World Metrology Day

[pete]

Well what I meant by there being no such physical object as being exactly that inch, foot, meter is caused by multiple problems. The uncertainty of your measurement issue to name just one. The master one meter physical standard is I think kept in a temperature controlled vault in Europe and it's variation from it being that exact meter is known. The deviation is very small but it's there.

Degrees, minutes and seconds works ok for anything that does work out evenly some where into those 1296000 seconds, but degrees and decimal degrees works fine when they don't if your accuracy needs require something like that. I'd guess but don't know for sure that NASA uses degrees and decimal degrees in there calculations. Earthy locations for general navigation is probably more than close enough with the degrees, minutes, and seconds old school method. If I remember the numbers correctly that gets you within 1/10th of a nautical mile.

[Steggy]

And then there is the easy divisibility of fraction based measures.

That's why the circle is 360 degrees instead of 100.

Imagine trying to calculate the compass heading in degrees for north by northwest?

Steve

In engineering, we mostly use radians, not degrees.

[BadDog]

Yes, radians makes SO much more sense for many things, but deg/min/sec has a variety of important ramifications (both historical and practical) as well.

And geospatial coordinates are most often processed as decimal degrees of lat/lon. How many digits of precision are used gives you a rough implicit accuracy. For instance, if I recall correctly, 7 digits of precision in lat/lon provide roughly sub-meter (or better) location anywhere on the globe. Of course just because you take a reading to that precision doesn't necessarily get that accuracy with GPS coordinates (at least not without 3 or more independent sources to use for calibration). Somewhat like trying to read a digital caliper to 3 decimal places.

[WesHowe]

IDK if anyone bothered to look it up, but the U.S. was one of the original 17 signatories of the Treaty of the Meter, in 1875. Argue about the lack of progress on implementation all you want, but Metric is a legal system of measurement in the U.S.

[RONALD]

Here is a still available great book on measurement, it has every unit you can think of and a few more, like Twaddell!

The World of Measurements by H. Arthur Klein 1974 SBN 671-21565-5

https://www.amazon.com/World-Measuremen ... s+by+Klein

[SteveHGraham]

Radians are the greatest thing ever, but imagine trying to use them in daily life. Picture a table saw with a radian miter gauge. The degree system is pretty impressive in terms of ease of use.

As for the accuracy of GPS, I still remember the says when the government degraded it and made it annoying to use. If you got within a city block, you were doing great. Trying to put a boat over a wreck was not much easier than using LORAN, and LORAN was pathetic. I knew a guy who said he hated LORAN so much he used a sextant.

[mklotz]

SOME THINGS YOU SHOULD THINK ABOUT BEFORE ATTEMPTING TO DISCUSS MEASUREMENT SYSTEMS INTELLIGENTLY

The main advantage of the metric system is NOT just that its units are based on powers of ten. Briefly, the key advantages are:

Only one unit used to measure each fundamental or derived physical quantity.

These fundamental units are, as much as possible, simply related to each other, e.g., a liter of water weighs a kilogram.

All subdivisions of fundamental units are related to the fundamental by the SAME sequence of prefixes, e.g., micro, milli, kilo, mega..., each of which denotes a power of ten multiplier.

Derived units of convenience are simply related to the fundamental units, e.g., a hectare = 10000 m^2, a Newton = 1 kg-m/sec^2, etc..

As much as possible, the standards which define the fundamentals are based on phenomena that can be accurately duplicated in laboratories around the world, e.g. the meter is defined in terms of a certain wavelength of light rather than the distance between marks on a bar kept in Paris. Thus the inevitable errors incurred in transferring a standard can be avoided.

Just because something uses a powers of ten sequence does not automatically make it metric. Your inch micrometer has markings for tenths and thousandths of inches but it isn't metric. Most money is subdivided into a hundred "pence" of some sort but it isn't metric.

The fact that something is expressed in metric units does not make it part of the METRIC MEASUREMENT SYSTEM. European thread standards are naturally expressed in metric units because they originate from nations using that system but you will find no mention of thread standards in the documents defining the metric measurement system. If you have a problem with metric threads, take it up with the agencies that set thread standards; don't use it as an excuse to condemn the metric system.

Some folks want to argue that the metric system is flawed because the length standard it uses is "wrong". The French set out to make the meter one ten-millionth of the distance from the Earth's equator to the North Pole. Their technique for measuring this quantity was inspired and they did an admirable job considering the tools available to them at the time. Still, the value they obtained was very slightly in error. But, NONE OF THIS MATTERS IN THE LEAST. All measurement systems are based on a choice of some arbitrary standard. Whether it's the length of the King's arms or the distance traveled by light in a vacuum in a given period of time, it's the fact that everyone agrees to use that length that's important and the real test of a measurement system is how well it satisfies the criteria outlined at the beginning of this text. THE NOTION OF "ACCURACY" OF A MEASUREMENT SYSTEM IS COMPLETE NONSENSE. Accuracy is a function of the measurement tools and techniques, not the system in which the measurements are expressed.

Another argument to avoid is the "we put a man on the moon using..." farce. Everything was built using the measurement system in force at the time. The ancient Egyptians built the pyramids using the cubit (and had problems with multiple, competing definitions) but that's not an argument for returning to their system. If you doubt me, try working a problem using Egyptian fractions. Building achievements are the product of the genius of their designers and fabricators, not the markings on their rulers, scales and buckets.

An insidious so-called argument to avoid is the exceedingly narrow-minded idea that only the aspect of a measurement system you personally use is a proper basis for deciding which measurement system is best for the society at large. It goes something like this...

I don't need another system to machine my parts. The difference between inferial and metric is just a factor of 2.54, why bother changing; I can make accurate parts using my present inferial system.

There might be some validity to this approach IF THE JOB OF EVERYONE IN THE SOCIETY INVOLVED NOTHING MORE THAN MAKING LINEAR MEASUREMENTS. People have many jobs involving all the aspects of the measurement system. Using one that's antiquated, un-necessarily complex and confusing leads to inefficiency, lost time and dangerous mistakes.

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Nomenclature systems are not a part of measurement systems but they deserve mention here. A sensible nomenclature system should satisfy the following criteria...

* Should provide information about the thing being named. A #43 or size 'K' drill carries no information about its most important feature, the size hole it will drill. This requirement pretty much means that things be labeled by their size in the measurement system in use. A 6 x 1 metric screw tells you OD and pitch directly; a 6-40 inferial screw makes you work to get the same information.

* Should have an intuitive progression. Smaller names should correlate with smaller entries in the progression; larger names with larger entries. Labeling wire with a number corresponding to how often it's been through the drawing dies may be useful on the wire mill floor but it should never be let loose in the real world where people only care about its diameter.

* Should be open-ended so that a new item larger or smaller than the original set can be sensibly named. This avoids the idiocy of things like 000-120 screws and AAAA batteries. What do you do if you want to add a drill slightly larger than a 'Z'? Again, labeling by actual size avoids most of these problems.

[SteveM]

Marv,

Thanks for those bits of information. Very interesting and informative.

BTW, don't try ordering a footlong sandwich at this place:


Steve

[SteveHGraham]

You can tell Marv is a physicist. Marv, did you have to learn physics using the imperial system? That would certainly explain your rage. I rarely saw imperial when I was studying, but I have seen old texts with slugs and feet per second.

My E&M prof told the class about calculating dimensions for waveguides using imperial. Gives me the willies to think about it.

[mklotz]

You can tell Marv is a physicist. Marv, did you have to learn physics using the imperial system?

Most everything was in metric but, for comic relief, the profs would occasionally do a calculation in inferial.

The mental massturbation (deliberately misspelled to defeat the prudish PC monitor) required to understand slugs was a real treat.

I've long contested that, were the citizens of the USA to really adopt the metric system, they would have two measurement systems, neither of which they understood. To demonstrate my point, I made up two problems...

Muzzle velocity of a gun is typically measured in ft/sec; bullet weight in grains. Muzzle energy is quoted in ft-lbs. This leads to an equation of this form...

Energy (ft-lb) = (muzzle velocity (ft/sec))^2 * bullet weight (grains) / K

where K is a constant. Your job is to calculate K and show your work to justify how you arrived at your answer.


Without using references, internet, pencil and paper or a calculator, calculate the number of gallons of water in an acre-ft of water.

The equivalent metric problem (liters in a hectare-m) can be done easily in one's head.

Note that neither of these problems involve any of the squirrely units of the inferial system, e.g. furlongs, firkins, jeroboams, etc.

[SteveHGraham]

Machinists have a very limited view of the horrors of the imperial system, because virtually all of the units machinists deal with are units of length, and that almost always means the decimal inch. Working with the decimal inch is not that bad; after all, it was adopted in order to ape the metric system. When you start getting into grains, drams, acre-feet, slugs, nautical miles (not statute miles), pecks, pennyweights, and Troy ounces, you realize the true extent of the imperial insanity.

I reload ammunition, and I have a grain scale. I have no idea what a grain is. From time to time, I look it up, but I always forget it again, because it's a nearly useless unit from an asinine measuring system. If gun people had any common sense, they'd use milligrams.

Here's a great riddle I would have gotten wrong if someone hadn't clued me in: which weighs more? A pound of gold, or a pound of feathers? The answer is a pound of feathers. A Troy pound of gold weighs 5760 grains, and a pound of feathers weighs 7000 grains.

Interesting note for everyone: the inch is actually a metric unit. For about half a century, the inch has been exactly (to the nanometer) 2.54 cm. I wish they had had the guts to make it 2.50; it would make lathe dials and screws easier to build. The yard and pound are also metric (international yard and pound). If you're an imperial holdout, you lost the war in 1959 and you probably don't even know it.

https://en.wikipedia.org/wiki/Internati ... _and_pound

Imperialists are like vegetarians. With the decimal inch, they admitted defeat. The world is full of gross vegetarian food tarted up to taste (not very much) like meat, but no one ever tries to make meat taste like celery. The decimal inch is machining tofurkey.

[mklotz]

Interesting note for everyone: the inch is actually a metric unit. For about half a century, the inch has been exactly (to the nanometer) 2.54 cm. I wish they had had the guts to make it 2.50; it would make lathe dials and screws easier to build.

If they had made it 25.6 mm all those stupid fractional dimensions wouldn't be irrational numbers when converted to metric.

17/64 * 25.6 = 6.8 mm

Of course, if you're going to revamp the inch, why not throw the whole mess out and use a sensibly constructed system.