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{{Image|Hydrometer.png|right|223px|A schematic of a specific gravity hydrometer}}


A '''hydrometer''' is an instrument used to measure the [[specific gravity]] (SG) (or ''relative density'') of [[Liquid/Definition|liquid]]s; that is, the ratio of the [[Density (chemistry)|density]] of the liquid to the density of [[water]] with both at the same [[temperature]]. It is usually made of glass and consists of a small diameter cylindrical stem and a larger diameter float section weighted with [[mercury]] or [[Lead|lead shot]] (sealed with [[wax]]) to make it float upright. The larger diameter float section is necessary to provide the displacement volume needed for the proper [[buoyancy]] of the hydrometer
The stem contains a rolled paper marked with the scale being used. There are a great many different hydrometer scales commonly used to measure liquid densities in: [[petroleum crude oil]] marketing and refining; making [[wine]], brewing [[beer]] and making [[whiskey]]; refining [[sugar]]; producing [[sulfuric acid]] and other industrial [[chemical]]s.
In the past, hydrometers have also been referred to as ''gravimeters'', ''densimeters'' or ''areometers''. If the float section (see adjacent drawing) includes a built-in thermometer, the instrument is then referred to as a ''thermohydrometer''.
== Scales ==
The many hydrometer scales that are commonly used include:
;Specific gravity (SG): The specific gravity of a liquid or solid is the dimensionless ratio of the density of the liquid or solid at a given reference temperature to the density of a given reference material at a given reference temperature. The given reference material is usually water and unless the two reference temperatures are explicitly stated, they are generally taken to be 4 [[Celsius (unit)|°C]].
:The specific gravity scale is widely used in [[chemistry]], [[engineering]] and [[physics]] as well as by [[geologist]]s, [[mineralogist]]s and [[gemologist]]s. It is often referred to as ''relative density'' and it is the one shown in the adjacent drawing. The specific gravity scale is expressed as:
::<math>\mathrm{SG}_{4\,^\circ \mathrm{C}}^{4\,^\circ \mathrm{C}} = \frac {\mathrm{density\; of\; the\; tested\; liquid\; at\; 4\,^\circ \mathrm{C}}} {\mathrm{density\; of\; water\; at\; 4\,^\circ \mathrm{C}}}</math>
:Note 1: The specific gravity of solids can be determined by using a [[pycnometer]] rather than a hydrometer.
:Note 2: Hydrometers are not used to determine the specific gravity of [[gas]]es and this article does not include any discussion of the specific gravity of gases.
;[[API gravity]]: This scale was developed by the [[American Petroleum Institute]] (API) in 1921 for use in the [[petroleum industry]] and it is now universally used by the petroleum industry worldwide. It is expressed as:<ref>[http://www.sizes.com/units/hydrometer_api.htm API Gravity] References the publication: {{cite book|author=Ernest L. Ruh, James J. Moran and Robert D. Thompson|title=Measurement problems in the instrument and laboratory apparatus fields|edition=|publisher=American Association for the Advancement of Science (AAAS)|year=1959|pages=Page 29|id=AAAS Publication No. 57}}</ref>
::<math>^\circ \mathrm{API} = \frac{141.5}{\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}}} - \mathrm{131.5}</math> &nbsp; &nbsp; and &nbsp; &nbsp; <math>\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}} = \frac {\mathrm{density\; of\; petroleum\; liquid\; at\; 60\,^\circ \mathrm{F}}} {\mathrm{density\; of\; water\; at\; 60\,^\circ \mathrm{F}}}</math>
:Note: 60 [[Fahrenheit (unit)|°F]] is equivalent to 15.56 °C.
;[[Baumé gravity]]: These two scales, one for liquids lighter than water and one for liquids heavier than water,  were developed by the [[France|French]] chemist [[Antoine Baumé]] in 1768. It is widely used in industrial chemistry, [[pharmacology]], [[sugar refining]] and other industries. The two scales are expressed below as:<ref name=Perry>{{cite book|author=Perry, R.H. and Green, D.W.|title=Perry's Chemical Engineers' Handbook |edition=6th Edition| publisher=McGraw Hill, Inc.|year=1984|pages=page 1-19|id=ISBN 0-07-049479-7}}</ref><ref>[http://antoine.frostburg.edu/chem/senese/101/measurement/faq/baume-scale.shtml What is a "degree Baume'?] Professor Frederick A. Senese, Chemistry Department, [[Frostburg State University]], [[Maryland]]</ref>
::<math>^\circ \mathrm{B\acute{e}} = \frac{140}{\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}}} - \mathrm{130}</math>  &nbsp; &nbsp; for liquids lighter than water and &nbsp; &nbsp; <math>\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}} = \frac {\mathrm{density\; of\; the\; tested\; liquid\; at\; 60\,^\circ \mathrm{F}}} {\mathrm{density\; of\; water\; at\; 60\,^\circ \mathrm{F}}}</math>
::<math>^\circ \mathrm{B\acute{e}} = \mathrm{145} - \frac{145} {\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}}}</math> &nbsp; &nbsp; for liquids heavier than water and &nbsp; &nbsp; <math>\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}} = \frac {\mathrm{density\; of\; the\; tested\; liquid\; at\; 60\,^\circ \mathrm{F}}} {\mathrm{density\; of\; water\; at\; 60\,^\circ \mathrm{F}}}</math>
:Note: Many literature sources present the above equations with the specific gravity reference temperatures being 20°C, which ignores the small difference between specific gravities at 60 °F and 20 °C.<ref>{{cite book|author=U.S. National Bureau of Standards|title=Bureau of Standard Circular No. 41, Testing and Properties of Textile Materials|edition=|publisher=Government Printing Office|year=1913|pages=page 160|id=}} (See second paragraph below Table 32 for comment about the small difference between specific gravities at 60 °F and 20 °C.)</ref>
;[[Brix gravity]]: This scale was developed in the 1854 by [[Adolf Ferdinand Wenceslaus Brix]], a [[Germany|German]] or [[Austria|Austrian]] engineer and mathematician. It is widely used in [[beer brewing]], [[Winery|wineries]], sugar refining and [[fruit juice]] industry. It is expressed as:<ref name=Jacobson>{{cite book|author=Jean L. Jacobson|title=Introduction to Wine Laboratory Practices and Procedures|edition=1st Edition|publisher=Springer|year=2005|id=0-387-24377-1}}</ref><ref>[http://www.ciagnik.net/en/Brix.html Degrees Brix]</ref>
::<math>^\circ \mathrm{Bx} = \mathrm{261.3} - \frac{261.3} {\mathrm{SG}_{20\,^\circ \mathrm{C}}^{20\,^\circ \mathrm{C}}}</math> &nbsp; &nbsp; and &nbsp; &nbsp; <math>\mathrm{SG}_{20\,^\circ \mathrm{C}}^{20\,^\circ \mathrm{C}} = \frac {\mathrm{density\; of\; the\; tested\; liquid\; at\; 20\,^\circ \mathrm{C}}} {\mathrm{density\; of\; water\; at\; 20\,^\circ \mathrm{C}}}</math>
;[[Oechsle gravity]]: This scale was developed in the 1830's by [[Christian Ferdinand Oechsle]], a German pharmacist and goldsmith. It is used in [[Germany]], [[Austria]] and [[Switzerland]] in wineries and beer brewing. It is expressed as:<ref name=Jacobson/>
::<math>^\circ \mathrm{Oe} = 1000\, (\mathrm{SG}_{20\,^\circ \mathrm{C}}^{20\,^\circ \mathrm{C}} -1)</math>
;[[Plato gravity]]: This scale was developed in 1918 by [[Fritz Plato|Dr. Fritz Plato]] , a German scientist. It is primarily used in the beer brewing industry and it is expressed as:<ref>{{cite book|author=Roger B. Boulton, Vernon L. Singleton, Linda F. Bisson and Ralph E. Kunkee|title=Principles and Practices of Winemaking|edition=1st Edition |publisher=Springer|year=1996|id=ISBN 0-8342-1270-6}}</ref>
::<math>^\circ \mathrm{P} = \mathrm{260} - \frac{260} {\mathrm{SG}_{17.5\,^\circ \mathrm{C}}^{17.5\,^\circ \mathrm{C}}}</math> &nbsp; &nbsp; and &nbsp; &nbsp; <math>\mathrm{SG}_{17.5\,^\circ \mathrm{C}}^{17.5\,^\circ \mathrm{C}} = \frac {\mathrm{density\; of\; the\; tested\; liquid\; at\; 17.5\,^\circ \mathrm{C}}} {\mathrm{density\; of\; water\; at\; 17.5\,^\circ \mathrm{C}}}</math>
:The origin of the Plato scale lies in the Balling Scale developed in 1835 by [[Carl Joseph Napoleon Balling]] which was recalibrated by Brix in 1854 and renamed the Brix scale. In 1918, Dr. Plato then developed his scale by improving and correcting Balling's original work. Basically, the Balling, Brix and Plato scales are identical up to the fifth and sixth decimal place.<ref>{{cite book|author=Peter Hull|title=Glucose Syrups: Technology and Applications|edition=1st Edition|publisher=Wiley-Blackwell|year=2010|id=ISBN 1-4051-7556-7}} Chapter 1 is available online at [http://media.wiley.com/product_data/excerpt/67/14051755/1405175567.pdf Chapter 1, History of Glucose Syrups]]</ref>
;[[Twaddell gravity]]: In the 19th century, this scale was developed in [[Glasgow]], [[Scotland]] by [[William Twaddell]], an instrument maker.<ref>[http://nms.scran.ac.uk/database/record.php?usi=000-190-004-180-C Hydrometers] From the website of the [[National Museums of Scotland]].</ref> It is used in many industries and it is expressed as:<ref name=Perry/>
::<math>^\circ \mathrm{Tw} = 200\, (\mathrm{SG}_{60\,^\circ \mathrm{F}}^{60\,^\circ \mathrm{F}} -1)</math> &nbsp; &nbsp; used only for liquids heavier than water
;Others:
*The leather tanning industry uses a Barkometer that expresses specific gravity in Barkometer degrees.
*The dairy industry uses a Lactometer calibrated in  Quevenne degrees in testing milk.
*The alcohol industry uses the Sikes, Richter, or Tralles scales on their Alcoholometers. Each of them reads the volumetric percentage of [[ethyl alcohol]] in water.
== History ==
Knowledge of relative density or specific gravity has been with us since the days of [[Archimedes]] in 250 BC, with the observation that light objects can float while heavier ones will sink in water.<ref>{{cite book|author=T. L. Heath|title=The Works of Archimedes|edition=|publisher=Cambridge University Press|year=1897|pages=page 253|id=}} Full text available at [http://www.archive.org/stream/worksofarchimede00arch#page/n5/mode/2up www.archive.org]</ref>  [[Hypatia]] (born ca. 350 and died 415 AD), a [[Greece|Greek]] scholar from [[Alexandria]] in [[Egypt]] and considered to be the first notable female mathematician, is reputed to have invented the hydrometer.<ref>[http://www.inventions.org/culture/female/hypatia.html Mothers of Invention] Ethlie Ann Vare and Greg Ptacek, 1988, pp. 24-26.</ref>
Several key figures in the history of science have mentioned the hydrometer in their work, including Galileo in 1612. In the 18th and 19th centuries, industrial development in Europe spurred the need for the hydrometer. It gained fame due to public controversy over [[alcohol]] taxation since the hydrometer was used in the [[Continuous distillation|distillation industry]] to measure alcohol content and determine excise taxes in [[England]].
A great deal more detailed history is provided in the book, edited by Holmes and Lever, about the history of instruments in chemistry from the days of the alchemists through the creation of the modern chemistry laboratory.<ref>{{cite book|author=Frederick L. Holmes and Trevor H. Levere (Editors)|title=Instruments and Experimentation in the History of Chemistry|edition=1st Edition, 2nd Printing|publisher=The MIT Press|year=2002|id=ISBN 0-262-08282-9}}</ref>
== References ==
{{reflist}}

Revision as of 22:42, 3 February 2010

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