Temperature measurement

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

Attempts of standardized temperature measurement have been reported as early as 170 AD by Claudius Galenus.[1] The modern scientific field has its origins in the works by Florentine scientists in the 17th century. Early devices to measure temperature were called thermoscopes. The first sealed thermometer was constructed in 1641 by the Grand Duke of Toscani, Ferdinand II.[1] The development of today's thermometers and temperature scales began in the early 18th century, when Gabriel Fahrenheit adapted a thermometer using mercury and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

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 increase causes 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 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 Gibbs 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-painted instrument shelter. The thermometers should be positioned 1.25–2 m above the ground. Important note.[2] 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).[3]

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.

Comparison of temperature scales

Comparison of temperature scales
Comment Kelvin
K		 Celsius
°C		 Fahrenheit
°F		 Rankine
°Ra (°R)		 Delisle
°D ¹		 Newton
°N 		 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
('Aziziya, Libya - 13 September 1922)
But that reading is queried.		 331 58 136 596 63 19 46 38
Water boils (at standard pressure) 373.15 100 212 672 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.

Standards

The American Society of Mechanical Engineers (ASME) has developed two separate and distinct standards on temperature Measurement. B40.200 and PTC 19.3. B40.200 provides guidelines on Bimetalic Actuated, Filled System, and Liquid-in-Glass Thermometers. It also provides guidelines for Thermowells. PTC 19.3 provides guidelines for temperature measurement as related to Performance Test Codes with particular emphasis on basic sources of measurement errors and means for coping with them.

See also

US (ASME) Standards

  • B40.200-2008: Thermometers, Direct Reading and Remotes Reading
  • PTC 19.3-1974(R2004): Performance test code for temperature measurement.

References

  1. ↑ 1.0 1.1 T. J. Quinn (1983). Temperature. London: Academic Press. 
  2. ↑ GISS Surface Temperature Analysis: The Elusive Absolute Surface Air Temperature (SAT) Quote: "...Q. What exactly do we mean by SAT ? A. I doubt that there is a general agreement how to answer this question...Q. If SATs cannot be measured, how are SAT maps created ? A. This can only be done with the help of computer models..."
  3. ↑

External links

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