Understanding Your Hydrometer
A hydrometer is a sealed, hollow, cylindrical glass tube with weight at the bottom and a graduated scale running up the tube. Like a thermometer, your hydrometer tells you something about your wine--and it has even more tricks than plain old temperature.
'Hollow' and 'sealed' mean that hydrometers float in liquid, and the weight at the bottom means they float sticking straight up, until the weight of the displaced liquid equals the weight of the hydrometer. This leaves the scale poking out to show you just how dense the liquid is, compared to pure water, and because sugar dissolved in liquid makes it more dense, it can tell you how much sugar is in your wine kit when you start, and because alcohol dissolved in liquid makes it less dense than water, it can tell you when the fermentation is finished.
How hydrometers help in winemaking
Hydrometers tell us four crucial things, but only if we pay attention and write down what the readings are every time we take them.
First, your hydrometer reading on day one lets you know your wine has the right amount of sugar to ferment out and give you an alcohol content appropriate to style. If the reading is too low or too high, double-check that the wine is at the right level (6-US gallons/22.7 litres). If it is, then give the wine a brutal stirring. Sugar can settle out on the bottom and throw a reading off.
Second, readings taken during fermentation will show that fermentation is proceeding. Watching for foam or bubbles in the airlock doesn't tell you anything except that CO2 gas is coming out of the wine, and the gravity may not be changing while that happens. Your hydrometer never lies!
Next, your hydrometer will tell you when the fermentation is finished. After it gets below 1.000 on the scale and then stays at that point for three consecutive days, the kit is finished fermenting and it's safe to proceed to the next step.
Figuring out your alcohol level
Finally your hydrometer will let you calculate how much alcohol your wine contains. If you wrote down the SG number at the beginning, and compare it to the number at the end, you'll see how much sugar was used up. To figure out how much alcohol that made, all you have to do is multiply the change in gravity by 131. Here's how:
|Finishing Gravity(for example)||0.998|
|0.900 x 131||11.79% ABV|
Hydrometers need to be cleaned and sanitised like all pieces of winemaking equipment. Hydrometers need to be treated gently: all of them used to be made from glass, just like thermometers. This meant that a minor bump or drop and they broke instantly. Nowadays, however, we've got an alternative, the Herculometer. Made of Polycarbonate plastic, it's much tougher than glass--it's not indestructible, but it can take a few bumps and knocks and still be there when you need it.