## Thursday, September 23, 2010

### 2.6f Temperature Calculations - Celsius/Fahrenheit/Kelvin

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2.6f: Update 14 Oct 2018

In recording temperature, Celsius C (degree Celsius) is the international measurement. In explaining the Celsius, it is useful to mention the centigrade scale it replaced. The boiling point of water at standard pressure, dry condition is 1000 centigrade and its freezing point 0centigrade. Thus the 100 equal gradations between 0 and 100 are each a single degree centigrade, and these gradations are continued above 100 as far as is necessary, and below 0 with minus sign.
Celsius scale replaced centigrade because the freezing (or ice) point of water, at standard pressure, dry is most accurately measured in reference to what is called ‘the triple point’ of water. The triple point is a temperature at which the gas, liquid and solid of water are in equilibrium (in dynamic, fixed proportion to each other). The Celsius scale uses the 1C gradation like centigrade but its reference point is the triple point, most closely 0.010 centigrade.
The U.S. and other English-speaking countries use the Fahrenheit scale. Its zero point, 0F (-17.8C), was the lowest temperature that Daniel Gabriel Fahrenheit could record with his primitive thermometer and a mixture of ice and salt in mid winter in 1724. The 100F (equiv. 37.8C), seems to have been chosen as the human body temperature. On the F scale, water boils at 212F (100C). The conversion formula is C for Celsius = 5/9(F minus 32) and F = (9/5)C + 32.
There are two “magic" equality conversion numbers for Celsius to F. The minus, –40C = –400 F exactly ; then (+)11F = (-)11C (Actually (-)11.666C)
A frequent conversion is for body temperature in illness. Easily memorized reference points are the 35 to 40 C range (350 C = 95F and 40C = 104F).  Then for each 1.0C gradation, you add 1.8F; thus 36C = 96.8F, etc. For scientific calculation, an absolute, or Kelvin scale uses the triple point of water for reference but bases its zero point on the theoretical lowest temperature in the Universe where atoms cease motion. Approached closely but never reached actually, the zero Kelvin (0K) is minus 273.15C. Thermometer measurements on this scale are zeroed at –273.15C, written 0K (not the common “OK” but “zero Kelvin”). Thus to convert to Kelvin from Celsius, the K= C + 273.15. The K measurement must be used when temperature enters into formulas in the metric system. Note that the K measurement does not use the degree superscript mark, as the F and C measurements do although that rule is not infrequently broken even by scientists.
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