely take the drop in sugar, divide by 2 because yeast makes half the sugar into alcohol, the other half into CO2. The answer is in ABW, multiply times 1.2 to get ABV.

If 12.2 is close enough to 12 to be good, this will work too.

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Old 02-23-2012, 12:46 AM   #10
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There are a lot of spreadsheets and other tools that help account for alcohol in a refractometer reading. More wine.com has one on their site.
I prefer refractometer brix readings and rarely use a hydrometer. A drop sized sample is just far more convenient that filling an entire tube for a hydrometer test.

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