Hydrometer
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A hydrometer is an instrument used for determining the specific gravity of liquids. It is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or shot to make it float upright. The liquid is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely.
The point where the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity (or density) can be read directly in grams per cubic centimeter.
In light liquids like kerosene, gasoline, and alcohol, the hydrometer must sink deeper to displace its weight of liquid than in heavy liquids like brine, milk, and acids. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top, and one for light liquids, on which the mark 1.000 is near the bottom of the stem.
The function of the hydrometer is based on Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink.
JJ==Commercial uses== Because the commercial value of many liquids, such as sugar solutions, sulfuric acid, alcohol, and wine, depends directly on the specific gravity, hydrometers are used extensively.
Perhaps the best-known form of hydrometer is the kind used in testing milk, called a lactometer. The specific gravity of cow's milk varies from 1.027 to 1.035. Since only the last two digits are important, the scale of a lactometer runs from 20 to 40, which means from 1.020 to 1.040. The specific gravity of milk does not give us a conclusive test as to its worth. In addition to water (which is about 87%), milk contains some substances that are heavier than water, such as albumen, sugar, and salt, and others that are lighter than water, such as butter fat. So, in addition to the specific gravity, one needs to determine the amount of fat, and, if possible, of other solids in the milk, to know its richness. Of course, the very important question as to the cleanliness of milk must be left to the bacteriologist.
An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids.
A saccharometer is a hydrometer used for determining the amount of sugar in a solution. It is primarily used by brewers and winemakers.
A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section.
hydrometer used in an actual historical context 1854
433. The Brussels Conference of 1853 recommended the systematic use of the hydrometer at sea. Captain [John] Rodgers, Lieutenant Porter, and Dr. [William Samuel Waithman] Ruschenberger, all of the United States Navy, with Dr. Raymond, in the American steamer Golden Age, and Captain Toynbee, of the English East Indiaman the Gloriana, have all returned to me valuable observations with this instrument hydrometer. Rodgers, however, has afforded the most extended series. It embraces 128° of latitude, extending from 71° in one hemisphere to 57° in the other. And here I beg to remark, that those navigators who use the hydrometer systematically and carefully at sea are quietly enlarging for us the bounds of knowledge and our field of research. These observations have already led to the discovery of new and beneficent relations in the workshops of the sea. In the physical machinery of the universe there is no compensation to be found that is more exquisite or beautiful than that which, by means of this little instrument, has been discovered in the sea between its salts, the air, and the sun.
434. The observations made with it by Captain Rodgers, on board the U. S. ship Vincennes, have shown that the specific gravity of sea water varies but little in the trade-wind regions, notwithstanding the change of temperature. The temperature was a little greater in the southeast trade-wind region of the Pacific; less in the Atlantic. But, though the sea at the equatorial borders of the trade-wind belt is some 20° or 25° warmer than it is on the polar edge, yet the specific gravity of its waters at the two places in the Atlantic differs but little. Though the temperature of the water was noted, his observations on its specific gravity have not been corrected for temperature.
The object which the Brussels Conference had in view when the specific gravity column was introduced into the sea-journal was, that 'hydrographers might find in it data for computing the dynamical force which the sea derives for its currents from the difference in the specific gravity of its waters in different climes'. The Conference held, and rightly held, that a given difference as to specific gravity between the water in one part of the sea and the water in another would give rise to certain currents, and that the set and strength of these currents would be the same, whether such difference of specific gravity arose from difference of temperature or difference of saltiness, or both.
Source: Physical Geography of the Sea sections 433, 434 by Matthew Fontaine Maury, USNO.
--Maury 18:24, 29 March 2006 (UTC)
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See also
Elevator paradox (physics)cs:Hustoměr de:Aräometer es:Hidrómetro mk:Ареометар nl:Hydrometer pl:Areometr sl:Areometer sr:Ареометар sv:Hydrometer tr:Hidrometre