Talk:International Temperature Scale of 1990
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[edit] From Greg L:
[edit] ITS–90 is not a “scale”
“International Temperature Scale of 1990” has the word “scale” in its name but this is a misnomer and can be misleading to many people. Only scales like kelvin, Celsius, Fahrenheit, and Rankine are scales. There is no such thing as “99.987 °ITS.”
ITS–90 is an equipment calibration standard for making accurate measurements of temperatures that can then be expressed as values on various temperature scales such as Celsius, kelvin, or Fahrenheit.
The lasting effect of making a temperature measurement using instruments calibrated per ITS–90 is the measured value will likely be very slightly different from those produced using a different calibration standard—regardless of which scale is used to express the value of the measurement. For instance, the boiling point of water can be said to occur at 211.9710 °F when calibrated to the two-point definition of absolute thermodynamic temperature. It is just as valid to say the boiling point of water is 211.953 °F when calibrated per ITS–90. Clearly, the scale on which both temperatures are being expressed is Fahrenheit even though the later was per a kelvin-based calibration standard that has the word “scale” in its title.
The important point is that when documenting high-precision temperature measurements (to a resolution of 10 mK or better), it becomes increasingly important to specify the calibration standard employed. And further, the calibration standard employed places no restrictions as to which scale the temperature may be expressed.
[edit] Temperature formating conventions
The name “kelvin” is not capitalized unless it begins a sentence. With the exception of the kelvin, all the scale symbols are immediately preceded by a ° (degree) symbol. Note also that a space is inserted between the value and the degree symbol (or in the case of the kelvin, before the “K”. Only angular measurements such as 180° have the degree symbol placed immediately after the value. Also, the scale's symbol is the always capitalized; thusly, we have…
The freezing point of water (expressed as kelvins) is 273.15 K
The freezing point of water (expressed as degrees Celsius) is 0 °C
The freezing point of water (expressed as degrees Fahrenheit) is 32 °F
and
The freezing point of water (expressed as degrees Rankine) is 459.67 °R
Remember that it is important to capitalize the kelvin symbol (K) because the lowercase version (k) is the SI prefix for 1 x 103
Various reasons can be found for why the name “kelvin” is lowercase and its symbol, “K,” is not preceded with a ° (degree) symbol. For instance, some sources say that kelvin has no degree symbol because it has the distinction of being an absolute scale. However, the definition of “degree” is simply “a unit division or step” so it doesn’t matter where a temperature scale’s null point is. Furthermore, the symbol for Rankine (also an absolute scale) is properly preceded by the degree symbol. There are two simple reasons for why the unit name kelvin and its symbol are written as they are: Notwithstanding the naming convention historically used for the other temperature scales, SI unit names are never capitalized. Also, the 13th General Conference on Weights and Measures (CGPM) in 1967 declared that the kelvin symbol (K) would no longer be preceded by a degree symbol because kelvin would no longer refer to the entire scale; it would thereafter be recognized as referring to an individual unit comprising the scale. Thus, the numeric value preceding the unit symbol would simply specify “how many kelvins there are” (just like 15 km or 4 µg). This is why the kelvin is the only unit measure of temperature that may be written in its plural form (e.g. “…is 12 kelvins greater than…”) in the same fashion as with all the other SI units such as mass (e.g. “…is 12 grams greater than…”). Thus, since kelvin has not referred to the entire scale since 1967, it is not appropriate to refer to a degree-related position on it, nor is it proper to precede the “K” with a ° (degree) symbol.
For more information on how the full word “kelvin” is not capitalized, see this article by the NIST as well as Wikipedia's kelvin article.
For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST.
Still more information on SI (International System of Units) conventions from the NIST can be found here.
[edit] Issues regarding freezing v.s. melting point
ITS-90 differentiates between “freezing” and “melting". See Table 1 in this link for more information.
[edit] Why do you see the mixed use of the kelvin and Celsius scales?
In science and engineering, the Celsius and kelvin scales are often used simultaneously in the same article. This is because Celsius is used to express a specific temperature (e.g. “The melting point of gallium is 29.7646 °C.”)
The kelvin scale, on the other hand, can be used two ways. It is uniquely used to express differences between temperatures and their uncertainties (e.g. “The error is 10 mK,” and “Agar exhibited a melting point hysteresis of 25 K.”) And of course, the kelvin may also be used to express specific temperatures along its absolute scale (e.g. “The triple point of hydrogen is 13.8033 K.”)
Celsius is not used to express differences between temperatures or their uncertainties. In part, this prohibition eliminates ambiguity. In a chart, the value “25 °C” would be confusing if it was actually intended to represent a difference between two temperatures.
[edit] Typography hints
In any document that has variable-width pages and where word-wrap can occur at almost any point on a line, try using a hard-space (option-space on a Mac) between a temperature value and its degree or scale symbol.
Thusly, you will obtain: “A temperature of
99.987 °C at the end of a line instead of 99.987
°C, which is fragmented and is harder to read.”
Similarly, try to use an en-dash (option-hyphen on a Mac) rather than a simple hyphen when writing “ITS–90” to prevent the same thing.
Thusly, you will obtain: “A calibration standard per
ITS–90, rather than a less controlled hyphenated ITS-
90 which is fragmented and is harder to read.”
“Typographers quotes” should be used rather than simple "typewriter quotes."
For further information on writing style and typography issues, see Wikipedia’s article Manual of style.
