Fahrenheit & Celsius Temperatures

While I was growing up, I lived in a world of Fahrenheit temperatures with an occasional reference to Centigrade. In 1948, the scale of degrees for the metric system was officially defined and along the way, Centigrade was designated Celsius to honor the creator of the scale. Fahrenheit and Celsius are both people's names.

Fahrenheit and Centigrade, are/were based on natural points easily handing in the lab, but later further refined to reference the Triple Point of water, while the Celsius was keyed to the Triple Point in the first place. The Triple Point is slightly above 32F/0C (32.01F) and instead of water and ice in equilibrium is water-ice-vapor equilibrium. Actual measurements must be adjusted for altitude and barometric pressure. Originally Fahrenheit used a water-ice-salt solution for 0F and blood temp for 96 (first calling it 12), finding boiling to be 212, later adjusted to use the freezing of pure water to 32 to give 180 degrees from freezing water to boiling (more details at: MORE)

The Kelvin scale uses Celsius sized degrees but starts at Absolute Zero so water freezes at 273K (273.16K being the triple point). (Rankin is the equivalent with Fahrenheit-sized degrees.)   The significance of Absolute Zero and Absolute temperature scales is that gases expand and chemical reactions increase in proportion to absolute temps - in other words if a gas stayed a gas all the way down, it would have no motion and least volume at 0K - of course, most turn liquid well above 0K, no longer being perfect gases.

[By the way, technically F & C should always be written °F or °C while K is just K, but I cheat a lot because ° (degree symbol) comes out differently on different browsers and it is awkward to type it/insert it repeatedly. (alt 248 on the keypad on MS systems, ° in HTML.)]

A point of confusion exists because people remember that there is a +32 involved somewhere and use it in the wrong place.
Because the two scales put zero at different points on a thermometer, a C reading converts to F by taking 9/5 (1.8) times the C reading and adding 32. (for example, boiling water at 100C, times 1.8 is 180, plus 32 is 212F). Going from F degrees to C, involves first subtracting the 32 then multiplying by 5/9.

The conversion of a temperature range just involves degree size without offset. The latter is important in glass work.  For example, annealing over a range of 300ºF for 3 hours. A C degree is almost twice (9/5) the size of a F degree. So 300F change is 166.67C.

  For a rough conversion to be done in the head while reading: double C and subtract 10% of the double (166.67C times 2 is 333.34 minus 33 gives 300F); or halve F and add 10% of half (300F divide by 2 gives 150, add 10% - 15 gives 165)
The following table shows several temperatures in F and C over the range that glass is commonly worked with a few comments. Note that because of the size of the degrees and the offset zero points, -40 is the same temperature on each scale.

More Melting Points  Viscosity