You need a hydrometer to measure the alcohol content of a drink and determine its proof, or twice the percentage of alcohol it contains when tested at 60 degrees Fahrenheit. In addition, the type of drink -- distilled spirit, wine or beer -- determines the scale needed to interpret the hydrometer's specific-gravity measurement. A hydrometer, a simple device based on Archimedes' principle, consists of a piece of stemmed glass with a bulbous base that measures the force exerted on it by the drink. You need a proof and Tralles hydrometer for distilled spirits, and a triple-scale hydrometer for wine and beer. A precision-finished hydrometer, which has smaller gradients, measures finished drinks.
Using the Hydrometer
Set the hydrometer's glass cylinder on a level table. Draw the distilled spirit, wine or beer into a food-grade pipette and release it into the glass cylinder -- add enough of the drink to fill the cylinder about 3/4 full. Insert the hydrometer in the drink gently and spin it as you let go of it -- this dissipates any bubbles touching its base. Wait for the hydrometer to come to a rest in the drink.
Reading the Hydrometer
Examine the hydrometer's specific gravity reading at the bottom of the meniscus -- the concave surface of the liquid surrounding the hydrometer -- and record it. Next, compare the specific-gravity measurement to the potential-alcohol scale supplied with the hydrometer to arrive at the alcohol percentage. Multiply the alcohol percentage by 2 to determine the proof.
For example, if a wine has a specific gravity of 1.1009 as measured on the hydrometer, it has 13.8 percent alcohol according to the potential-alcohol scale, which equates to 27.4 proof.