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Temperature measurement

 
Sci-Tech Encyclopedia:

Temperature measurement

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Measurement of the hotness of a body relative to a standard scale. The fundamental scale of temperature is the thermodynamic scale, which can be derived from any equation expressing the second law of thermodynamics. Efforts to approximate the thermodynamic scale as closely as possible depend on relating measurements of temperature-dependent physical properties of systems to thermodynamic relations expressed by statistical thermodynamic equations, thus in general linking temperature to the average kinetic energy of the measured system. Temperature-measuring devices, thermometers, are systems with properties that change with temperature in a simple, predictable, reproducible manner. See also Temperature; Thermodynamic principles.

In the establishment of a useful standard scale, assigned temperature values of thermodynamic equilibrium fixed points are agreed upon by an international body (General Conference of Weights and Measures), which updates the scale about once every 20 years. Thermometers for interpolating between fixed points and methods for realizing the fixed points are prescribed, providing a scheme for calibrating thermometers used in science and industry.

The scale now in use is the International Temperature Scale of 1990 (ITS-90). Its unit is the kelvin, K, arbitrarily defined as 1/273.16 of the thermodynamic temperature T of the triple point of water (where liquid, solid, and vapor coexist). For temperatures above 273.15 K, it is common to use International Celsius Temperatures, t90 (rather than International Kelvin Temperatures, T90), having the unit degree Celsius, with symbol °C. The degree Celsius has the same magnitude as the kelvin. Temperatures, t90, are defined as t90/°C = T90/K − 273.15, that is, as differences from the ice-point temperature at 273.15 K. The ice point is the state in which the liquid and solid phases of water coexist at a pressure of 1 atm (101,325 pascals). [The Fahrenheit scale, with symbol °F, still in common use in the United States, is given by tF/°F = (t90/°C × 1.8) + 32, or tF/°F = (T90/K × 1.8) − 459.67.] The ITS-90 is defined by 17 fixed points. See also Triple point.

Primary thermometers are devices which relate the thermodynamic temperature to statistical mechanical formulation. The fixed points of ITS-90 are all based on one or more types of gas thermometry or on spectral radiation pyrometry referenced to gas thermometry. Secondary thermometers are used as reference standards in the laboratory because primary thermometers are often too cumbersome. It is necessary to establish standard secondary thermometers referenced to one or more fixed points for interpolation between fixed points. Lower-order thermometers are used for most practical purposes and, when high accuracy is required, can usually be calibrated against reference standards maintained at laboratories, such as the U.S. National Institute of Standards and Technology, or against portable reference devices (sealed boiling or melting point cells). See also Gas thermometry; Low-temperature thermometry; Pyrometer; Thermistor; Thermocouple; Thermometer.

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Wikipedia:

Temperature measurement

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A medical/clinical thermometer showing the temperature of 38.7 °C

Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Røemer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

The world's average surface air temperature is about 15 °C. For information on temperature changes relevant to climate change or Earth's geologic past see: Temperature record.

Contents

History

Further information: Timeline of temperature and pressure measurement technology

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 thermometer. 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 thermometer.

Other important devices for measuring temperature include:

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 thermodynamics 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 point 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 radiation. 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 plasma 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 screen, 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 Organization (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).[1]

Comparison of temperature scales

Main article: Comparison of temperature scales
Comparison of temperature scales
Comment Kelvin
K		 Celsius
°C		 Fahrenheit
°F		 Rankine
°Ra (°R)		 Delisle
°D ¹		 Newton
°N ¹		 Réaumur
°R (°Ré, °Re) ¹		 Rømer
°Rø (°R) ¹
Absolute zero    0 −273.15 −459.67     0   559.725  −90.14 −218.52 −135.90
Lowest recorded natural temperature on Earth
(Vostok, Antarctica - 21 July 1983)		  184  −89 −128   331   284  −29  −71  −39
Celsius / Fahrenheit's "cross-over" temperature  233.15  −40  –40   419.67   210  –13.2  –32  –13.5
Fahrenheit's ice/salt mixture  255.37  −17.78    0   459.67   176.67   −5.87  −14.22   −1.83
Water freezes (at standard pressure)  273.15    0   32   491.67   150    0    0    7.5
Average surface temperature on Earth  288   15   59   519   128    5   12   15
Average human body temperature ²  310.0 ±0.7   36.8 ±0.7   98.2 ±1.3   557.9 ±1.3    94.8 ±1.1   12.1 ±0.2   29.4 ±0.6   26.8 ±0.4
Highest recorded surface temperature on Earth
(Al 'Aziziyah, Libya - 13 September 1922)		  331   58  136   596    63   19   46   38
Water boils (at standard pressure)  373   100  212   672     0   33   80   60
Gas flame 1773 ~ 1500 ~ 2732 ~
Titanium melts 1941 1668 3034  3494 −2352  550 1334  883
The surface of the Sun 5800 5526 9980 10440 −8140 1823 4421 2909

1 The temperature scale is in disuse, and of mere historical interest.
2 Normal human body temperature is 36.8 ±0.7 °C, or 98.2 ±1.3 °F. The commonly given value 98.6 °F is simply the exact conversion of the nineteenth-century German standard of 37 °C. Since it does not list an acceptable range, it could therefore be said to have excess (invalid) precision. See Temperature of a Healthy Human (Body Temperature) for more information.
Some numbers in this table have been rounded off.

See also temperature conversion formulas.

Negative temperatures

See main article: Negative temperature.

For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).

See also

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Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Temperature measurement".  Read more

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