Temperature measurement using modern scientific
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
s and
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
scales goes back at least as far as the early 18th century, when
Gabriel FahrenheitDaniel Gabriel Fahrenheit was a physicist and engineer who determined a temperature scale now named after him.- Biography :...
adapted a thermometer (switching to
mercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
) and a scale both developed by
Ole Christensen RøemerOle Christensen Rømer was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light...
. Fahrenheit's scale is still in use, alongside the
CelsiusCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
scale and the
KelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
scale.
The world's average surface air temperature is about 15 °C. For information on temperature changes relevant to
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
or
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's
geologicGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
past see:
Temperature recordThe temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. The most detailed information exists since 1850, when methodical thermometer-based records began. There are numerous estimates of temperatures since the end of the...
.
Many methods have been developed for measuring temperature.
Temperature measurement using modern scientific
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
s and
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
scales goes back at least as far as the early 18th century, when
Gabriel FahrenheitDaniel Gabriel Fahrenheit was a physicist and engineer who determined a temperature scale now named after him.- Biography :...
adapted a thermometer (switching to
mercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
) and a scale both developed by
Ole Christensen RøemerOle Christensen Rømer was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light...
. Fahrenheit's scale is still in use, alongside the
CelsiusCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
scale and the
KelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
scale.
The world's average surface air temperature is about 15 °C. For information on temperature changes relevant to
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
or
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's
geologicGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
past see:
Temperature recordThe temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. The most detailed information exists since 1850, when methodical thermometer-based records began. There are numerous estimates of temperatures since the end of the...
.
Technologies
Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the
glass thermometerA mercury-in-glass thermometer, invented by German physicist Daniel Gabriel Fahrenheit, is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the heat given to it...
. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the
gas thermometerA gas thermometer measures temperature by the variation in volume or pressure of a gas. One common apparatus is a constant volume thermometer. It consists of a bulb connected by a capillary tube to a manometer. The bulb is filled with a gas such that the volume of the gas in the bulb remains...
.
Other important devices for measuring temperature include:
- Thermocouple
A thermocouple or thermocouple thermometer is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat into electric power...
s
- Thermistor
A thermistor is a type of resistor whose resistance varies with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.Thermistors differ...
s
- Resistance Temperature Detector (RTD)
- Pyrometers
- Langmuir probe
A Langmuir probe is a device named after Nobel Prize winning physicist Irving Langmuir, used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential...
s (for electron temperature of a plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
)
- Infrared
Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
- Other thermometer
A thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
s
One must be careful when measuring temperature to ensure that the measuring instrument (thermometer, thermocouple, etc) is really the same temperature as the material that is being measured. Under some conditions heat from the measuring instrument can cause a temperature gradient, so the measured temperature is different from the actual temperature of the system. In such a case the measured temperature will vary not only with the temperature of the system, but also with the heat transfer properties of the system. An extreme case of this effect gives rise to the wind chill factor, where the weather feels colder under windy conditions than calm conditions even though the temperature is the same. What is happening is that the wind increases the rate of heat transfer from the body, resulting in a larger reduction in body temperature for the same ambient temperature.
The theoretical basis for thermometers is the
zeroth law of thermodynamicsThe zeroth law of thermodynamics is a generalization about the thermal equilibrium among bodies, or thermodynamic systems, in contact. It results from the definition and properties of temperature.It can be stated as:...
which postulates that if you have three bodies, A, B and C, if A and B are at the same temperature, and B and C are at the same temperature then A and C are at the same temperature. B, of course, is the thermometer.
The practical basis of thermometry is the existence of
triple pointIn thermodynamics, the triple point of a substance is the temperature and pressure at which three phases of that substance coexist in thermodynamic equilibrium...
cells. Triple points are conditions of pressure, volume and temperature such that three phases (matter) are simultaneously present, for example solid, vapor and liquid. For a single component there are no degrees of freedom at a triple point and any change in the three variables results in one or more of the phases vanishing from the cell. Therefore, triple point cells can be used as universal references for temperature and pressure. (See Gibb's phase rule)
Under some conditions it becomes possible to measure temperature by a direct use of the
Planck's law of black body radiationFor a general introduction, see black body.In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature . As a function of frequency , Planck's law is written as:...
. For example, the cosmic microwave background temperature has been measured from the spectrum of photons observed by satellite observations such as the WMAP. In the study of the
quark-gluon plasmaA quark–gluon plasma is a phase of quantum chromodynamics which exists at extremely high temperature and/or density. This phase consists of free quarks and gluons, which are the basic building blocks of matter...
through heavy-ion collisions, single particle spectra sometimes serve as a thermometer.
Surface air temperature
Meteorological observatories measure the temperature and humidity of the air near the surface of the Earth usually using thermometers placed in a
Stevenson screenA Stevenson screen or instrument shelter is an enclosure to shield meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard weather station...
, a standardized well-ventilated white-pained instrument shelter. The thermometers should be positioned 1.25–2 m above the ground. Details of this setup are defined by the
World Meteorological OrganizationThe World Meteorological Organization is an intergovernmental organization with a membership of 188 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
(WMO).
The true daily mean, obtained from a thermograph, is approximated by the mean of 24 hourly readings (which is not the same as the mean of the daily minimum and maximum readings).
http://eobglossary.gsfc.nasa.gov/Library/glossary.php3?mode=alpha&seg=s
See also
temperature conversion formulas- Comparison :-Comparison of temperature scales:...
.
Negative temperatures
- See main article: Negative temperature
In physics, certain systems can achieve negative temperatures; that is, their thermodynamic temperature can be of a negative quantity. Negative temperatures can be expressed as negative numbers on the kelvin scale....
.
For some systems and specific definitions of temperature, it is possible to obtain a
negative temperatureIn physics, certain systems can achieve negative temperatures; that is, their thermodynamic temperature can be of a negative quantity. Negative temperatures can be expressed as negative numbers on the kelvin scale....
. A system with a negative temperature is not colder than
absolute zeroAbsolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible and cannot be reached by artificial or natural means, because it is impossible to decouple a system fully from the rest of the universe...
, but rather it is, in a sense,
hotter than infinite temperature (
sic).