Units will drive you nuts

Back in the Energy Currency post, I mentioned that energy can be confusing because it can be represented with such a wide variety of units.

British Thermal Units, Watts, Joules, Horsepower-Hours.  All talking about the same thing, but it’s like converting from Dollars to Chinese Yuan to Czech Crowns to Euros when you want to buy a beer in Mexico.  More confusing than ideal.

Well, really it’s worst than that.  Aside from different units, there are two different systems of units in use in engineering.

The International System of Units (the SI Units) is more or less the metric version of things, giving us units like meters, kilograms, pascals (unit of pressure) and newtons (a unit of mass that, appropriately enough, weighs about as much as an apple here on earth – 1/4 lb roughly.)

English Units were derived largely from old measurement standards from Britain.   Some, such as yards, miles and pounds (weight) are familiar.  Others, like slugs (mass, and equal to 32 pound-mass) and grains (weight) are pretty obscure, but still around to drive us crazy.

According to Wikipedia, only Burma, Liberia and the United States have not adopted SI units as their official system of weights and measures.  This is somewhat unfortunate because SI units are simpler to work with, but given the extent of our legacy systems, you really have no choice but to learn how to manipulate numbers with both systems anyway.

In the energy world, we really have a strange mix of units that are run into all the time.  Let’s start a list right here of the big ones.  I will come back and add to this as things occur to me:

  • Gallons (volume)
  • Gallons per Minute (GPM – Flow Rate)
  • Cubic Feet (volume)
  • Cubic Feet per Minute (CFM – Flow Rate)
  • Degrees Fahrenheit (DegF – Temperature)
  • British Thermal Units (Btu – Energy)
  • Thousand British Thermal Units (MBtu, sometimes KBtu – Energy)
  • Million British Thermal Units (MMBtu – Energy)
  • British Thermal Units per Hour (Btuh – Power)
  • Watts (Power)
  • Watt-Hours (Energy)
  • Tons (cooling, 12,000 Btuh – Refrigeration Power)
  • Ton-hours (12,000 Btu – Refrigeration Energy)
  • MLb (heating, 1,000 pounds of steam)
  • Pounds per Hour (PPH, commonly used for steam power)
  • Volts (V)
  • Amperes (Electrical Current, I)
  • Ohms (Electrical Resistance, R)
  • Power Factor (pf)

As I think of more I’ll add them.  But we need these because we are going to derive the eight or ten essential equations that allow you to do energy calculation quickly and accurately for wide array of situations.

Hang in there.  What I find about engineering, at first, is that you have to learn a bunch of seemingly random stuff, but only after you know enough of that random stuff can you string it together and make a sensible whole out of it.


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