What IS a Grain?

Learning all the different units of measurement HVAC/R technicians must deal with can be a daunting task for students. The inability to accurately describe what you need is a major barrier to performing your job. Understanding the different units of measurement used in the HVAC/R trade simply makes your job easier. However, all of us run into specifications or terms used in technical literature that we are not really familiar with. Knowing why we use certain units for particular measurements or knowing the historical background of the units helps me remember them. Ever wondered about the origin, definition, or use of an unusual unit of measurement? Maybe even one you use often but don’t really know much about? For me, the grain had been an enigma for years. Although I used it often when working with humidity and the psychrometric chart, it was always a bit of a mystery to me. The grain is a very small unit of weight: 7000 grains equals one pound. Grains are used to measure the weight of water in each pound of dry air when working with the psychrometric chart. Grains are a perfect unit of measure for that purpose, giving us reasonable whole numbers to work with when discussing very small quantities of weight. But why the name grain, and what measurement system did this odd 7000 quantity come from? I found the answers to those questions and more at a web site called How Many? A Dictionary of Units by Russ Rowlett of the University of North Carolina at Chapel Hill. It turns out that the grain was originally based on the weight of a grain of barleycorn in England, or a grain of wheat in France. The name is literal. Many of our common measurements are based on concepts and systems that go back hundreds of years. To introduce your students to the site, ask them to look on the site and find some air conditioning specific information, like the difference between American Wire Gauge and Metric Wire Gauge, or how the American Wire Gauge came up with the particular wire diameters it uses. Here is another challenge: how many of you know what the gauge of a shotgun is based on? To find out go to Mr. Rowlett’s site, How Many? A Dictionary of Units. However, I warn you that you can get lost for hours reading the intriguing background to common measurements.