A hydrometer or lactometer is an instrument used for measuring density or

relative density Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

liquids A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...

based on the concept of

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

. They are typically

calibrated In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known a ...

and graduated with one or more scales such as

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

. A hydrometer usually consists of a sealed hollow glass tube with a wider bottom portion for

, a

ballast Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship ...

such as

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...

or 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 A graduated cylinder, also known as a measuring cylinder or mixing cylinder, is a common piece of laboratory equipment used to measure the volume of a liquid. It has a narrow cylindrical shape. Each marked line on the graduated cylinder represent ...

, 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

spirits Spirit or spirits may refer to: Liquor and other volatile liquids * Spirits, a.k.a. liquor, distilled alcoholic drinks * Spirit or tincture, an extract of plant or animal material dissolved in ethanol * Volatile (especially flammable) liquids, ...

. The hydrometer makes use of

Archimedes' principle Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes ...

: 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

upright=0.6, Hydrometer from Practical Physics The hydrometer probably dates back to the Greek philosopher

Archimedes Archimedes of Syracuse (;; ) was a Greek mathematician, physicist, engineer, astronomer, and inventor from the ancient city of Syracuse in Sicily. Although few details of his life are known, he is regarded as one of the leading scientists i ...

(3rd century BC) who used its principles to find the density of various liquids. 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 fluid displacement used by Archimedes to determine the gold content of Hiero II's crown.

Hypatia of Alexandria Hypatia, Koine pronunciation (born 350–370; died 415 AD) was a neoplatonist philosopher, astronomer, and mathematician, who lived in Alexandria, Egypt, then part of the Eastern Roman Empire. She was a prominent thinker in Alexandria where ...

(4th-5th century AD), an important female Greek mathematician, is the first person traditionally associated with the hydrometer. In a letter,

Synesius of Cyrene Synesius (; el, Συνέσιος; c. 373 – c. 414), was a Greek bishop of Ptolemais in ancient Libya, a part of the Western Pentapolis of Cyrenaica after 410. He was born of wealthy parents at Balagrae (now Bayda, Libya) near Cyrene between ...

asks Hypatia, his teacher, to make a hydrometer for him:

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.

According to the ''Encyclopedia of the History of Arabic Science'', it was used by

Abū Rayhān al-Bīrūnī Abu Rayhan Muhammad ibn Ahmad al-Biruni (973 – after 1050) commonly known as al-Biruni, was a Khwarazmian Iranian peoples, Iranian in scholar and polymath during the Islamic Golden Age. He has been called variously the "founder of Indolog ...

in the 11th century and described by

Al-Khazini Abū al-Fath Abd al-Rahman Mansūr al-Khāzini or simply al-Khāzini (, flourished 1115–1130) was an Iranian astronomer of Greek origin from Seljuk Persia. His astronomical tables written under the patronage of Sultan Sanjar (', 1115) is co ...

in the 12th century. It was rediscovered in 1612 by Galileo and his circle of friends, and used in experiments especially at the Accademia del Cimento. It appeared again in the 1675 work of Robert Boyle (who coined the name ''"hydrometer"''), with types devised by Antoine Baumé (the

Baumé scale The Baumé scale is a pair of hydrometer scales developed by French pharmacist Antoine Baumé in 1768 to measure density of various liquids. The unit of the Baumé scale has been notated variously as ''degrees Baumé'', ''B°'', ''Bé°'' and simpl ...

), William Nicholson, and Jacques Alexandre César Charles in the late 18th century, 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.

Ranges

The hydrometer sinks deeper in low-density liquids such as

kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regis ...

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic com ...

, and

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

, and less deep in high-density liquids such as

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for b ...

milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modulatin ...

, and

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

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 (1.0–0.95, 0.95–.) to have instruments covering the range of specific gravities that may be encountered.

Scales

Modern hydrometers usually measure

but different scales were (and sometimes still are) used in certain industries. Examples include: *

API gravity The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks ...

, universally used worldwide by the petroleum industry. *

, formerly used in industrial chemistry and

pharmacology Pharmacology is a branch of medicine, biology and pharmaceutical sciences concerned with drug or medication action, where a drug may be defined as any artificial, natural, or endogenous (from within the body) molecule which exerts a biochemica ...

Brix Degrees Brix (symbol °Bx) is a measure of the dissolved solids in a liquid, and is commonly used to measure dissolved sugar content of an aqueous solution. One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength ...

scale, primarily used in fruit juice, wine making and the sugar industry *

Oechsle scale The Oechsle scale is a hydrometer scale measuring the density of grape must, which is an indication of grape ripeness and sugar content used in wine-making. It is named for Ferdinand Oechsle (1774–1852) and it is widely used in the German, Swi ...

, used for measuring the density of

grape must Must (from the Latin ''vinum mustum'', "young wine") is freshly crushed fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace and typically makes up 7–23% of th ...

Plato scale When drinking beer, there are many factors to be considered. Principal among them are bitterness, the variety of flavours present in the beverage and their intensity, alcohol content, and colour. Standards for those characteristics allow a more o ...

, primarily used in

brewing Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and fermenting the resulting sweet liquid with yeast. It may be done in a brewery by a commercial brewer, ...

* Twaddell scale, formerly used in the bleaching and dyeing industries

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.

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; if it is adulterated or impure, the lactometer sinks.

Saccharometer

A saccharometer is a hydrometer used for determining the amount of sugar in a solution, invented by

Thomas Thomson Thomas Thomson may refer to: * Tom Thomson (1877–1917), Canadian painter * Thomas Thomson (apothecary) (died 1572), Scottish apothecary * Thomas Thomson (advocate) (1768–1852), Scottish lawyer * Thomas Thomson (botanist) (1817–1878), Scottish ...

. It is used primarily by

winemaker A winemaker or vintner is a person engaged in winemaking. They are generally employed by wineries or wine companies, where their work includes: *Cooperating with viticulturists *Monitoring the maturity of grapes to ensure their quality and to dete ...

s and brewers, and it can also be used in making

sorbet Sorbet (), also called "water ice", is a frozen dessert made using ice combined with fruit juice, fruit purée, wine, liqueur, honey, etc. Generally sorbets do not contain dairy ingredients, while sherbets do. Etymology The word "sorbet" en ...

s and ice-creams. The first brewers' saccharometer was constructed by Benjamin Martin (with distillation in mind), and initially used for brewing by James Baverstock Sr in 1770. Henry Thrale adopted its use and it was later popularized by John Richardson in 1784. 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 A thermometer is a device that temperature measurement, measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a merc ...

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 °C. Very light oils with many volatile components are measured in a variable volume container using a floating

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...

sampling device to minimize light end losses.

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 An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

. A hydrometer calibrated to read specific gravity relative to water at is a standard tool for servicing 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 An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, all ...

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 (sometimes spelled acidimeter) is a hydrometer used to measure the specific gravity of an

Barkometer

A barkometer is calibrated to test the strength of tanning liquors used in tanning

leather Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hogs ...

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 Urinalysis, a portmanteau of the words ''urine'' and ''analysis'', is a panel of medical tests that includes physical (macroscopic) examination of the urine, chemical evaluation using urine test strips, and microscopic examination. Macroscopic e ...

. 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

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 charge of the battery (~1985). Coolant indicator.jpg, Antifreeze tester measures the freezing point of engine coolant.

Use in soil analysis

A hydrometer analysis is the process by which fine-grained soils,

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel whe ...

s and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay parti ...

s, are graded. Hydrometer analysis is performed if the grain sizes are too small for

sieve analysis A sieve analysis (or gradation test) is a practice or procedure used in civil engineering and chemical engineering to assess the particle size distribution (also called ''gradation'') of a granular material by allowing the material to pass throu ...

. The basis for this test is

Stoke's Law In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by so ...

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.Fakhry A. Assaad, Philip Elmer LaMoreaux, Travis H. Hughes (ed.), '' Field Methods for Geologists and Hydrogeologists'', Springer Science & Business Media, 2004 , p.299

References

Sources

Hypatia of AlexandriaHydrometer InformationGuide to Brewing Hydrometers
*Jurjen Draaijer

''Milk Producer Group Resource Book'', Food and Agriculture Organization of the United Nations
Using Your Hydrometer
Winemaking Home Page.

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