Categories

 Loading... Please wait...

Blog

I forgot to take a Starting / Original Gravity reading and I am bottling today. Is it too late to determine the alcohol percentage?

Posted by Don Christian on

I forgot to take a Starting / Original Gravity reading and I am bottling today. Is it too late to determine the alcohol percentage?


The most accurate method available to most home brewers and winemakers is to calculate alcohol by volume is the Spirit Indication Test. This method involves boiling a known sample of mead (wine or beer) to remove all its alcohol, reconstituting the solution to the original sample volume, and then comparing the SG of the two samples to determine the alcohol by volume.

These steps are defined below:

1. Measure the SG of the finished beer, wine or mead (a.k.a. "sample.")

2. Measure 250 mL of the sample with a graduated cylinder and pour into a flask, beaker, or pot that can be boiled.

3. Boil the sample to half its volume. As alcohol’s boiling point is less than that of water, all of the alcohol in the sample will have evaporated.

4. Reconstitute the sample to its original volume, 250 mL, using distilled water. Distilled water has an SG of 1.000, so we can ensure that the only changes to SG from the original sample to the final sample are the reduction of alcohol and replacement with water. The reason for using distilled water is to ensure the water being added has a specific gravity of 1.0. If it does not the calculations will be thrown off and you will not get accurate results. Also, to ensure accuracy, the post-boil level needs to be exactly the same level as was there pre-boil. The degree to which you don’t get back to the same level drives a difference in the ratio of material that produces a deflection in specific gravity to water which will throw off the final measurement. If we do bring the level back precisely then we will have replaced the water boiled off with water and the alcohols boiled off with water. Hey, that is it; replacing the alcohol with water.


5. Calculate ABV

The difference between these two specific gravities multiplied by 1000 is known as the spirit indication. If the specific gravity taken after replacement is SG2 and the initial specific gravity is SG1, then the equation [1] is:

Spirit indication (SI) = (SG2 – SG1)*1000

The best formula I have found to give the percentage alcohol by volume (%ABV) is:

%ABV = (0.008032927443 * SI^2) + (0.6398537044 * SI) – 0.001184667159

Note: There is an equation that has been picked up on the net but is incorrect. I mention it only so you will not be confused by it. Please do not use %ABV =(SI/1000)/2.11*1000.

Jon Musther’s site at www.musther.net has an online calculator that will handle the actual calculations for you so you don’t really need to create your own if you don’t want to.


One great feature of this method is that it only requires measurements of the final beer, wine, mead, cider, not a measure of the initial wort/must. However, that in itself presents issues, as the brewer/winemaker does not know the status of the mead during its fermentation, and may schedule nutrient additions at inopportune times.

This method also involves the sacrifice of a portion of your finished product. So, for those looking to calculate ABV without sacrificing their mead, a method using only measurements is ideal. 250 mL is only 1.25% of a 5 gallon batch (6.25% of a 1 gallon batch), but that is still a cup you can’t drink!

A similar method involves using an ebulliometer, but for home hobbyist use, this is an unnecessary investment. Instead, keep reading below to find ways that don’t require a significant sacrifice of the finished mead to determine alcohol by volume.



[1] I got this equation from Jonathan Musther at http://www.musther.net. I don’t know where he got it or if he created it but it matches the table values from William Honneyman,B.Sc.,Ph.D. discussed in http://valleyvintner.com/Tips&Links/MeasuringAlcohol.pdf.

Back to Top