A medical thermometer (also called clinical thermometer) is a device which is used for
measuring
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determining how large or small a physical quantity is as compared t ...
human or animal
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
. The tip of the thermometer is inserted into the
mouth
In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
under the
tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
(''oral'' or ''sub-lingual temperature''), under the
armpit
The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superiorl ...
(''axillary temperature''), into the
rectum
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
via the
anus
The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
(''rectal temperature''), into the
ear
An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...
(''tympanic temperature''), or on the
forehead
In human anatomy, the forehead is an area of the head bounded by three features, two of the skull and one of the scalp. The top of the forehead is marked by the hairline, the edge of the area where hair on the scalp grows. The bottom of the fore ...
(''temporal temperature'').
History
The medical thermometer began as an instrument more appropriately called a water
thermoscope
A thermoscope is a device that shows changes in temperature. A typical design is a tube in which a liquid rises and falls as the temperature changes. The modern thermometer gradually evolved from it with the addition of a scale in the early 17th c ...
, constructed by
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
circa 1592–1593. It lacked an accurate scale with which to measure temperature and could be affected by changes in atmospheric pressure.
Italian
Italian(s) may refer to:
* Anything of, from, or related to the people of Italy over the centuries
** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom
** Italian language, a Romance language
*** Regional Ita ...
physician
Santorio Santorio
Santorio Santori (29 March, 1561 – 25 February, 1636) also called Santorio Santorio, Santorio de' Sanctoriis, or Sanctorius of Padua and various combinations of these names, was an Italian physiologist, physician, and professor, who introduc ...
is the first known individual to have put a measurable scale on the thermoscope and wrote of it in 1625, though he possibly invented one as early as 1612. His models were bulky, impractical and took a fair amount of time to take an accurate oral reading of the patient's temperature.
Two individuals switched from water to alcohol in the thermometer.
* The earliest is
Ferdinando II de' Medici, Grand Duke of Tuscany
Ferdinando II de' Medici (14 July 1610 – 23 May 1670) was grand duke of Tuscany from 1621 to 1670. He was the eldest son of Cosimo II de' Medici and Maria Maddalena of Austria. He was remembered by his contemporaries as a man of culture and ...
(1610–1670), who created an enclosed thermometer that used alcohol circa 1654.
*
Daniel Gabriel Fahrenheit
Daniel Gabriel Fahrenheit FRS (; ; 24 May 1686 – 16 September 1736) was a physicist, inventor, and scientific instrument maker. Born in Poland to a family of German extraction, he later moved to the Dutch Republic at age 15, where he spent ...
(1686–1736), a Polish-born Dutch physicist, engineer, and glass blower, made contributions to thermometers as well. He created an alcohol thermometer in 1709 and later innovated the mercury thermometer in 1714.
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, he found, responded more quickly to temperature changes than the previously used water.
Fahrenheit also created the
temperature scale which is named after him, having recorded the system in 1724. The scale is still only mainly used for everyday applications in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, its
territories and associated states (all served by the
U.S. National Weather Service) as well as
the Bahamas
The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the Atlantic Ocean, North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to ...
,
Belize
Belize (; bzj, Bileez) is a Caribbean and Central American country on the northeastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a wate ...
, and
the Cayman Islands
The Cayman Islands () is a self-governing British Overseas Territory—the largest by population in the western Caribbean Sea. The territory comprises the three islands of Grand Cayman, Cayman Brac and Little Cayman, which are located to the ...
.
[Encyclopædia Britannica "Science & Technology: Daniel Gabriel Fahrenheit]
/ref>
Prominent Dutch mathematician, astronomer and physicist Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
created a clinical thermometer in 1665, to which he added an early form of the Celsius
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
scale by setting the scale to the freezing and boiling points of water. By 1742 Swedish astronomer Anders Celsius
Anders Celsius (; 27 November 170125 April 1744) was a Swedish astronomer, physicist and mathematician. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Germa ...
created the Celsius
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
temperature scale that was the reverse of the modern scale, in that ''0'' was the boiling point of water, while ''100'' was freezing. It was later reversed by Swedish botanist Carolus Linnaeus (1707–1778) in 1744.[Citation: Uppsala University (Sweden)]
''Linnaeus' thermometer''
/ref>
Working independently of Celsius, the Lyon
Lyon,, ; Occitan: ''Lion'', hist. ''Lionés'' also spelled in English as Lyons, is the third-largest city and second-largest metropolitan area of France. It is located at the confluence of the rivers Rhône and Saône, to the northwest of t ...
nais physicist Jean-Pierre Christin
Jean-Pierre Christin (31 May 1683 – 19 January 1755) was a French physicist, mathematician, astronomer, and musician. His proposal in 1743 to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degre ...
, permanent secretary of the ''Académie des sciences, belles-lettres et arts de Lyon'' FR, developed a similar scale in which ''0'' represented the freezing point of water and ''100'' represented boiling.Don Rittner
Don Rittner is an American historian, archeologist, anthropologist, environmental activist, educator, author and film maker living in the Capital District, Schenectady County, New York. He is the former Schenectady County Historian, responsible f ...
; Ronald A. Bailey (2005)
''Encyclopedia of Chemistry.''
Facts On File
Infobase Publishing is an American publisher of reference book titles and textbooks geared towards the North American library, secondary school, and university-level curriculum markets. Infobase operates a number of prominent imprints, includin ...
, Manhattan
Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...
, New York City
New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...
. pp. 43. On 19 May 1743 he published the design of a mercury thermometer
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Mercury ...
, the "Thermometer of Lyon" built by the craftsman Pierre Casati that used this scale.['']Mercure de France
The was originally a French gazette and literary magazine first published in the 17th century, but after several incarnations has evolved as a publisher, and is now part of the Éditions Gallimard publishing group.
The gazette was published f ...
'' (1743)
''MEMOIRE sur la dilatation du Mercure dans le Thermométre.''
Chaubert; Jean de Nully, Pissot, Duchesne, Paris
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
. pp. 1609–1610.[''Journal helvétique'' (1743)]
''LION.''
Imprimerie des Journalistes, Neuchâtel
, neighboring_municipalities= Auvernier, Boudry, Chabrey (VD), Colombier, Cressier, Cudrefin (VD), Delley-Portalban (FR), Enges, Fenin-Vilars-Saules, Hauterive, Saint-Blaise, Savagnier
, twintowns = Aarau (Switzerland), Besançon (France), ...
. pp. 308-310.[''Memoires pour L'Histoire des Sciences et des Beaux Arts'' (1743)]
''DE LYON.''
Chaubert, París. pp. 2125-2128.
The medical thermometer was used by Dutch
Dutch commonly refers to:
* Something of, from, or related to the Netherlands
* Dutch people ()
* Dutch language ()
Dutch may also refer to:
Places
* Dutch, West Virginia, a community in the United States
* Pennsylvania Dutch Country
People E ...
chemist and physician Hermann Boerhaave
Herman Boerhaave (, 31 December 1668 – 23 September 1738Underwood, E. Ashworth. "Boerhaave After Three Hundred Years." ''The British Medical Journal'' 4, no. 5634 (1968): 820–25. https://www.jstor.org/stable/20395297.) was a Dutch botanist ...
(1668–1738), as well as his notable students Gerard van Swieten
Gerard van Swieten (7 May 1700 – 18 June 1772) was a Dutch physician who from 1745 was the personal physician of the Holy Roman Empress Maria Theresa and transformed the Austrian health service and medical university education. He was the fa ...
(1700–72) and Anton de Haen
Anton de Haen (The Hague December 8, 1704 – Vienna September 5, 1776) was a Dutch physician who worked in Vienna as a professor at the University of Vienna and was the director of its medical department. He became a very influential physician in ...
(1704–76). It was also utilized around the same time by Scottish physician George Martine (1700–1741). De Haen made particular strides in medicine with the thermometer. By observing the correlation in a patient's change in temperature and the physical symptoms of the illness, he concluded that a record of one's temperature could inform the doctor of a patient's health. However, his proposals were not met with enthusiasm by his peers and the medical thermometer remained a scarcely used instrument in medicine.
Thermometers remained cumbersome to transport and use. By the mid 19th century, the medical thermometer was still a foot long (30.28 cm) and took as long as twenty minutes to take an accurate temperature reading. Between 1866 and 1867, Sir Thomas Clifford Allbutt
Sir Thomas Clifford Allbutt (20 July 183622 February 1925) was an English physician best known for his role as president of the British Medical Association 1920, for inventing the clinical thermometer, and for supporting Sir William Osler in fou ...
(1836–1925) designed a medical thermometer that was much more portable, measuring only six inches long and taking only five minutes to record a patient's temperature.
In 1868, German physician, pioneer psychiatrist, and medical professor Carl Reinhold August Wunderlich
Carl Reinhold August Wunderlich (4 August 1815, Sulz am Neckar – 25 September 1877, Leipzig) was a German physician, pioneer psychiatrist, and medical professor. He is known for his measurement of mean normal human body temperature of 37&nbs ...
published his studies that consisted of over one million readings from twenty-five thousand patients' temperatures, taken in the underarm
The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superiorl ...
. With his findings, he was able to conclude a healthy human's temperature fell within the range of 36.3 to 37.5 °C (97.34 to 99.5 °F).
Dr. Theodor H. Benzinger (13 April 1905 - 26 October 1999) invented the ear thermometer in 1964. Born in Stuttgart
Stuttgart (; Swabian: ; ) is the capital and largest city of the German state of Baden-Württemberg. It is located on the Neckar river in a fertile valley known as the ''Stuttgarter Kessel'' (Stuttgart Cauldron) and lies an hour from the ...
, Germany, he immigrated to the U.S. in 1947 and became a naturalized citizen
Naturalization (or naturalisation) is the legal act or process by which a non-citizen of a country may acquire citizenship or nationality of that country. It may be done automatically by a statute, i.e., without any effort on the part of the in ...
in 1955. He worked from 1947 to 1970 in the bioenergetics
Bioenergetics is a field in biochemistry and cell biology that concerns energy flow through living systems. This is an active area of biological research that includes the study of the transformation of energy in living organisms and the study of ...
division at the Naval Medical Research Center
The Naval Medical Research Center (NMRC) is an agency that performs basic and applied biomedical research to meet the needs of the United States Navy and United States Marine Corps. Its areas of focus include study of infectious diseases, biodef ...
in Bethesda, Maryland.
Classification by location
The temperature can be measured in various locations on the body which maintain a fairly stable temperature (mainly oral, axillary, rectal, tympanic, or temporal). The normal temperature varies slightly with the location; an oral reading of 37 °C does not correspond to rectal, temporal, etc. readings of the same value. When a temperature is quoted the location should also be specified. If a temperature is stated without qualification (e.g., typical body temperature) it is usually assumed to be sub-lingual. The differences between core temperature and measurements at different locations, known as ''clinical bias'', is discussed in the article on normal human body temperature
Normal human body-temperature (normothermia, euthermia) is the typical temperature range found in humans. The normal human body temperature range is typically stated as .
Human body temperature varies. It depends on sex, age, time of day, exert ...
. Measurements are subject to both site-dependent clinical bias and variability between a series of measurements (standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
s of the differences). For example, one study found that the clinical bias of rectal temperatures was greater than for ear temperature measured by a selection of thermometers under test, but variability was less.
Oral
Oral temperature may only be taken from a patient who is capable of holding the thermometer securely under the tongue, which generally excludes small children or people who are unconscious or overcome by coughing, weakness, or vomiting. (This is less of a problem with fast-reacting digital thermometers, but is certainly an issue with mercury thermometers, which take several minutes to stabilise their reading.) If the patient has drunk a hot or cold liquid beforehand time must be allowed for the mouth temperature to return to its normal value.
The typical range of a sub-lingual thermometer for use in humans is from about 35 °C to 42 °C or 90 °F to 110 °F.
Armpit
The armpit (axilla
The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superior ...
) temperature is measured by holding the thermometer tightly under the armpit. One needs to hold the thermometer for several minutes to get an accurate measurement. The axillary temperature plus 1 °C is a good guide to the rectal temperature in patients older than 1 month. The accuracy from the axilla is known to be inferior to the rectal temperature.
Rectal
Rectal thermometer temperature-taking, especially if performed by a person other than the patient, should be facilitated with the use of a water-based personal lubricant. Although rectal temperature is the most accurate, this method may be considered unpleasant or embarrassing ; in 1966, Time Magazine
''Time'' (stylized in all caps) is an American news magazine based in New York City. For nearly a century, it was published weekly, but starting in March 2020 it transitioned to every other week. It was first published in New York City on Mar ...
noted "what for many remains a humiliating procedure ... insertion of a rectal thermometer. Also, if not taken the correct way, rectal temperature-taking can be uncomfortable and in some cases painful for the patient. Rectal temperature-taking is considered the method of choice for infant
An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
s.
Ear
The ear thermometer was invented by Dr. Theodor H. Benzinger in 1964. At the time, he was seeking a way to get a reading as close to the brain's temperature as possible, since the hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
at the brain's base regulates the core body temperature. He accomplished this by using the ear canal's ear drum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air ...
's blood vessels
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
, which are shared with the hypothalamus. Before the ear thermometer's invention, easy temperature readings could only be taken from the mouth, rectum, or underarm
The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superiorl ...
. Previously, if doctors wanted to record an accurate brain temperature, electrodes needed to be attached to the patient's hypothalamus.
This tympanic thermometer has a projection (protected by a one-time hygienic sheath) that contains the infrared probe; the projection is gently placed in the ear canal and a button pressed; the temperature is read and displayed within about a second. These thermometers are used both in the home and in medical facilities.
There are factors that make readings of this thermometer to some extent unreliable, for example faulty placement in the external ear canal by the operator, and wax blocking the canal. Such error-producing factors usually cause readings to be below the true value, so that a fever can fail to be detected.
Forehead
Temporal artery
Temporal artery thermometers, which use the infrared principle report temperature, are becoming increasingly common in clinical practice because of their ease of use and minimal invasiveness. Because of the variability of technique and environmental considerations, measurements by temporal artery thermometers may suffer issues of precision
Precision, precise or precisely may refer to:
Science, and technology, and mathematics Mathematics and computing (general)
* Accuracy and precision, measurement deviation from true value and its scatter
* Significant figures, the number of digit ...
, and to a lesser degree accuracy
Accuracy and precision are two measures of ''observational error''.
''Accuracy'' is how close a given set of measurements (observations or readings) are to their ''true value'', while ''precision'' is how close the measurements are to each other ...
. Temporal thermometers have been found to have a low sensitivity of around 60–70%, but a very high specificity of 97–100% for detecting fever and hypothermia. Because of this, it is suggested that they should not be used in acute care settings like the ICU, or in patients with a high suspicion of temperature imbalance. Evidence supports higher accuracy and precision amongst pediatric patients.
Plastic strip thermometer
The thermometer is applied to the patient's brow. It is typically a band coated with different temperature-sensitive markings using plastic strip thermometer
A liquid crystal thermometer, temperature strip or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures.
Liquid cry ...
or similar technology; at a given temperature the markings (numerals indicating the temperature) in one region are at the right temperature to become visible. This type may give an indication of fever, but is not considered accurate.
Classification by technology
Liquid-filled
The traditional thermometer is a glass tube with a bulb at one end containing a liquid which expands in a uniform manner with temperature. The tube itself is narrow (capillary) and has calibration markings along it. The liquid is often mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, but alcohol thermometer
The alcohol thermometer or spirit thermometer is an alternative to the mercury-in-glass thermometer and has similar functions. Unlike the mercury-in-glass thermometer, the contents of an alcohol thermometer are less toxic and will evaporate qui ...
s use a colored alcohol. Medically, a maximum thermometer
The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam (1714). It consists of a bulb containing mercury attached to a glass tube of narrow diameter; the volume of mercury in the tube is much l ...
is often used, which indicates the maximum temperature reached even after it is removed from the body.
To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
—typically five minutes in the mouth and ten minutes under the armpit. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction.
Mercury
Mercury-in-glass thermometer
The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam (1714). It consists of a bulb containing mercury attached to a glass tube of narrow diameter; the volume of mercury in the tube is much le ...
s have been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube.
The tube must be very narrow to minimise the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimise the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible. Visibility is less of a problem with a coloured liquid.
It has been decided by many states to prohibit the use and sale of mercury thermometers due to the risk of handling and spilling, and the potential to cause mercury poisoning; the vigorous swinging needed to "reset" a mercury maximum thermometer makes it easy to accidentally break it and release poisonous mercury vapors. Mercury thermometers have largely been replaced by electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as galinstan
Galinstan (R) is a brand name for a alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. However, it is not a eutectic alloy but a near eutectic alloy. In scientific literature, galinstan is also used ...
, coloured 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 ...
s and heat-sensitive liquid crystals).
Gallium
One company markets a thermometer of this sort which it bills as "the first analogue thermometer without mercury," as it instead uses the liquid metal
A liquid metal is a metal or a metal alloy which is liquid at or near room temperature.
The only stable liquid elemental metal at room temperature is Mercury (element), mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9&nbs ...
gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
. Gallium is considered non-toxic and when disposed of does not face environmental concerns. Like mercury, gallium is a liquid at body temperature (melting point 29.7 °C), but according to the manufacturer an alloy of gallium, indium and tin is actually used, resulting in a lower melting temperature.
Phase-change (dot matrix) thermometers
Phase-change thermometers use samples of inert chemicals which melt at progressively higher temperatures from 35.5 °C to 40.5 °C in steps of 0.1 °C. They are mounted as small dots in a matrix on a thin plastic spatula with a protective transparent cover. This is placed under the patient's tongue. After a short time the spatula is removed and it can be seen which dots have melted and which have not: the temperature is taken as the melting temperature of the last dot to melt.
These are cheap disposable devices and avoid the need for sterilizing for re-use.
Liquid crystal
A liquid crystal thermometer
A liquid crystal thermometer, temperature strip or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures.
Liquid cry ...
contains heat-sensitive (thermochromic
Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, such as baby bottles which change to a differen ...
) liquid crystals in a plastic strip that change color to indicate different temperatures.
Electronic
Since compact and inexpensive methods of measuring and displaying temperature became available, electronic thermometers (often called ''digital'', because they display numeric values) have been used. Many display readings with a resolution of only 0.1 °C (.2 °F), but this should not be taken as a guarantee of accuracy: the specified accuracy must be checked in the documentation and maintained by periodic recalibration. A typical inexpensive electronic ear thermometer for home use has a displayed resolution of 0.1 °C, but a stated accuracy within ±0.2 °C (±0.35 °F) when new. The first electronic clinical thermometer, invented in 1954, used a flexible probe that contained a Carboloy thermistor.
Types of Digital Thermometer
Resistance temperature detectors (RTDs)
RTDs are wire windings or other thin film serpentines that exhibit changes in resistance with changes in temperature. They measure temperature using the positive temperature coefficient of electrical resistance of metals. The hotter they become, the higher the value of their electrical resistance. Platinum is the most commonly used material because it is nearly linear over a wide range of temperatures, is very accurate, and has a fast response time. RTDs can also be made of copper or nickel. Advantages of RTDs include their stable output for long periods of time. They are also easy to calibrate and provide very accurate readings. Disadvantages include a smaller overall temperature range, higher initial cost, and a less rugged design
Thermocouples
Thermocouples are accurate, highly sensitive to small temperature changes, and quickly respond to changes to the environment. They consist of a pair of dissimilar metal wires joined at one end. The metal pair generates a net thermoelectric voltage between their opening and according to the size of the temperature difference between the ends.
•Advantages of thermocouples include their high accuracy and reliable operation over an extremely wide range of temperatures. They are also well-suited for making automated measurements both inexpensive and durable.
•Disadvantages include errors caused by their use over an extended period of time, and that two temperatures are required to make measurements. Thermocouple materials are subject to corrosion, which can affect the thermoelectric voltage
Thermistor
Thermistor elements are the most sensitive temperature sensors available. A thermistor is a semiconductor device with an electrical resistance that is proportional to temperature. There are two types of products.
•Negative temperature coefficient (NTC) devices are used in temperature sensing and are the most common type of thermistor. NTCs have temperatures that vary inversely with their resistance, so that when the temperature increases, the resistance decreases, and vice versa. NTCs are constructed from oxides of materials such as nickel, copper, and iron.
• Positive temperature coefficient (PTC) devices are used in electric current control. They function in an opposite manner than NTC in that the resistance increases as temperature increases. PTCs are constructed from thermally sensitive silicons or polycrystalline ceramic materials.
• There are several advantage and disadvantages to using an NTC thermistor thermometer.
• Advantages include their small size and high degree of stability. NTCs are also long lasting and very accurate.
• Disadvantages include their non-linearity, and unsuitability for use in extreme temperatures
Contact
Some electronic thermometers may work by contact (the electronic sensor is placed in the location where temperature is to be measured, and left long enough to reach equilibrium). These typically reach equilibrium faster than mercury thermometers; the thermometer may beep when equilibrium has been reached, or the time may be specified in the manufacturer's documentation.
Remote
Other electronic thermometers work by remote sensing: an infrared sensor responds to the radiation spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
emitted from the location. Although these are not in direct contact with the area being measured, they may still contact part of the body (a thermometer which senses the temperature of the eardrum without touching it is inserted into the ear canal). To eliminate the risk of patient cross-infection, disposable probe covers and single-use clinical thermometers of all types are used in clinics and hospitals.
Accuracy
According to a 2001 research, electronic thermometers on the market significantly underestimate higher temperatures and overestimate lower temperatures. The researchers conclude that "the current generation of electronic, digital clinical thermometers, in general, may not be sufficiently accurate or reliable to replace the traditional glass/mercury thermometers""An investigation into the accuracy of different types of thermometers"
''Nursing Times.net'', 1 October 2002.
Basal thermometer
A basal thermometer is 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 ...
used to take the basal (base) body temperature, the temperature upon waking. Basal body temperature
Basal body temperature (BBT or BTP) is the lowest body temperature attained during rest (usually during sleep). It is usually estimated by a temperature measurement immediately after awakening and before any physical activity has been undertaken. ...
is much less affected than daytime temperature by environmental factors such as exercise and food intake. This allows small changes in body temperature to be detected
Glass oral thermometers typically have markings every 0.1 °C or 0.2 °F. Basal temperature is stable enough to require accuracy of at least 0.05 °C or 0.1 °F, so special glass basal thermometers are distinct from glass oral thermometers. Digital thermometers which have sufficient resolution (0.05 °C or 0.1 °F is sufficient) may be suitable for monitoring basal body temperatures; the specification should be checked to ensure absolute accuracy, and thermometers (like most digital instruments) should be calibrated at specified intervals. If only the variation of basal temperature is required, absolute accuracy is not so important so long as the readings do not have large variability (e.g., if real temperature varies from 37.00 °C to 37.28 °C, a thermometer which inaccurately but consistently reads a change from 37.17 °C to 37.45 °C will indicate the magnitude of the change). Some digital thermometers are marketed as "basal thermometers" and have extra features such as a larger display, expanded memory functions, or beeping to confirm the thermometer is placed properly.
Smart and wearable thermometers
A smart thermometer is able to transmit its readings so that they can be collected, stored and analysed. Wearable thermometers can provide continuous measurement but it is difficult to measure core body temperature in this way.
See also
*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 ...
*Liquid crystal thermometer
A liquid crystal thermometer, temperature strip or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures.
Liquid cry ...
Footnotes
References
Allbutt, T.C., "Medical Thermometry", ''British and Foreign Medico-Chirurgical Review''
Vol.45, No.90, (April 1870), pp.429-441
Vo.46, No.91, (July 1870), pp.144-156.
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