Temperature!

As touched upon before, fooling around with energy, particularly in the United States, also means fooling around with a menagerie of different units to express power (energy per unit time) and energy itself.  Temperature is no different.  There are four temperature scales that are useful to know if you work with energy:

  • Fahrenheit
  • Celsius
  • Rankine
  • Kelvin

According to Wikipedia, the Fahrenheit scale is used in the United States, Palau, the Cayman Islands the Bahamas and Belize.  So once again, we (Americans) employ a legacy measurement system which adds a bit of complexity to our engineering efforts.

In the Fahrenheit (°F) scale, pure water freezes at 32 degrees and pure water boils at 212 degrees.   Absolute zero is -459.7 degrees F.  Absolute zero is an interesting topic that deserves, and will have, it’s own brief post.

The Rankine (°R) scale uses the same measurement increments as the Fahrenheit scale (for example, the temperature difference between boiling water and freezing it is 180 degrees in either system), but it defines absolute zero to be 0 degrees.  This means that water freezes at approximately 492 degrees Rankine (32 + 460) and boils at approximately 672 degrees R (212 + 460.)

The Celsius (°C) scale is intuitively a bit more obvious to us, defining the temperature at which water freezes to be 0 degrees and the temperature at which it boils to be 100 degrees.  Absolute zero in degrees Celsius is -273.2 degrees C.

The Kelvin (°K) scale is to the Celsius scale as Rankine is to Fahrenheit.  It employs the same measurement increment as the Celsius scale, but it establishes absolute zero to be 0 degrees.  This means that water freezes at 273 degrees Kelvin, and boils at 373 degrees K.

Though it is easy to convert from one temperature scale to another, I can never remember the conversions and always need to look them up.  Here’s a summary that hopefully will help others not have to do the same.  Note that I am rounding the absolute zero terms.

°F to °R:               °R = °F + 460

°F to °C:               °C = 5/9 * (°F -32)

°F to °K:               °K = 5/9 * (°F + 460)

°C to °F:               °F = 9/5 * °C + 32

°C to °R:               °R = 9/5 * °C + 492

°C to °K:               °K = °C + 273

Most energy work in the U.S. employs the Fahrenheit scale, although you do see a lot of equipment specifications (e.g. allowable temperature operating ranges) called out in degrees Celsius.   While Rankine and Kelvin temperatures are not commonly used, these temperature scales are necessary in calculations of radiant heat transfer and thermal efficiency.

Please note that one degree Celsius or Kelvin is always equal to 9/5 of a degree Fahrenheit or Rankine.

It is also worth noting that the increments employed by these temperature scales are in a certain sense arbitrary (although the value of absolute zero is not.)  There could be, and are, other exotic temperature scales that can also be used.

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