View source for Hydrometer

{{Short description|Device used to measure density of liquids}}
{{distinguish|Hygrometer}}
[[File:Areometer.svg|thumb|upright=0.65|Schematic drawing of a hydrometer. The lower the density of the fluid, the deeper the weighted float '''B''' sinks. The depth is read off the scale '''A'''.]]

A '''hydrometer''' or '''lactometer''' is an instrument used for measuring density or [[relative density]] of [[liquids]] based on the concept of [[buoyancy]]. They are typically [[Calibration|calibrated]] and [[Graduation (instrument)|graduated]] with one or more scales such as [[specific gravity]].

A hydrometer usually consists of a sealed hollow glass tube with a wider bottom portion for [[buoyancy]], a [[ballast]] such as [[lead]] or [[Mercury (element)|mercury]] for stability, and a narrow stem with graduations for measuring. The liquid to test is poured into a tall container, often a [[graduated 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 correlates to relative density. Hydrometers can contain any number of scales along the stem corresponding to properties correlating to the density.

Hydrometers are calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in [[Distilled beverage|spirits]].

The hydrometer makes use of [[Archimedes' principle]]: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. The lower the density of the fluid, the deeper a hydrometer of a given weight sinks; the stem is calibrated to give a numerical reading.

==History==
[[File : Hydrometer6455.png|thumb|upright=0.6|Hydrometer from ''Practical Physics'']]

The hydrometer probably dates  back to the Greek philosopher [[Archimedes]] (3rd century BC) who used its principles to find the density of various liquids.<ref>{{Cite book |last=Hornsey |first=Ian S. |title=A history of beer and brewing |date=2003 |publisher=Royal Society of Chemistry |others= |isbn=978-0-85404-630-0 |series= |location=Cambridge |page=429 |language=en}}</ref><ref>{{Cite book |last=Bendick |first=Jeanne |title=Archimedes And The Door Of Science |publisher=Literary Licensing, LLC |year=2011 |isbn=9781258014889 |pages=63-64 |language=en |orig-date=1962}}</ref> An early description of a hydrometer comes from a Latin poem, written in the 2nd century AD by Remnius, who compared the use of a hydrometer to the method of [[Displacement (fluid)|fluid displacement]] used by Archimedes to determine the gold content of [[Hiero II of Syracuse|Hiero II's]] crown.<ref name="bensaude-vincent">{{cite book |last=Bensaude-Vincent |first=Bernadette |title=Instruments and Experimentation in the History of Chemistry |date=2000 |publisher=Massachusetts Institute of Technology Press |isbn=9780262082822 |editor1-last=Holmes |editor1-first=Frederic L. |page=153 |author-link=Bernadette Bensaude-Vincent |editor2-last=Levere |editor2-first=Trevor H.}}</ref>

[[Hypatia of Alexandria]] (4th-5th century AD), an important female Greek mathematician, is the first person traditionally associated with the hydrometer.<ref name="bensaude-vincent"/> In a letter, [[Synesius|Synesius of Cyrene]] asks Hypatia, his teacher, to make a hydrometer for him:

<blockquote>
The instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. It has notches in a perpendicular line, by means of which we are able to test the weight of the waters. A cone forms a lid at one of the extremities, closely fitted to the tube. The cone and the tube have one base only. This is called the baryllium. Whenever you place the tube in water, it remains erect. You can then count the notches at your ease, and in this way ascertain the weight of the water.<ref>{{cite book |last=FitzGerald |first=Augustine |url= |title=The Letters of Synesius of Cyrene |date=1926 |publisher=Oxford University Press |page=99 |chapter=Letter 15 - To the Philosopher (Hypatia) |chapter-url=https://hdl.handle.net/2027/mdp.39015008616180?urlappend=%3Bseq=105%3Bownerid=13510798888320432-109}}</ref>
</blockquote>

According to the ''Encyclopedia of the History of Arabic Science'', it was used by [[Abū Rayhān al-Bīrūnī]] in the 11th century and described by [[Al-Khazini]] in the 12th century.<ref>{{cite book |author1=Rozhanskaya |first=Mariam |title=[[Encyclopedia of the History of Arabic Science|Encyclopedia of the History of Arabic Science, Volume 2]] |author2=Levinova |first2=I. S. |publisher=[[Routledge]] |year=1996 |isbn=978-0-415-12411-9 |editor1=Rushdī Rāshid |pages=614–642 [ [https://books.google.com/books?id=Dy897SeErOQC&pg=PA639 639] ] |chapter=Statics |access-date=2019-03-26 |editor2=Régis Morelon |chapter-url=https://books.google.com/books?id=Dy897SeErOQC&pg=PA614 |name-list-style=}}</ref> It was rediscovered in 1612 by Galileo and his circle of friends, and used in experiments especially at the Accademia del Cimento.<ref>{{cite web |title=In-depth Hydrometer |url=https://catalogue.museogalileo.it/indepth/Hydrometer.html |access-date=2024-01-01 |website=Museo Galileo}}</ref> It appeared again in the 1675 work of Robert Boyle (who coined the name ''"hydrometer"''),<ref name="bensaude-vincent"/> with types devised by [[Antoine Baumé]] (the [[Baumé scale]]), [[William Nicholson (chemist)|William Nicholson]], and [[Jacques Charles|Jacques Alexandre César Charles]] in the late 18th century,<ref>{{cite journal |author=Blondel |first=Claude-Joseph |year=2003 |title=Un enfant illustre de Beaugency : le physicien et aéronaute Jacques Charles (1746-1823) |trans-title=An illustrious child of Beaugency: the physicist and aeronaut |url=https://gallica.bnf.fr/ark:/12148/bpt6k65454933 |journal=Les Publications de l'Académie d'Orléans, Agriculture, Sciences, Belles-lettres et Arts |language=fr |publisher=Académie d'Orléans |page=37 |number=4}}</ref> more or less contemporarily with [[Benjamin Sikes]]' discovery of the device by which the alcoholic content of a liquid can be automatically determined. The use of the Sikes device was made obligatory by British law in 1818.<ref>{{cite book |last=Denison |first=Merrill |date=1955 |title=The Barley and the Stream: The Molson Story |location=Toronto |publisher=McClelland & Stewart |page=132}}</ref>

==Ranges==
The hydrometer sinks deeper in low-density liquids such as [[kerosene]], [[gasoline]], and [[ethanol|alcohol]], and less deep in high-density liquids such as [[brine]], [[milk]], and [[acid]]s. It is usual for hydrometers to be used with dense liquids to have the mark 1.000 (for water) near the top of the stem, and those for use with lighter liquids to have 1.000 near the bottom. In many industries a set of hydrometers is used (1.0&ndash;0.95, 0.95&ndash;.) to have instruments covering the range of specific gravities that may be encountered.

==Scales==
[[File:Brine concentration measurement.jpg|thumb|upright|A [[NASA]] worker using a hydrometer to  measure the [[brine]] density of a salt evaporation pond]]
Modern hydrometers usually measure [[specific gravity]] but different scales were (and sometimes still are) used in certain industries.  Examples include:

* [[API gravity]], universally used worldwide by the petroleum industry.
* [[Baumé scale]], formerly used in industrial chemistry and [[pharmacology]]
* [[Brix]] scale, primarily used in fruit juice, wine making and the sugar industry
* [[Oechsle scale]], used for measuring the density of [[grape must]]
* [[Plato scale]], primarily used in [[brewing]]
* [[Twaddell scale]], formerly used in the bleaching and dyeing industries<ref>{{Cite book |last=Dick |first=William Brisbane |url= |title=Dick's Encyclopedia of Practical Receipts and Processes |date=1975 |publisher=Dick and Fitzgerald, Publishers |isbn=9780308101579 |location=New York |language=en |chapter=Degrees of Baum'e |chapter-url=https://chestofbooks.com/reference/Encyclopedia-Of-Practical-Receipts-And-Processes/Degrees-of-Baum-e.html}}</ref>

==Specialized hydrometers==
Specialized hydrometers are frequently named for their use: a lactometer, for example, is a hydrometer designed especially for use with dairy products. They are sometimes referred to by this specific name, sometimes as hydrometers.

===Alcoholometer===
An alcoholmeter is a hydrometer that indicates the alcoholic strength of liquids which are essentially a mixture of alcohol and water.  It is also known as a ''proof and Tralles hydrometer'' (after [[Johann Georg Tralles]], but commonly misspelled as '''traille''' and '''tralle'''). It measures the density of the fluid. Where no sugar or other dissolved substances are present, the specific gravity of a solution of ethanol in water can be directly correlated to the concentration of alcohol. Saccharometers for measuring sugar-water mixtures measure densities greater than water. Many 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 or carbohydrate based material.  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.<ref>{{cite book |url=https://books.google.com/books?id=XRyxWu8rRnQC&q=Alcoholometer&pg=PA12 |title=The dictionary of beer and brewing|first1=Dan|last1=Rabin|first2=Carl|last2=Forget|publisher=Fitzroy Dearborn|place=London|date=1998 |isbn=9781579580780|access-date=2009-10-11 }}</ref>

These were important instruments for determining tax, and specific maker's instruments could be specified. [[Bartholomew Sikes]] had a monopoly in the UK and [[Mary Dicas]] and her family enjoyed a similar monopoly in the US.<ref>{{Cite book |last=Morrison-Low |first=A. D. |url=http://www.oxforddnb.com/view/10.1093/ref:odnb/9780198614128.001.0001/odnb-9780198614128-e-49542 |title=Dicas [married name Arstall], Mary (fl. 1800–1815), maker of scientific instruments |date=2004-09-23 |publisher=Oxford University Press |volume=1 |language=en |doi=10.1093/ref:odnb/49542}}</ref>

===Lactometer===
A lactometer is used to check purity of cow's 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. If the milk sample is pure, the lactometer floats higher than if it is adulterated or impure.<ref>{{Cite journal |last=Geisler |first=Joseph F. |date=1891 |title=Signification of Lactometer Tests |url=https://pubs.acs.org/doi/abs/10.1021/ja02124a009 |journal=Journal of the American Chemical Society |language=en |volume=13 |issue=2 |pages=93–98 |doi=10.1021/ja02124a009 |issn=0002-7863}}</ref><ref>{{Cite web |title=Glass lactometer, England, 1875-1900 |url=https://collection.sciencemuseumgroup.org.uk/objects/co154299/glass-lactometer-england-1875-1900-hydrometer-lactometer |access-date=2024-01-01 |website=Science Museum Group Collection |language=en}}</ref>

===Saccharometer===
{{about|the hydrometer|the optical instrument|Saccharimeter|section=yes}}
[[File:Saccharometer, Merseyside Maritime Museum.jpg|thumb|right|A 20th century saccharometer]]
A saccharometer is a type of hydrometer used for determining the amount of sugar in a solution, invented by [[Thomas Thomson (chemist)|Thomas Thomson]].<ref>{{Cite book |last=Thomson |first=Thomas |url=https://books.google.com/books?id=6bUoAQAAMAAJ&q=Thomas+Thomson+saccharometer |title=Explanation of Allan's Saccharometer: Appointed by Act of Parliament for the Use of Distillers, & C |date=1840 |publisher=D. & W. Millar |language=en}}</ref> It is used primarily by [[winemaker]]s and [[brewing|brewers]],<ref>{{cite book |last1=Sambrook |first1=Pamela |url=https://books.google.com/books?id=vDEwIR0ZHGYC&q=Saccharometer&pg=PA64 |title=Country House Brewing in England, 1500-1900 |date=1996 |publisher=A&C Black |isbn=9781852851279 |access-date=2009-10-11}}</ref> and it can also be used in making [[sorbet]]s and ice-creams.<ref>{{cite book |url=https://books.google.com/books?id=D6YuI15dT74C&q=Saccharometer&pg=PA250 |title=Patisserie |isbn=9780750604307 |access-date=2009-10-11 |last1=Hanneman |first1=Leonard J. |year=1993 }}</ref> The first brewers' saccharometer was constructed by Benjamin Martin (with distillation in mind), and initially used for brewing by James Baverstock Sr in 1770.<ref>{{cite book |last1=Mathias |first1=Peter |url=https://books.google.com/books?id=PyU9AAAAIAAJ&pg=PA71 |title=The Brewing Industry in England 1700-1830 |publisher=University Press |year=1959 |access-date=2012-03-16}}</ref> Henry Thrale adopted its use and it was later popularized by John Richardson in 1784.<ref>{{cite book |last1=Bud |first1=Robert |url=https://books.google.com/books?id=1AsFdUxOwu8C&q=Saccharometer+John+Richardson&pg=RA1-PA312 |title=Instruments of Science: An Historical Encyclopedia |last2=Warner |first2=Deborah Jean |last3=Chaplin |first3=Simon |last4=Johnston |first4=Stephen |last5=Peterson |first5=Betsy Bahr |publisher=Science Museum, London, and National Museum of American History, Smithsonian Institution |year=1998 |isbn=9780815315612 |access-date=2009-10-11}}</ref>

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. The higher the sugar content, the denser the solution, and thus the higher the bulb will float.

===Thermohydrometer===
A thermohydrometer is a hydrometer that has a [[thermometer]] enclosed in the float section. For measuring the density of petroleum products, such as 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&nbsp;°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.<ref>{{Cite book|title=Instrument and automation engineers' handbook: measurement and safety|publisher=Taylor & Francis Group, CRC Press|year=2017|isbn=978-1-4987-2764-8|editor-last=Béla G. Lipták|location=United States|pages=1314|editor-last2=Kriszta Venczel|edition=Fifth}}</ref>

===Battery hydrometer===
The state of charge of a [[lead-acid]] battery can be estimated from the density of the [[sulfuric acid]] solution used as [[electrolyte]]. A hydrometer calibrated to read specific gravity relative to water at {{Convert|60|F}} is a standard tool for servicing [[car battery|automobile batteries]].  Tables are used to correct the reading to the standard temperature. Hydrometers are also used for maintenance of wet-cell nickel-cadmium batteries to ensure the electrolyte is of the proper strength for the application; for this battery chemistry the specific gravity of the electrolyte is not related to the state of charge of the battery.

A battery hydrometer with thermometer (thermohydrometer) measures the temperature-compensated specific gravity and electrolyte temperature.

===Antifreeze tester===
Another automotive use of hydrometers is testing the quality of the [[antifreeze]] solution used for engine cooling. The degree of freeze protection can be related to the density (and so concentration) of the antifreeze; different types of antifreeze have different relations between measured density and freezing point.

===Acidometer===
An acidometer, or acidimeter, is a hydrometer used to measure the specific gravity of an [[acid]].<ref>{{Cite web |title =Acidometer |work =The Free Dictionary |publisher =Farlex |year =2003 |url =http://www.thefreedictionary.com/acidometer |access-date =18 September 2010 }}</ref>

===Barkometer===
A barkometer is  calibrated to test the strength of tanning liquors used in tanning [[leather]].<ref>{{cite book |last=Davis |first=Charles Thomas |url=https://archive.org/details/manufactureoflea00davi |title=The manufacture of leather: being a description of all of the processes for the tanning and tawing with bark, extracts, chrome and all modern tannages in general use |publisher=H. C. Baird & co. |year=1897 |quote=Barkometer. |access-date=2009-10-11}}</ref>

===Salinometer===
A salinometer is a hydrometer used to measure the salt content of the feed water to a marine steam boiler.

===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.

===Gallery===
<gallery mode="packed" heights="440px">
Hydrometer in a still (cropped).jpg|An alcohol meter in a still
Densimetro o galattometro con custodia - Musei del cibo - Parmigiano - 314a.jpg|Lactometer
Battery condition indicator.jpg|Battery condition indicator to measure the density of the electrolyte in a lead–acid battery, indicating the charge of the battery (~1985)
Coolant indicator.jpg|Antifreeze tester measures the density of engine coolant, indicating its freezing point.
</gallery>

==Use in soil analysis==
A hydrometer analysis is the process by which fine-grained soils, [[silt]]s and [[clay]]s, 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 fraction of particles of a certain equivalent particle diameter to be calculated.<ref>Fakhry A. Assaad, Philip Elmer LaMoreaux, Travis H. Hughes (ed.), '' Field Methods for Geologists and Hydrogeologists'', Springer Science & Business Media, 2004 {{ISBN|3540408827}}, p.299</ref>

==See also==
{{div col|colwidth=18em}}
* [[Density meter]]
* [[Densitometer]]
* [[Dasymeter]]
* [[Elevator paradox (physics)]]
* [[Fahrenheit hydrometer]]
* [[Gravity (beer)]]
* [[Hydrostatic bubbles]]
* [[Oscillating U-tube]]
* [[Pyknometer]]

{{div col end}}

==References==
{{reflist}}

==Sources==
{{commons category|Hydrometers}}
*[https://web.archive.org/web/20100906035321/http://www.inventions.org/culture/female/hypatia.html Hypatia of Alexandria]
*[https://web.archive.org/web/20110715112525/http://www.pressedforwine.com/equipment/hydrometer.shtml Hydrometer Information]
*[http://www.homebrewing.com/articles/hydrometers.php Guide to Brewing Hydrometers]
*Jurjen Draaijer. [http://www.fao.org/ag/AGA/AGAP/LPS/dairy/DAP/mpo/chap4-2.htm Milk Testing]. ''Milk Producer Group Resource Book'', Food and Agriculture Organization of the United Nations
*[http://winemaking.jackkeller.net/hydrom.asp Using Your Hydrometer], Winemaking Home Page.
*[http://www.home-winemaking.com/winemaking-2b.html How The Hydrometer Works], Home Winemaking Techniques

{{Geotechnical engineering|state=collapsed}}
{{Authority control}}

[[Category:Ancient Roman technology]]
[[Category:Brewing]]
[[Category:Laboratory equipment]]
[[Category:Laboratory glassware]]
[[Category:Density meters]]