hydrometer/abv question

hey all,
i have a newb question regarding hydrometers that im pretty sure i know the answer to, but if i could get your guys expert opinions that would be great. my hydrometer came with my starter kit. when i took the original gravity reading on brew day, it read that the approximate ABV would be between 5-7%. obviously now the gravity readings have gone down and and so has the approximate ABV. now it says the abv would be around 3%. is this because the hydrometer only tells you the approximate abv when you take the original gravity reading? the recipe i brewed from gave a math formula regarding OG and FG to figure out the final ABV. is this what i would use to figure out the final ABV?

This may not answer your question, however I calculate by taking the OG and subtracting the FG then multiplying the result by 131. I never used the ABV thingy on my hydrometer. There's a good calculator at Brewersfriend.com.

If you're reading the "Potential Alcohol" scale on the hydrometer- yes, it only gives you a meaningful reading before fermentation (O.G.)- hence, potential alcohol. It doesn't really give you numbers that mean anything after fermentation. You need to use the Specific Gravity scale for O.G. and F.G. and do the math to calculate ABV. The Potential Alcohol scale just gives you a ballpark for what kind of ABV range is possible given your O.G., but there are lots of other variables that are going to affect your actual ABV.

Hope this helps. Brew on!

HeadyKilowatt said:

If you're reading the "Potential Alcohol" scale on the hydrometer- yes, it only gives you a meaningful reading before fermentation (O.G.)- hence, potential alcohol. It doesn't really give you numbers that mean anything after fermentation. You need to use the Specific Gravity scale for O.G. and F.G. and do the math to calculate ABV. The Potential Alcohol scale just gives you a ballpark for what kind of ABV range is possible given your O.G., but there are lots of other variables that are going to affect your actual ABV.

Hope this helps. Brew on!

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awesome thank you for clearing that up for me!

Copbrew133 said:

This may not answer your question, however I calculate by taking the OG and subtracting the FG then multiplying the result by 131. I never used the ABV thingy on my hydrometer. There's a good calculator at Brewersfriend.com.

Click to expand...

ok awesome thanks! thats exactly the formula my recipe told me to use: (OG-FG)x131.25 = ABV

thanks for the knowledge gentlemen!

You can also use the potential alcohol scale to figure out abv. If you take the initial minus the final of that scale it will also give you abv. For example it was at 7% and finished at 2% mark your abv is 5%.

Brad2287 said:

You can also use the potential alcohol scale to figure out abv. If you take the initial minus the final of that scale it will also give you abv. For example it was at 7% and finished at 2% mark your abv is 5%.

Click to expand...

awesome i was unaware of this method. thank you!

One thing that was not mentioned in this thread (that is critical to getting accurate gravity readings), is how temperature impacts gravity. Hydrometers are calibrated to a certain temperature and the hotter the wort, the more the reading drifts.

First, record the gravity reading and the temperature of the sample. From there, do a temperature/calibration correction on your hydrometer using this tool:
http://www.brewersfriend.com/hydrometer-temp/

Then use the ABV calculator, and select your preferred equation:
http://www.brewersfriend.com/abv-calculator/

That will give you greater accuracy, and hopefully make your brewing better too!

The "Potential Alchohol" scale on a hydrometer is really used more for winemaking. It is giving the ABV with the assumption that the FG will be 1.000. Most beers finish in the 1.01-1.02 gravity range, so you have to use the formula. Most wines finish closer to 1.000, so this Potential Alchohol scale is using the same (OG-FG)x131.25 = ABV formula, it is just assuming that FG is constant 1.000 instead of considering that a variable.

Use the actual gravity scale on the other side of the hydometer for beer to record the OG and FG, then apply the formula.