thermometer

Definition

A thermometer is a device used to measure temperature.

Purpose

A thermometer is used in health care to measure and monitor body temperature. In an office, hospital or other health care facility, it allows a caregiver to record a baseline temperature when a patient is admitted. Repeated measurements of temperature are useful to detect deviations from normal levels. Repeated measurements are also useful in monitoring the effectiveness of current medications or other treatments.

The patient's temperature is recorded to check for pyrexia or monitor the degree of hypothermia present in the body.

Demographics

All health care professionals use thermometers. All health care facilities have thermometers. Most homes also have thermometers.

Description

A thermometer can use any of several methods to register temperature. These include mercury; liquid-in-glass; electronic with digital display; infrared or tympanic; and disposable dot matrix. A thermometer can be used in a clinical or emergency setting or at home. Thermometers can record body temperatures in the mouth (oral), armpit (axillary), eardrum (tympanic membrane), or anus (rectal).

A mercury thermometer consists of a narrow glass stem approximately 5 in (12.7 cm) in length with markings along one or both sides indicating the temperature scale in degrees Fahrenheit, Centigrade or both. Liquid mercury is held in a reservoir bulb at one end and rises through a capillary tube when the glass chamber is placed in contact with the body. Mercury thermometers are not used in modern clinical settings.

Electronic thermometers can record a wide range of temperatures between 94°F and 105°F, (35°C and 42°C) and can record oral, axillary, or rectal temperatures. They have temperature sensors inside round-tipped probes that can be covered with disposable guards to prevent the spread of infection. The sensor is connected to a container housing the central processing unit. The information gathered by the sensor is then shown on a display screen. Some electronic models have such other features as memory recall of the last recording or a large display screen for easy reading. To use an electronic thermometer, the caregiver places the probe under the patient's arm or tongue, or in the patient's rectum. The probe is left in place for a period of time that depends on the model used. The device will beep when the peak temperature is reached. The time required to obtain a reading varies from 3–30 seconds.

A tympanic thermometer has a round-tipped probe containing a sensor that can be covered with a disposable guard to protect against the spread of ear infections. It is placed in the ear canal for 1 sec while an infrared sensor records the body heat radiated by the eardrum. The reading then appears on the unit's screen.

Digital and tympanic thermometers should be used in accordance with the manufacturer's guidelines.

Disposable thermometers are plastic strips with dots on the surface that have been impregnated with temperature-sensitive chemicals. The strips are sticky on one side to adhere to the skin under the armpit and prevent slippage. The dots change color at different temperatures as the chemicals in them respond to body heat. The temperature is readable after two to three minutes, depending on the instrument's guidelines. These products vary in length of use; they may be disposable, reusable, or used continuously for up to 48 hours. Disposable thermometers are useful for children, as they can record temperatures while children are asleep.

Diagnosis/Preparation

Training

Caregivers should be given training appropriate for the type of device used in their specific clinical setting.

Operation

The patient should sit or lie in a comfortable position to ensure that temperature readings are taken in similar locations each time and to minimize the effects of stress or excitement on the reading.

The manufacturer's guidelines should be followed when taking a patient's temperature with a digital, tympanic, or disposable thermometer. Dot-matrix thermometers are placed next to the skin and usually held in place by an adhesive strip. With the tympanic thermometer, caregivers should ensure that the probe is properly inserted into the ear to allow an optimal reading. The reading will be less accurate if the sensor cannot accurately touch the tympanic membrane or if the ear canal is clogged by wax or debris.

A mercury thermometer can be used to monitor a temperature in three body locations:

• Axillary.
• Oral or sublingual. This placement is never used with infants.
• Rectal. This method is used with infants. The tip of a rectal mercury thermometer is usually colored blue to distinguish it from the silver tip of an oral/axillary thermometer.

Before recording a temperature using a mercury thermometer, the caregiver shakes the mercury down by holding the thermometer firmly at the clear end and flicking it quickly a few times with a downward wrist motion toward the silver end. The mercury should be shaken down below 96°F (35.5°C) before the patient's temperature is taken.

In axillary placement, the silver tip of the thermometer is placed under the patient's right armpit, with the patient's arm pressing the instrument against the chest. The thermometer should stay in place for six to seven minutes. The caregiver can record the patient's other vital signs during this waiting period. After the waiting period has elapsed, the caregiver removes the thermometer and holds it at eye level to read it. The mercury will have risen to a level indicating the patient's temperature.

The procedure for taking a patient's temperature by mouth with a mercury thermometer is similar to the axillary method except that the silver tip of the thermometer is placed beneath the tongue for four to five minutes before being read. In both cases, the thermometer is wiped clean and stored in an appropriate container to prevent breakage.

To record the patient's rectal temperature with a mercury thermometer, a rectal thermometer is shaken down as described earlier. A small amount of water-based lubricant is placed on the colored tip of the thermometer to make it easier to insert. Infants must be positioned lying on their stomachs and held securely by the caregiver. The tip of the thermometer is inserted into the rectum no more than 0.5 in (1.3 cm) and held there for two to three minutes. The thermometer is removed, read as before, and cleansed with an antibacterial wipe. It is then stored in an appropriate container to prevent breakage. This precaution is important as mercury is poisonous when swallowed.

Liquid-in-glass thermometers contain alternatives to mercury (such as colored alcohol), but are used and stored in the same manner as mercury thermometers.

Maintenance

Many digital and infrared thermometers are self-calibrating and need relatively little care. To ensure accuracy, mercury thermometers should be shaken down prior to every use and left in place for at least three minutes. They require careful storage to prevent breakage and thorough cleaning after each use to prevent cross-infection.

As of early 2003, there is a nationwide initiative to ban the sale of thermometers and blood pressure monitors containing mercury. Health activists are concerned about mercury from broken or unwanted instruments contaminating the environment. A mercury thermometer contains 0.7g (0.025 oz) of mercury; 1 g of the substance is enough to contaminate a 20-acre lake. Several states have banned the use of products containing mercury. Most retail stores have stopped selling mercury thermometers. In October 1999, the Environmental Protection Agency (EPA) advised using alternative products to avoid the need for increased regulations in years to come and to protect human health and wildlife by reducing unnecessary exposure to mercury. According to a 2001 study by the Mayo Clinic, mercury-free devices can monitor information without compromising accuracy.

Aftercare

A thermometer should be cleaned, disinfected and placed in an appropriate container for storage.

Risks

Breakage of a glass thermometer creates a risk of cuts from broken glass and possible mercury poisoning. Improper operation of a tympanic thermometer can cause injury to the middle ear. As digital devices have replaced glass thermometers, however, the number of injuries has declined.

An additional risk is that old or broken thermometers may give inaccurate results.

Normal Results

A normal body temperature is defined as approximately 98.6°F (37°C). Body temperature is not constant throughout a 24-hour period. Some variation (0.3°F) is normal. Individuals also vary in their basal temperatures (0.3°F). A fever is defined as a temperature of 101°F or higher in an infant younger than three months or above 102°F for older children and adults. Hypothermia is recognized as a temperature below 96°F (35.5°C).

Morbidity and Mortality Rates

Injuries caused by properly inserted and normally functioning thermometers are extremely rare.

Alternatives

There are no convenient alternatives to using a thermometer to measure body temperature.

See also Health history; Physical examination; Vital signs.

Resources

Books

Bickley, L. S., P. G. Szilagyi, and J. G. Stackhouse, eds. Bates'Guide to Physical Examination & History Taking, 8th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2002.

Chan, P. D., and P. J. Winkle. History and Physical Examination in Medicine, 10th ed. New York, NY: Current Clinical Strategies, 2002.

Seidel, Henry M. Mosby's Physical Examination Handbook, 4th ed. St. Louis, MO: Mosby-Year Book, 2003.

Swartz, Mark A., and William Schmitt. Textbook of PhysicalDiagnosis: History and Examination, 4th ed. Philadelphia, PA: Saunders, 2001.

Periodicals

Dowding, D., S. Freeman, S. Nimmo, et al. "An Investigation Into the Accuracy of Different Types of Thermometers." Professional Nurse 18 (November 2002): 166-168.

Drake-Lee, A., I. Mantella, and A. Bridle. "Infrared Ear Thermometers Versus Rectal Thermometers." Lancet 360 (December 7, 2002): 1883-1886.

Moran, D. S., and L. Mendal. "Core Temperature Measurement: Methods and Current Insights." Sports Medicine 32 (2002): 879-885.

Pompei, F. "Insufficiency in Thermometer Data." Anesthesia and Analgesia 96 (March 2003): 908-909.

Organizations

American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 66211-2672. (913) 906-6000. www.aafp.org. E-mail: fp@aafp.org.

American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 60007-1098. (847) 434-4000; FAX: (847) 434-8000. www.aap.org. E-mail: kidsdoc @aap.org .

American College of Physicians. 190 N. Independence Mall West, Philadelphia, PA 19106-1572. (800) 523-1546, x2600 or (215) 351-2600. www.acponline.org.

American Medical Association. 515 N. State Street, Chicago, IL 60610. (312) 464-5000. www.ama-assn.org.

American Nurses Association. 600 Maryland Avenue, SW, Suite 100 West, Washington, DC 20024. (202) 651-7000 or (800) 274-4262. www.nursingworld.org.

Other

About.com. [cited March 1, 2003]. www.inventors.about.com/library/inventors/blthermometer.htm.

AskLynnRN. [cited March 1, 2003]. www.asklynnrn.com/html/healthmon_bbt_thermometer.htm.

How Stuff Works. [cited March 1, 2003]. www.howstuffworks.com/therm.htm.

Rice University. [cited March 1, 2003]. www.es.rice.edu/ES/humsoc/Galileo/Things/thermometer.html.

— L. Fleming Fallon, Jr., MD, DrPH

Background

A thermometer is a device used to measure temperature. The thermoscope, developed by Galileo around 1592, was the first instrument used to measure temperature qualitatively. It was not until 1611 that Sanctorius Sanctorius, a colleague of Galileo, devised and added a scale to the thermoscope, thus facilitating quantitative measurement of temperature change. By this time the instrument was called the thermometer, from the Greek words therme ("heat") and metron ("measure"). About 1644 it became obvious, however, that this instrument—comprising a large bulb flask with a long, open neck, using wine to indicate the reading—was extremely sensitive to barometric pressure. To alleviate the problem, Grand Duke Ferdinand II of Tuscany developed a process to hernetically seal the thermometer, thereby eliminating outside barometric influence. The basic form has varied little since.

There are many types of thermometers in use today: the recording thermometer uses a pen on a rotating drum to continuously record temperature readings; the digital readout thermometers often coupled with other weather measuring devices; and the typical household types hung on a wall, post, or those used for medical purposes.

With a thermometer, temperature can be measured using any of three primary units: Fahrenheit, Celsius, or Kelvin. At one point during the eighteenth century, nearly 35 scales of measure had been developed and were in use.

In 1714 Gabriel Daniel Fahrenheit, a Dutch instrument maker known for his fine craftsmanship, developed a thermometer using 32 (the melting point of ice) and 96 (the standard temperature of the human body) as his fixed points. It has since been determined that 32 and 212 (the boiling point of water) are the scale's fixed points, with 98.6 being accepted as the healthy, normal body temperature.

Swedish scientist Anders Celsius, in 1742, assigned 0 degrees as the point at which water boiled and 100 degrees as the point at which ice melted. These two figures were eventually switched—creating the scale we know today—with 0 degrees as the freezing point of water and 100 degrees as the boiling point. Use of this scale quickly spread through Sweden and to France, and for two centuries it was known as the centigrade scale. The name was changed in 1948 to Celsius to honor its inventor.

In 1848 another scientist, Lord Kelvin (William Thomson), proposed another scale based on the same principles as the Celsius thermometer, with the fixed point of absolute zero set at the equivalent of -273.15 degrees Celsius (the units used on this scale are called Kelvin [K]). The freezing and boiling points of water are registered at 273 K and 373 K respectively. The Kelvin scale is most often used in scientific research studies.

Design

The operating principle of a thermometer is quite simple. A known measure of liquid (mercury, alcohol, or a hydrocarbon-based fluid) is vacuum-sealed in a glass tube. The liquid expands or contracts when air is heated or cooled. As the liquid level changes, a corresponding temperature scale can be read to indicate the current temperature.

Thermometers are designed according to predefined standards identified by the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) and standard manufacturing practices. Within the regulatory guidelines there are provisions for the custom manufacture of thermometers. Custom thermometers can be as varied as those who use them. Different sizes exist for the amount, weight, and length of glass used, the type of liquid filled into the glass, the frequency of gradations laid onto the glass tube or enclosure, and even the color of the gradation scale marks.

A design engineer will look at the travel limits for the liquid to be used in the thermometer. Once precise limits are established, the dimensions of the glass tube and size of the glass bulb can be determined.

Use of electronic components in thermometers has grown. Many of today's broadly used thermometers contain digital readouts and sample program cycles to feed back the current temperature to a light-emitting diode (LED) or liquid crystal display (LCD) panel. For all the electronic wizardry available, a thermometer must still contain a heat-cold sensitizing element in order to respond to environmental changes.

Raw Materials

Thermometers consist of three basic elements: spirit-filled liquid, which responds to changes in heat and cold; a glass tube to house the temperature-measuring liquid; and black ink to color in the engraved scale marks with legible numbers. In addition, other elements are necessary for the manufacture of thermometers, including a wax solution used to engrave the scale marks on the glass tube; an engraving engine that makes permanent gradations on the glass tube; and a hydrofluoric acid solution into which the glass tube is dipped to seal the engraving marks.

The glass material forming the body of the thermometer is usually received from an outside manufacturer. Some thermometer products are made with an enclosure, which can be made of plastic or composites and may contain scale gradations as opposed to having these on the glass tube itself. The enclosure also serves to protect and mount the thermometer on a wall, post, or in a weather shelter box.

The Manufacturing
Process

Although there are numerous types of thermometers, the production process for the most common of these—the classic household variety—is described below.

The glass bulb

• First, the raw glass material is received from an outside manufacturer. The tube is made with a fine passage, or bore, throughout its length. The bored tubes are checked for quality; any rejected parts are sent back to the manufacturer for replacement.
• The bulb reservoir is formed by heating one end of the glass tube, pinching it closed, and using glassblowing and the application of an air-driven torch to complete it. Alternately, the bulb can be made by blowing a separate piece of lab material that is then joined with one end of the glass tube. The bulb is sealed at its bottom, leaving an open tube at the top.

Adding the fluid

• With the open end down in a vacuum chamber, air is then evacuated from the glass tube, and the hydrocarbon fluid is introduced into the vacuum until it penetrates the tube about 1 inch (2.54 centimeters). Due to environmental concerns, contemporary thermometers are manufactured less with mercury and more with a spirit-filled hydrocarbon liquid. Such a practice is mandated (with tolerance for a limited use of mercury) by the Environmental Protection Agency (EPA).

  The vacuum is then gradually reduced, forcing the fluid down near the top of the tube. The process is the same when mercury is used, except heat is also applied in the vacuum chamber.

• Once full, the tube is placed upon its bulb end. A heating-out process is then conducted by placing the thermometer into a warm bath and raising the temperature to 400 degrees Fahrenheit (204 degrees Celsius). Next, the temperature is reduced to room temperature to bring the residual liquid back to a known level. The open end of the thermometer is then sealed by placing it over a flame.

Applying the scale

• After the tube is sealed, a scale is applied based on the level at which the fluid rests when inserted into a water bath of 212 degrees Fahrenheit (100 degrees Celsius) versus one at 32 degrees Fahrenheit (0 degrees Celsius). These reference points for the desired scale are marked on the glass tube before engraving or silkscreening is done to fill gradations.
• The range lengths vary according to the design used. A scale is picked that best corresponds to even marks between the reference points. For accuracy purposes, engraving is the preferred method of marking. The marks are made by an engraving engine after the thermometer is placed in wax. The numbers are scratched onto the glass and, once complete, the thermometer is dipped in hydrofluoric acid to seal the engraved markings. Ink is then rubbed into the marks to highlight the scale values. When enclosures are used on the scales, a silkscreening process is used to apply the marks.
• Finally, the thermometers are packaged accordingly and shipped to customers.

Quality Control

The manufacturing process is controlled by widely adopted industry standards and specific in-house measures. Manufacturing design considerations include quality control checks throughout the production process. The equipment used to perform fabrication tasks must also be carefully maintained, especially with updated design protocol.

Waste materials accrued during manufacturing are disposed of according to environmental regulatory standards. During the manufacturing cycle, equipment used to heat, evacuate, and engrave the thermometer must be checked and calibrated regularly. Tolerance tests are also performed, using a known standard, to detennine the accuracy of the temperature readings. All thermometers have a tolerance for accuracy. For the common household, this tolerance is usually plus or minus 2 degrees Fahrenheit (16 degrees Celsius). For laboratory work, plus or minus 1 degree is generally acceptable.

The Future

Although the longstanding simple glass thermometer is unlikely to change, other thermometer forms continue to evolve. With technological advances and the more widespread use of lighter and stronger materials, manufacturers of electronically integrated temperature instruments can provide more accurate measurements of temperature with minimal equipment bulk and at an affordable price. Analog box thermometers, for example, were once used with a long wire and probe tip for in-ground temperature measurements, among other uses. Today, the probe tips are made of lighter materials, and the boxes, loaded with digital electronics, are not as bulky and square. Looking ahead, further work with the microchip may provide the impetus to fully digitize the temperature measuring process. Also, it may eventually be possible to direct an infrared beam into soil and extract a temperature reading from a target depth without even touching the soil.

Where To Learn More

Books

Gardner, Robert. Temperature and Heat. Simon & Schuster, 1993.

McGee, Thomas D. Principles and Methods of Temperature Measurement. John Wiley & Sons, 1988.

Pavese, F., ed. Modern Gas-Based Temperature and Pressure Measurements. Plenum Publishing, 1992.

Periodicals

Alderman, Lesley. "Stick It In An Ear," Money. January, 1993, p. 19.

"Fever Thermometers," Consumer Reports. December, 1988, p. 214.

DiChristina, Mariette. "Thermometer You Swallow," Popular Science. March, 1990, p. 113.

"Taking the Heat from Inside," Discover. June, 1988, p. 12.

Joyce, Mary E. "Thermometer Assists in Cancer Therapy," Design News. September 21, 1992, p. 46.

[Article by: Matthew Fogel]

An instrument that measures temperature. Although this broad definition includes all temperature-measuring devices, they are not all called thermometers. Other names have been generally adopted. For a discussion of two such devices .See also Pyrometer; Thermocouple.

For a general discussion of temperature measurement See also Temperature measurement.

Liquid-in-glass thermometer

This thermometer consists of a liquid-filled glass bulb and a connecting partially filled capillary tube. When the temperature of the thermometer increases, the differential expansion between the glass and the liquid causes the liquid to rise in the capillary. A variety of liquids, such as mercury, alcohol, toluene, and pentane, and a number of different glasses are used in thermometer construction, so that various designs cover diverse ranges between about −300°F and +1200°F (−184°C and +649°C).

Bimetallic thermometer

In this thermometer the differential expansion of thin dissimilar metals, bonded together into a narrow strip and coiled into the shape of a helix or spiral, is used to actuate a pointer. In some designs the pointer is replaced with low-voltage contacts to control, through relays, operations which depend upon temperature, such as furnace controls.

Filled-system thermometer

This type of thermometer has a bourdon tube connected by a capillary tube to a hollow bulb. When the system is designed for and filled with a gas (usually nitrogen or helium) the pressure in the system substantially follows the gas law, and a temperature indication is obtained from the bourdon tube. The temperature-pressure-motion relationship is nearly linear. Atmospheric pressure effects are minimized by filling the system to a high pressure. When the system is designed for and filled with a liquid, the volume change of the liquid actuates the bourdon tube.

Vapor-pressure thermal system

This filled-system thermometer utilizes the vapor pressure of certain stable liquids to measure temperature. The useful portion of any liquid-vapor pressure curve is between approximately 15 psia (100 kilopascals absolute) and the critical pressure, that is, the vapor pressure at the critical temperature, which is the highest temperature for a particular liquid-vapor system. A nonlinear relationship exists between the temperature and the vapor pressure, so the motion of the bourdon tube is greater at the upper end of the vapor-pressure curve. Therefore, these thermal systems are normally used near the upper end of their range, and an accuracy of 1% or better can be expected.

Resistance thermometer

In this type of thermometer the change in resistance of conductors or semiconductors with temperature change is used to measure temperature. Usually, the temperature-sensitive resistance element is incorporated in a bridge network which has a reasonably constant power supply. Although a deflection circuit is occasionally used, almost all instruments of this class use a null-balance system, in which the resistance change is balanced and measured by adjusting at least one other resistance in the bridge. Metals commonly used as the sensitive element in resistance thermometers are platinum, nickel, and copper.

Thermistor

This device is made of a solid semiconductor with a high temperature coefficient of resistance. The thermistor has a high resistance, in comparison with metallic resistors, and is used as one element in a resistance bridge. Since thermistors are more sensitive to temperature changes than metallic resistors, accurate readings of small changes are possible. See also Thermistor.

Instrument used for taking temperature readings. Varying designs of the thermometer allow the temperature to be taken in the mouth, rectum, or externally at the axillary or groin areas.

Thermometers. (Zakus, 2001)

A device for measuring temperature.

An instrument for determining temperatures, in principle making use of a substance (such as alcohol or mercury) with a physical property that varies with temperature and is susceptible of measurement on some defined scale.

• Celsius t. — one employing the Celsius scale, that is, with the ice point at 0 (0°C) and the normal boiling point of water at 100 degrees (100°C).
• centigrade t. — one having the interval between two established reference points divided into 100 equal units, as the Celsius thermometer.
• clinical t. — one used to determine the temperature of the patient in clinical situations.
• electronic t. — a clinical thermometer using a sensor based on thermistors, solid-state electronic devices whose electrical characteristics change with temperature. The reading is recorded within seconds, some having a red light or other device to indicate when maximum temperature is reached. Available models include handheld, desk-top and wall-mounted units, all having probes that are inserted orally or rectally. It is expected that electronic thermometers worn by the patient will have some use.
• Fahrenheit t. — one employing the Fahrenheit scale, that is, with the ice point at 32 and the normal boiling point of water at 212 degrees (212°F).
• Kelvin t. — one employing the kelvin scale.
• recording t. — a temperature-sensitive instrument by which the temperature to which it is exposed is continuously recorded.
• rectal t. — a clinical thermometer that is inserted in the rectum for determining body temperature.
• resistance t. — one that uses the electric resistance of metals for determining temperature (thermocouple).
• self-registering t. — recording thermometer.

A clinical mercury thermometer

A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer). Thermometers increasingly use electronic means to provide a digital display or input to a computer.

Thermometers can be divided into two separate groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities. For primary thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity of sound in a gas, on the thermal noise (see Johnson–Nyquist noise) voltage or current of an electrical resistor, and on the angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field. Primary thermometers are relatively complex.

Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple points and superconducting transitions, occur reproducibly at the same temperature.

There is an absolute thermodynamic temperature scale. Internationally agreed temperature scales are designed to approximate this closely, based on fixed points and interpolating thermometers. The most recent official temperature scale is the International Temperature Scale of 1990. It extends from 0.65 K (−272.5 °C; −458.5 °F) to approximately 1,358 K (1,085 °C; 1,985 °F).

Contents 1 Early history 2 Types of thermometers 3 Calibration 4 Accuracy 5 Special uses of thermometers 6 See also 7 Notes 8 References 9 Further reading 10 External links

Early history

Galileo thermometer

Various thermometers from the 19th century.

Various authors have credited the invention of the thermometer to Avicenna, Cornelius Drebbel, Robert Fludd, Galileo Galilei or Santorio Santorio. The thermometer was not a single invention, however, but a development.

Philo and Hero of Alexandria knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water.^[1] The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

Such a mechanism was later used to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the air. These devices were developed by Avicenna in the early 11th century,^[2][3] and by several European scientists in the 16th and 17th centuries, notably Galileo Galilei.^[4]. As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it reflected the changes in sensible heat (the concept of temperature was yet to arise).^[4] The difference between a thermoscope and a thermometer is that the latter has a scale.^[5] Though Avicenna or Galileo are often said to be the inventor of the thermometer, what they produced were thermoscopes.

Galileo also discovered that objects (glass spheres filled with aqueous alcohol) of slightly different densities would rise and fall, which is nowadays the principle of the Galileo thermometer (shown). Today such thermometers are calibrated to a temperature scale.

The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani: the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, with a bulb at the top and the end immersed in water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now call an air thermometer.^[6]

The first person to put a scale on a thermoscope is variously said to be Francesco Sagredo^[7] or Santorio Santorio^[8] in about 1611 to 1613.

The word thermometer (in its French form) first appeared in 1624 in La Récréation Mathématique by J. Leurechon, who describes one with a scale of 8 degrees.^[9]

The above instruments suffered from the disadvantage that they were also barometers, i.e. sensitive to air pressure. In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany, made sealed tubes part filled with alcohol, with a bulb and stem, the first modern-style thermometer, depending on the expansion of a liquid, and independent of air pressure.^[9] Many other scientists experimented with various liquids and designs of thermometer.

However, each inventor and each thermometer was unique—there was no standard scale. In 1665 Christiaan Huygens suggested using the melting and boiling points of water as standards, and in 1694 Carlo Renaldini proposed using them as fixed points on a universal scale. In 1701 Isaac Newton proposed a scale of 12 degrees between the melting point of ice and body temperature. Finally in 1724 Daniel Gabriel Fahrenheit produced a temperature scale which now (slightly adjusted) bears his name. He could do this because he manufactured thermometers, using mercury (which has a high coefficient of expansion) for the first time and the quality of his production could provide a finer scale and greater reproducibility, leading to its general adoption. In 1742 Anders Celsius proposed a scale with zero at the boiling point and 100 degrees at the melting point of water,^[10] though the scale which now bears his name has them the other way around.^[11]

In 1866 Sir Thomas Clifford Allbutt invented a clinical thermometer that produced a body temperature reading in five minutes as opposed to twenty.^[12]

Types of thermometers

Mercury-in-glass thermometer

Cooking thermometers used to measure the temperature of steamed milk

Thermometers have been built which utilise a range of physical effects to measure temperature. Most thermometers are originally calibrated to a constant-volume gas thermometer.^{[citation needed]} Temperature sensors are used in a wide variety of scientific and engineering applications, especially measurement systems. Temperature systems are primarily either electrical or mechanical, occasionally inseparable from the system which they control (as in the case of a mercury thermometer).

• Alcohol thermometer
• Beckmann differential thermometer
• Bi-metal mechanical thermometer
• Coulomb blockade thermometer
• Galileo thermometer
• Infrared thermometer
• Liquid crystal thermometer
• Medical thermometer (e.g. oral thermometer, rectal thermometer, basal thermometer)
• Mercury-in-glass thermometer
• Pill thermometer
• Recording thermometer
• Resistance thermometer
• Reversing thermometer
• Silicon bandgap temperature sensor
• Six's thermometer- also known as a maximum minimum thermometer
• Thermistor
• Thermocouple
• Phosphor thermometry, phosphorescence can be used to detect the temperature of an object eg a gas turbine component ^[13],^[14],^[15],^[16]

Calibration

Thermometers can be calibrated either by comparing them with other certified thermometers or by checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. (Note that the boiling point of water varies with pressure, so this must be controlled.)

The traditional method of putting a scale on a liquid-in glass or liquid-in-metal thermometer was in three stages:

1. Immerse the sensing portion in a stirred mixture of pure ice and water and mark the point indicated when it had come to thermal equilibrium.
2. Immerse the sensing portion in a steam bath at 1 standard atmosphere (101.3 kPa; 760.0 mmHg) and again mark the point indicated.
3. Divide the distance between these marks into equal portions according to the temperature scale being used.

Other fixed points were used in the past are the body temperature (of a healthy adult male) which was originally used by Fahrenheit as his upper fixed point (96 °F (36 °C) to be a number divisible by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the definition of 0 °F (−18 °C).^[17] (This is an example of a Frigorific mixture). As body temperature varies, the Fahrenheit scale was later changed to use an upper fixed point of boiling water at 212 °F (100 °C).^[18]

These have now been replaced by the defining points in the International Temperature Scale of 1990, though in practice the melting point of water is more commonly used than its triple point, the latter being more difficult to manage and thus restricted to critical standard measurement. Nowadays manufacturers will often use a thermostat bath or solid block where the temperature is held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put into the same bath or block and allowed to come to equilibrium, then the scale marked, or any deviation from the instrument scale recorded.^[19] For many modern devices calibration will be stating some value to be used in processing an electronic signal to convert it to a temperature.

Accuracy

Some digital thermometers give readouts to a tenth of a degree. However if the stated accuracy of the thermometer is +/- 1 or 2 or 3 degrees, it is very unlikely that a readout to a tenth of a degree will be accurate. Thus the stated precision is false.[1]

Special uses of thermometers

The "Boyce MotoMeter" radiator cap on a 1913 Car-Nation automobile. It is a thermometer for measuring temperature of vapor in 1910s and 1920s cars.

• Candy thermometer
• Meat thermometer

See also

• Automated airport weather station
• Timeline of temperature and pressure measurement technology
• Temperature conversion
• Thermogenerator

Notes

1. ^ T. D. McGee (1988) Principles and Methods of Temperature Measurement ISBN 0471627674
2. ^ Robert Briffault (1938). The Making of Humanity, p. 191
3. ^ Fatima Agha Al-Hayani (2005). "Islam and Science: Contradiction or Concordance", Zygon 40 (3), p. 565-576.
4. ^ ^{a b} R. S Doak (2005) Galileo: astronomer and physicist ISBN 0756508134 p36
5. ^ T. D. McGee (1988) Principles and Methods of Temperature Measurement page 3, ISBN 0471627674
6. ^ T. D. McGee (1988) Principles and Methods of Temperature Measurement, pages 2-4 ISBN 0471627674
7. ^ J. E. Drinkwater (1832)Life of Galileo Galilei page 41
8. ^ The Galileo Project: Santorio Santorio
9. ^ ^{a b} R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 4
10. ^ R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 6
11. ^ Linnaeus' thermometer
12. ^ Sir Thomas Clifford Allbutt, Encyclopædia Britannica
13. ^ X. Chen, Z. Mutasim, J. Price, J. P. Feist, A. L. Heyes and S. Seefeldt (2005), 'Industrial sensor TBCs: Studies on temperature detection and durability', International Journal of Applied Ceramic Technology, Vol. 2, No. 5, pp. 414-421.
14. ^ A. L. Heyes, S. Seefeldt, J. P Feist (2005), ‘Two-colour thermometry for surface temperature measurement’, Optics and Laser Technology, 38, pp.257-265.
15. ^ R.J.L.Steenbakker,J.P.Feist,R.G.Wellmann,J.R.Nicholls, (2008),SENSOR TBCs: REMOTE IN-SITU CONDITION MONITORING OF EB-PVD COATINGS AT ELEVATED TEMPERATURES, GT2008-51192,Proceedings of ASME Turbo Expo 2008: Power for Land, Sea and Air,June 9-13, 2008, Berlin, Germany.
16. ^ J. P. Feist, A. L. Heyes and J. R. Nicholls (2001), 'Phosphor thermometry in an electron beam physical vapour deposition produced thermal barrier coating doped with dysprosium', Proceedings of Institution of Mechanical Engineers, Vol. 215 Part G, pp. 333-340.
17. ^ R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8, page 5
18. ^ J. Lord (1994) Sizes ISBN 0 06 273228 5 page 293
19. ^ R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8, chapter 11 "Calibration of Temperature Sensors"

References

• History Channel - Invention - Notable Modern Inventions and Discoveries
• About - Thermometer - Thermometers - Early History, Anders Celsius, Gabriel Fahrenheit and Thomson Kelvin.
• Thermometers and Thermometric Liquids - Mercury and Alcohol.
• The NIST Industrial Thermometer Calibration Laboratory

Further reading

• Middleton, W. E. K. (1966). A history of the thermometer and its use in meteorology. Baltimore: Johns Hopkins Press. Reprinted ed. 2002, ISBN 0801871530.
• The NIST Industrial Thermometer Calibration Laboratory

External links

Look up thermometer in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Measuring instruments (temperature)

• History of Temperature and Thermometry
• The Chemical Educator, Vol. 5, No. 2 (2000) The Thermometer—From The Feeling To The Instrument

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Dansk (Danish)
n. - termometer

Nederlands (Dutch)
warmtemeter, thermometer

Français (French)
n. - thermomètre

Deutsch (German)
n. - Thermometer, Wärmemesser

Ελληνική (Greek)
n. - (ιατρ., φυσ.) θερμόμετρο

Italiano (Italian)
termometro

Português (Portuguese)
n. - termômetro (m)

Русский (Russian)
термометр, градусник

Español (Spanish)
n. - termómetro

Svenska (Swedish)
n. - termometer

中文（简体）(Chinese (Simplified))
温度计, 体温计

中文（繁體）(Chinese (Traditional))
n. - 溫度計, 體溫計

한국어 (Korean)
n. - 온도계

日本語 (Japanese)
n. - 温度計, 体温計, 検温器

العربيه (Arabic)
‏(الاسم) ميزان الحرارة, الترمومتر‏

עברית (Hebrew)
n. - ‮מדחום‬

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