A hydrometer is a tool to measure the density of a liquid in comparison to the density of water. In beer brewing, the hydrometer will show you to what degree the yeast is converting sugar to ethanol. The following are the steps involved in using a hydrometer:
Sampling and Inserting the Hydrometer
You take the first measurement after the cooldown. Retrieve a sample of the wort then place your hydrometer and allow it to rest. Wait until all the air has ascended. It is highly recommended that your hydrometer is positioned vertically.
Obtaining the Original Gravity Reading
There are specific gravity points that are represented by your hydrometer increments. Your gravity reading should occur on the level at which the liquid rises. This is called the liquid-air line.
You then record the number by which your hydrometer is being crossed by the liquid-air line. Usually, a wort OG will be around 1.035 and 1.060. To get maximum accuracy, read the bottom of the meniscus.
Calculating with Temperature
The standard temperature of hydrometer readings is assumed at 15 degrees Celsius. Therefore, knowing the actual temperature of your wort is important for your reading to be accurate. Calibrate should you feel the need to do so or if your temperature reading is different.