SPRING 2019

154 THE DISTRIBUTOR’S

154 THE DISTRIBUTOR’S LINK BENGT BLENDULF THE GOOD AND BAD USE (ABUSE) OF METRIC STANDARDS from page 98 The kilometer is pronounced with the accent on “i” and not on the “o” as most uneducated reporters say it. If you say kilo and then meter and just put them together like you just said it, it will be correctly pronounced. Area Again, the meter being the base unit, we can use mm², m², km² or whatever would suit conversion of in², ft², acre and so on. Temperature The kelvin unit, K, is the SI base unit, but should only be used in scientific contexts. Degree Celsius (°C) is the SI additional unit replacing degree Fahrenheit (°F). The Celsius scale is simply based on water either freezing at 0°C or boiling at 100°C. The Fahrenheit scale is pure nonsense in my opinion. Don’t use centigrade instead of Celsius, this was an old term used by pharmacists a long time ago. A couple of easy reference points: -40 is the same in °C and °F (much too cold either way), +10°C equals 50°F, +16°C = 61°F and “room temperature” +20°C is 68°F. Force N or newton (low case always) replaced all old “metric” terms like kilogram force and kilopond. It is a relatively small unit, 1 pound force (lbf) is approximately 4.5N (4.4482 exactly). For practical purposes we normally use a prefix like k for kN (thousand newtons) or M for MN (million newtons), etc. Mass (Weight) One kg (kilogram) equals 2.2046 lb (pound). Don’t use kilo to indicate kilogram, kilo is a prefix simply meaning 1000. The Europeans, who should know better by now, are very sloppy with this, let’s try to be better (shall we?). Why we use lb (libra from Latin) for pound and oz (onza also from Latin) I simply don’t know. Do you? Volume Use liter (L) for liquids (gasoline, wine, soda pop, etc.). Otherwise, in technical matters use mm³, m³ and similarly cubed metric units. 1 L = 1 dm³. 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/mm² was used by ISO and most industrial countries. For reasons unknown to me, ISO and ASTM have adapted the pascal unit as MPa instead of N/mm² for mechanical properties. The rational for using N/mm² is that the stress is defined as a force (in this case N) applied to a cross sectional area (in this case mm²). Since everyone in the world knows (??) that MPa = N/mm², 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 (1852m) 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). To my dear Link readers: 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 30 years overdue. We pay these people for this! BENGT BLENDULF

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