A hydrometer is an instrument used to measure the specific gravity

Hydrometers- A Guide to applications and usage A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall container, often a graduated cylinder, or specially designed hydrometer cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a scale inside the stem, so that the specific gravity can be read directly. A variety of scales exist, and are used depending on the context, this includes Specific Gravity (SG), Baume, Twaddle and mg/L. Below we explain a little about the more specialist types of hydrometers available. 

 Lactometer A lactometer (or galactometer) is a hydrometer used to test milk. The specific gravity of milk does not give a conclusive indication of its composition since milk contains a variety of substances that are either heavier or lighter than water. Additional tests for fat content are necessary to determine overall composition. The instrument is graduated into a hundred parts. Milk is poured in and allowed to stand until the cream has formed, then the depth of the cream deposit in degrees determines the quality of the milk. Range 1.025 to 1.035 g/ml 

 Alcoholometer An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a Proof and Tralles hydrometer. It only measures the density of the fluid. Certain assumptions are made to estimate the amount of alcohol present in the fluid. Alcoholometers have scales marked with volume percents of "potential alcohol", based on a pre-calculated specific gravity. A higher "potential alcohol" reading on this scale is caused by a greater specific gravity, assumed to be caused by the introduction of dissolved sugars. A reading is taken before and after fermentation and approximate alcohol content is determined by subtracting the post fermentation reading from the pre-fermentation reading. Range 0.98 to 1.12 °SG 

Saccharometer A saccharometer is a hydrometer used for determining the amount of sugar in a solution, invented by Thomas Thomson. It is used primarily by winemakers and brewers, and it can also be used in making sorbets and ice-creams. It consists of a large weighted glass bulb with a thin stem rising from the top with calibrated markings. The sugar level can be determined by reading the value where the surface of the liquid crosses the scale. It works by the principle of buoyancy. A solution with a higher sugar content is denser, causing the bulb to float higher. Less sugar results in a lower density and a lower floating bulb. 

Thermohydrometer A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section. For measuring the density of petroleum products, like fuel oils, the specimen is usually heated in a temperature jacket with a thermometer placed behind it since density is dependent on temperature. Light oils are placed in cooling jackets, typically at 15 °C. Very light oils with many volatile components are measured in a variable volume container using a floating piston sampling device to minimize light end losses. As a battery test it measures the temperature compensated specific gravity and electrolyte temperature. 

Urinometer A urinometer is a medical hydrometer designed for urinalysis. As urine's specific gravity is dictated by its ratio of solutes (wastes) to water, a urinometer makes it possible to quickly assess a patient's overall level of hydration. Range 1 to 1.06 °SG Soil Analysis A hydrometer analysis is the process by which fine-grained soils, silts and clays, are graded. Hydrometer analysis is performed if the grain sizes are too small for sieve analysis. The basis for this test is Stoke's Law for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the grain diameter and the densities of the grain in suspension and of the fluid. The grain diameter thus can be calculated from a knowledge of the distance and time of fall. The hydrometer also determines the specific gravity (or density) of the suspension, and this enables the percentage of particles of a certain equivalent particle diameter to be calculated. Range 0.995 to 1.038 °