What is a Hydrometer? A hydrometer is a scientific instrument used to measure the specific gravity or relative density of a liquid. This measurement is expressed as a ratio of the density of the liquid to the density of water. Hydrometers measure this relative density based on the principles of buoyancy and are essentially measuring dissolved solids in a liquid - the more dissolved solids the more buoyant the liquid.
There are several different types of hydrometers, the main difference between them being their scales. Each scale measures a different type of dissolved solid such as sugar, petroleum, alcohol, salt and soil. Thus, there are many different hydrometers with different applications - despite all hydrometers functioning in the same way on the same principle.
How do Hydrometers Work? Hydrometers can be made from glass or plastic and consists of a cylindrical stem with a scale inside and a bulb weighted with a heavy material to ensure it floats upright. The sample liquid to be tested is poured into a hydrometer jar or cylinder, and the hydrometer is lowered so it floats freely in the jar. Where the surface of the liquid touched the stem of the hydrometer is the point from which you take your reading.
Each type of hydrometer is calibrated to its particular scale, and is fabricated with graduated readings so you can easily read and calculate the approximate density of your sample to a reference sample - usually plain water. The sample must be the same temperature as the reference sample to accurately reflect density. Most hydrometers will have the temperature standard marked on them, and it can be handy to use a hydrometer with a built in thermometer to ensure accuracy.
Baume: older scale from industrial chemical and pharmaceutical industry, usually used to measure liquids heavier than water.
Brix: measures dissolved sugar. Commonly used in beer, wine, juice, and other food and beverage production. When used in beer brewing - a sample is taken of the wort and after fermentation has occurred. These two measurements can be used together to determine when fermentation is complete and to estimate the overall alcohol content of your brew. More accurate for final gravity than refractometers, Brix hydrometers can be helpful in verifying your results.
High Precision / Specific Gravity: measure in specific gravity, these hydrometers have small graduations with a high degree of accuracy. Depending on what beer you brew, you may need more than one range in this scale.
Plato: Scale used in brewing as designated by the American Society of Brewing Chemists