Introduction

The Alcohol Content panel has two functions:

• Correcting your hydrometer readings for temperature.
• Calculating the approximate alcohol content of your finished wine.

To calculate the alcohol content, you need to know the initial and final hydrometer readings. These calculations are only valid if you have not added any additional water or sugar between the initial and final readings.

Making a Calculation

To make a calculation, follow these steps:

1. Select Calculation > Alcohol Content from the menu, or select the Alcohol tab at the top of the main window.
2. Select the appropriate units for the input fields using the drop-down menus in the right-hand column.
3. Enter the initial and final Measured Gravity of the wine.
4. Enter the Temperature at which the initial and final gravities were measured. (If you don't know what these are or if you don't want to do a temperature correction, just leave them set equal to the hydrometer calibration temperature.)
5. Enter the Calibration Temperature of the hydrometer(s) used to measure each of the gravities.

The corrected specific gravities and the approximate alcohol content of your wine will appear in the output fields as you type.

Calculation Details - Temperature Corrections

According to tables published in the USDA Technical Inspection Procedures, temperature corrections to specific gravity are functions of both the temperature and the degrees Brix of the must. Based on these tables, I developed the following equations to correct hydrometer readings.

sg = sga[a(T - 20)2 + b(T - 20) + 1] / [a(Tc - 20)2 + b(Tc - 20) + 1]

(1)

where:

sg = corrected specific gravity
sg_a = apparent specific gravity at temperature T
T = must temperature, °C
T_c = hydrometer calibration temperature, °C
a = 5.0811·10^-6sg^-5.3924
b = -1.5056·10^-3sg² + 4.0977·10^-3sg - 2.4191·10^-3

The plot below compares the USDA data with the calculated specific gravity correction factors from equation (1) above.

Calculation Details - Alcohol Content

Alcohol content is calculated in FermCalc using methods proposed by Duncan & Acton and by Balling. Both methods require measurements of the initial and final specific gravities. Agreement between the two methods is generally excellent.

All gravity values are converted to specific gravity when they are entered, and are subjected to an upper limit of 1.55 (pure sugar) and a lower limit of 0.79 (ethanol).  If any of the entered values are outside this range, or if the final specific gravity is greater than the initial specific gravity, output fields are highlighted in red and an error message is displayed.

Standard Gravity Drop Method

This method estimates the alcohol content by dividing the drop in gravity by the factor 7.4, or

ap = 1000(sgi - sgf)/7.4

(2)

where

a_v = alcohol content (% by volume)
sg_i = initial specific gravity
sg_f = final specific gravity

Duncan & Acton Method

The Duncan & Acton method calculates the alcohol content from the initial and final specific gravities divided by a factor F that is a function of the corrected initial specific gravity. The equations are as follows.

ap = 1000(sgi - sgf)/F	(2)
F = 7.75 - 3000(sgc - 1.0)/800	(3)
sgc = sgi - 0.007	(4)

where

a_v = alcohol content (% by volume)
sg_i = initial specific gravity
sg_f = final specific gravity
F = conversion factor
sg_c = initial specific gravity corrected for non-sugar solutes

Combining equations (2) through (4) above yields the following equation:

av = 1000(sgi - sgf) / [7.75 - 3.75(sgi - 1.007)]

(5)

Balling Method

The Balling method is normally used for beer but gives results that agree very well with the other methods.  It requires the calculation of something called "Real Extract" as follows:

RE = 0.1808Bi + 0.8192Bf

(6)

where

RE = real extract
B_i = initial Brix
B_f = final Brix

The alcohol content (% by weight) is then calculated as:

aw = (Bi - RE) / (2.0665 - 0.010665Bi)

(7)

where a_w is the alcohol content in % by weight.

Combining equations (6) and (7) above yields the following equation:

aw = 0.8192(Bi - Bf) / (2.0665 - 0.010665Bi)

(8)

The result of equation (8) is then converted to % alcohol by volume using equation (19) under Proof Conversions.