Monthly Mechanics: Know Your Units

Let’s start with PSI – Pounds per Square Inch. If you’ve ever inflated a car or bicycle tire, then you already know about psi. It measures the pressure inside the tire; a reading of 30 psi means that every square-inch area of the tire has 30 pounds of air pushing on it.

psi

30 psi means 30 pounds of pressure on each square inch.

Lots of things cause pressure: your weight on a scale, wind blowing on a skyscraper, water held back by a dam. All of these can be measured in psi.

(Aside: Years ago when I visited the Hoover Dam, I told the tour guide I was a civil engineer. He said, “OK, you know what psi means, but how about PE-psi? I said I had never heard of it and he replied, “Well, that’s Pepsi,” much to the amusement of the tour group.)

Sometimes the pressure is relatively small. A strong wind on a 200-foot-tall building might measure 0.1 psi. To make the numbers easier to work with, we might convert to Pounds per Square Foot – PSF. There are 12*12=144 square inches in a square foot, so 0.1 psi is equivalent to 14.4 psf.

Other times the pressure is relatively large, and we’ll want to make the numbers smaller. A pressurized oxygen tank might measure 2500 psi. Here we might convert to Kips per Square Inch – KSI. (KIP is just an abbreviation of KIloPound – 1 kip equals 1000 pounds. Engineers use kips all the time; hardly anyone else uses them ever.) We can also convert to Kips per Square Foot – KSF. 2500 psi is equivalent to 2.5 ksi or 360 ksf.

Units

Density works similar to pressure. If some type of wood has a density of 80 PCF (pounds per cubic foot), then every cubic-foot piece of that wood has a weight of 80 pounds. Water has a density of 62.4 pcf, so this particular wood would sink, not float. We can convert density units to PCI, KCF, or KCI, although “per cubic inch” is rarely used.

Let’s say you have 2 feet of snow on your roof, and the density of the snow is 20 pcf. Then the pressure on your roof is 2 feet * 20 pcf = 40 psf. Pressure and density are easy to work with, and they form the foundation of almost all civil engineering calculations. Just don’t get duped by a snarky tour guide.

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