Here's what I've learne so far and I am simply looking for some knowledgable interaction:
Consistency vs. Practice (From an ASME Engineer); To the Editor: As an older, semi-retired mechanical engineer, I would like to offer the following comments on the article "Standard Measure" (April). I agree with the author's comment on page 72 stating, "Most people, once they understand it, like SI for its logic, consistency, and lack of conversion factors." I am one who likes the SI system. However, as the author also stated, there are inconsistencies in its use.
In the 1970s, I was a member of the national ASME Metric Study Committee. Our purpose was to evaluate various versions of the metric system, with the objective of making recommendations to the membership at large. Our primary focus was on the SI system. The presumption at that time was that the United States would adopt the metric system sooner, rather than later.
What I found then, as an engineer working in industry, was that there seemed to be various metric systems in use, with none of them being the true SI version. For example, German engineers used the kg/mm2 as the unit for pressure, the kilogram, of course, being the unit for mass rather than force. The French had some distinct units of their own, which I cannot recall.
A universal system of units is not worth much unless it is universally used. The worldwide adoption of the metric system as a tool of economy and everyday commerce was based to some extent on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardise the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with the decimal numeral system.
There are many units in everyday and scientific use that are not derived from the seven SI base units--metre, kilogram, second, ampere, kelvin, mole and candela--combined with the SI prefixes. In some cases these deviations have been approved by the BIPM.[2] Examples include:
The many units of time --minute (min), hour (h), day (d) -- in use besides the SI second, and are specifically accepted. The year is specifically not included but has a recommended conversion factor.
The Celsius temperature scale; kelvins are never employed in everyday use.
Electric energy is often billed in kilowatt-hours instead of megajoules.
Use of kilometres per hour (km/h) instead of the SI metres per second for automotive speed; fuel consumption is, in a few countries, often given in litres per 100 km (L/100 km).
The nautical mile and knot (nautical mile per hour) used to measure travel distance and speed of ships and aircraft (1 nautical mile = 1852 m ≅ 1 minute of latitude). In addition to these, Annex 5 of the Convention on International Civil Aviation permits the "temporary use" of the foot for altitude.
Astronomical distances measured in astronomical units, parsecs and light-years instead of, say, petametres (a light-year is about 9.461 Pm or about 9 461 000 000 000 000 m).
Atomic scale units used in physics and chemistry, such at the ångström, electron volt, atomic mass unit, and barn.
Some physicists still use the centimetre-gram-second (CGS) units.
In some countries, the informal cup measurement has become 250 ml. Likewise, a 500 g "metric pound" is used in many countries. Liquids, especially alcoholic ones, are often sold in units whose origins are historical, e.g. beer in pints in the UK, champagne in Jeroboams in France.
In the US, blood glucose measurements are recorded in milligrams per decilitre (mg/dL); in Canada and Europe, the standard is millimole per litre (mmol/L or mM (millimolar).
Blood pressure is measured in mmHg instead of Pa.
The fine-tuning that has happened to the metric base unit definitions over the past 200 years, as experts have tried periodically to find more precise and reproducible methods, does not affect the everyday use of metric units. Since most non-SI units in common use, such as the US customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.
The International System of Units (Système international d'unitès or SI) is the current international standard metric system and the system most widely used around the world. It is based on the metre, kilogram, second, ampere, kelvin, candela and mole.
Metre-tonne-second systems: The metre-tonne-second system of units (MTS) was based on the metre, tonne and second. It was invented in France and mostly used in the Soviet Union from 1933 to 1955.
Gravitational systems: Gravitational metric systems use the kilogram-force (kilopond) as a base unit of force, with mass measured in a unit known as the hyl, TME, mug or metric slug.
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