Stan, Bill Metrology is all about splitting hairs (and in this case, hairs I don't entirely grasp).
As to what other temperature scales, measurement on the thermodynamic scale is somewhat impractical, and there have been a succession of international practical scales, based on multiple fixed points, and official means of interpolating between them, the latest being ITS-90. Google throws a "may harm your computer" warning on the official ITS-90 site, but I believe this is a faithful copy of the text: http://www.omega.com/techref/intltemp.html A brief quote from its priciples 2. Principles of the International Temperature Scale of 1990 (ITS-90) The ITS-90 extends upwards from 0.65 K to the highest temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation. The ITS-90 comprises a number of ranges and sub-ranges throughout each of which temperatures T90 are defined. Several of these ranges or sub-ranges overlap, and where such overlapping occurs, differing definitions of T90 exist: these differing definitions have equal status. --------------------------------------------- This scale differs from previous scales attempting the same thing, differences are around ±0.02 K at ambient temperature ranges increasing to about -0.1 K at 600 K, and +0.4 K at 800 K. At one time freezing and boiling points of water at std. pressure may have precisely defined the Centigrade scale; however, under the modern ITS-90 scale, the melting point of water at standard pressure is around +0.0025 °C, not 0 °C. (The above link includes a table of pressure depressions). A more precise measure is given in a reference in this Wikipedia article (I don't have access to the reference itself): http://en.wikipedia.org/wiki/Melting_point ^ The melting point of purified water has been measured to be 0.002519 +/- 0.000002 degrees Celsius - see R. Feistel and W. Wagner (2006). "A New Equation of State for H2O Ice Ih". J. Phys. Chem. Ref. Data 35: 1021–1047. doi:10.1063/1.2183324. I would further point out, although irrelevant to this discussion, that the boiling point of water at standard pressure is around 99.98 °C. Your proposed explanation of the Celsius scale would have to become more convoluted, the elevation of temperature above the point which is 0.01 K below the triple point of water. Lets just leave it as an offset of precisely 273.15 K. The 0 °C and 100 °C explanations commonly used have fallen to the status of the quarter-meridian being precisely 10 Mm, useful approximations, but not definitions. ________________________________ From: Bill Hooper <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Mon, August 30, 2010 10:27:49 PM Subject: [USMA:48435] Re: kelvin On Aug 27 , at 10:27 AM, Stanislav Jakuba wrote: ... BIPM is neither infallible nor keen on removing its own contradictions. Several of those concern temperature and Celsius, particularly the thermodynamic temperature. > >Why the adjective thermodynamic with the temperature in the main table? What >are >the other temperatures and what are their units? Some justify the adjective on >the basis of the existence of the “Celsius temperature” (see SI10). But it is >not in any table. >For decades I have been fighting for the simple statement: >“Temperature – its unit is the kelvin in SI, symbol K.” > >Some (SI10 included) claim that degree Celsius is a derived unit. This cannot >be >true if one believes the definition of SI derived units and how they are >formed. >One or the other must be incorrect. Stan, I agree that there is something wrong ith the logic here and I think I have a reasonable alternative view of the situation that may be helpful. My view is that kelvins and degrees Celsius are NOT two different units for measuring the same thing. They are two IDENTICAL units used to measure DIFFERENT things. The two different things are (1) the elevation of temperature above absolute zero and (2) the elevation of temperature above (or below) freezing. (Throughout this note, when I write "freezing" I mean the freezing point of water with all the usual conditions that are normally prescribed.) Using the terminology "the temperature is 30 kelvins above freezing" is identical to the practice of measuring altitudes of geographic features or things in the Earth's atmosphere (or even above). One commonly measures the elevation above (or below) mean sea level. It would certainly be possible in this day and age to measure "absolute" elevations, meaning distance from the center of the Earth. No one would consider measuring both of these in metres but calling the unit by one name if the elevation is from the center of the planet while calling it by a DIFFERENT name when it is measured above sea level. The measures, both in metres, are measures of two different things (height above sea level vs. distance form Earth's center). My suggestion would be to emphasize that the phrase "degree Celsius" MEANS "kelvins above freezing". The proper usage would be to state "Today's high temperature was 30 kelvins above freezing" or writing "Today's high temperature was 30 K afw". I just invented the abbreviation "afw" to mean "above freezing water". This would be similar to using ASL to mean "above sea level" as in "The height of that mountain is 2224 m ASL", as is already done in some places. I would be the first to point out that it is improper (by SI rules) to combine the metre symbol with the sea level abbreviation (as it indeed is done in some places) so that it should not be written "The height of that mountain is 2224 mASL". So also it would be improper to write "The temperature is 30 Kafw"; it must be "The temperature is 30 K afw". Bill Hooper Temperature 37 K apf Fernandina Beach, Florida, USA ========================== SImplification Begins With SI. ==========================
