The Definition of a Hydrometer
A hydrometer is an instrument developed for the use of measuring the relative density or specific gravity of various liquids. The hydrometer measures the density in relation to its ratio compared against the density of water. The relative density of water is a constant 1.0, and in order to obtain an accurate reading, there must be equal parts of water and the other liquid to be measured.
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History
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Most scientists and historians credit Antoine Baumé as the original inventor of the hydrometer. He invented two hydrometers: one for measuring liquids heavier than water, and another for liquids lighter in density than water. The two Baumé hydrometers are calibrated using a scale that was also created by Baumé, while other hydrometers are calibrated at the specific temperature of 60 degrees F.
Function
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Hydrometers are used extensively in the commercial industry. Dairies, breweries and wineries frequently use hydrometers because the quality of the finished product relies greatly upon the liquid's density. Special hydrometers were created for use in specific industries. To measure the density of milk, a lactometer hydrometer is used, in addition to fat content testing. Breweries and wineries use alcoholometer hydrometers to measure the alcohol content of the finished product, and provide a proof percentage to consumers.
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Features
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Hydrometers come in different sizes and shapes, but most are cylindrical glass bulbs with glass stems. The bulb is the casing for a lead shot, or small amount of mercury that works to make the bulb float upright in the liquid being measured. Inside the stem a paper scale allows the user to take a direct measurement of the liquid's density. On the hydrometer, the calibration temperature will be listed, so that you will know at which temperature to measure the liquids' density; for example, if the calibration is set to 60 degrees, the liquid being measured should be heated or cooled to 60 degrees.
Operation
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To measure the density of a liquid, a large cylindrical test tube is filled midway with the liquid, and the hydrometer is then placed bulb first into the tube. The hydrometer will bob up and down in the water and finally settle. Once settled, the density can be read, by looking at where the water's surface rests on the scale inside the hydrometer's stem. The measurement should be read at the meniscus, or water's surface, not at the peak of the water curving up the side of the hydrometer.
Theories/Speculation
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The science that makes the measurement of density possible is the Archimedes' principal that states that an item dropped into a fluid is buoyed up by a force equaling the weight of the displaced fluid. Of course, this is also dependent upon what the dropped item is, and what it is comprised of; for example, a human body will float in water because our bodies are approximately 70 percent water.
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