8 years ago


Distributor's Link Magazine Summer Issue 2012 / VOL 35 / NO.3


122 THE DISTRIBUTOR’S LINK THE BAD USE (ABUSE) OF METRIC continued from page 112 Volume Use liter (L) for liquids (gasoline, wine, soda pop, etc.). Otherwise, in technical matters use mm 3 , m 3 and similarly cubed metric units. 1 L = 1 dm 3 . 1 US gallon is approximately 3.8 L (3.7854 exactly). Pressure/Stress The derived SI unit pascal (Pa) is used worldwide for barometric, hydraulic and other types of pressures. For mechanical properties, like strength of materials, N/mm 2 is used by ISO and most industrial countries. For reasons unknown to me, ASTM has adapted the pascal unit as MPa instead of N/mm 2 for mechanical properties. The rational for using N/mm 2 is that the stress is defined as a force (in this case N) applied to a cross sectional area (in this case mm 2 ). Since everyone in the world knows () that MPa = N/mm 2 , this should not be a problem, but why can’t we have the same expressions for all technical standards to avoid trouble The United States, is after all a major member of ISO, The International Organization for Standardization. Torque/Moment of force For tightening of most sizes of fasteners we use newton meter (Nm) or other suitable combinations of N and length of moment arm. A final note In this modern and high tech age, why would a pilot of a commercial airliner tell the passengers the speed of the airplane in knots! The knot (kn) is a term (not SI, of course) coming from the old sailing ship era where knots were tied on a line every 47 feet 3 inches apart. The line was attached to a chip log (weighted on one end to make the thing stand up – sort of) which was cast over the stern of a moving vessel/ship. A 30-second sand-glass (later 28 second) was then used to time how many such knots passed by to indicate speed. One knot, by the way, is based on 1.85166 m (one nautical mile) per hour. With the use of modern, precise information from GPS and Doppler radar, this knot business is pure nonsense. There is, unfortunately, a 1969 United States Federal Aviation Regulation for airworthiness standards mandating use of nautical miles (1852 m) and knots. To be meaningful it also has to be combined with the Mercator projection world map to make any sense for navigation. So, if a pilot reads this, please spare your passengers this unnecessary information and tell them your speed (in air and over ground) in km/h. Keep the knot inside the flight deck (used to be called cock-pit). I don’t mind if you make a copy of this article and send it to your Congress- and/or Senate representative. They passed the metric law and should get moving on implementing it, it is more than 20 years overdue! SFA SPRING CONFERENCE & EXPO Houston, Texas, March 22-25, 2012


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