Deviations in metrication?

Date 07-17-2007 13:36

A terrific question jon, of which I haven't got a clue. I really hope this one kicks off a good discussion. I'd like to know too.

By aevald

Date 07-17-2007 14:57

Hello Jon, I chuckle many times when the metric system is discussed in application here in the U.S. and other parts of the world. A few examples: we have a u.n.f. and u.n.c. system for most of our fasteners, coupled with a fractional sizing system, most would argue that this is a rather confusing system and doesn't make much sense. Yet, when you look at a metric fastener system they have what I call an inverse problem, you can have a 12mm fastener, but it can have a 1.75, 1.50, 1.25, 1.00, or possibly a .75mm pitch. So if you're looking for simplicifation does that promote a simpler system? I don't think so. I could be ignorant on this a bit so I'll include that this particular example may be a U.S. induced variation to metrics.
The reason that I bring this up in the first place has to do with the many times that a person goes to work on something and has to select a fastener to perform a task. With the U.S. system, you can generally look at the situation and make a reasonably good assessment of whether you will need a n.f. or n.c. type fastener, and most, after a bit of experience, can make a pretty good judgement on whether the fastener is 3/8", 1/2" or otherwise. With a metric system in a like fashion, an individual can probably tell with a bit of experience whether the fastener required is a 10mm or 12mm or something else, however, I feel fairly confident that you may not be able to tell with a great deal of consistent accuracy whether the pitch required can be readily discerned without the use of pitch gauges or other special tools, as some sizes can have possibly five different pitches applied. These are just a few thoughts of mine on metrics. I too look forward to hearing the different takes on your question. Regards, aevald

By jon20013 (*****)

Date 07-17-2007 16:04

Thanks Aevald. I know, I was a Secretary for the C4 Committee for a couple of years and there was lots of discussions on how to "standardize" torch tip sizing also... not a simple issue but any means. Same holds true for filler metal sizing and classification systems.

By bozaktwo1 (***)

Date 07-17-2007 16:54

Just a minor observation from my little corner of the manufacturing world: We build a lot of support equipment for older projects, such as C-130. Everything about a C-130 is measured in standard. Therefore, a foreign customer ordering our equipment might be inclined to order something in the wrong size because the dimensions were misinterpreted.

As far as fasteners go: We build a specific piece of equipment that was recently adopted by a foreign military customer, and part of the redesign involved replacing almost all the fasteners with metric. We never expected the lead time to be so outrageous for these items! Along with that, the cost is amazing.

Now, in my way of thinking, true global standardization is right next to impossible. C-130 is just one of thousands, probably millions of product lines that were originated in English/American standard; there's no way to convert everything to metric, so each product would have to be redesigned from the ground-up...or scrapped for new construction, and try telling that to someone who runs 100 C-130s!

By pipewelder_1999 (****)

Date 07-17-2007 17:00

As a person born with 10 fingers and toes I have often wondered why we are still hesitant to change. I don't understand what there is that can be confusing. Though I am NOT very good at t elling you how long or big something is in the metric system, I sure can add it to other dimensions and divide it up easily.

I am not familar with what is going on in the industry. What are the "variations" that you speak of? I figured the metric system is standard.

Thanks

Gerald

By aevald

Date 07-17-2007 18:01 Edited 07-17-2007 18:05

Hello Gerald, let me take a stab at explaining maybe just one small thing that has a lot to do with the welding industry. If you take a look at drive roll systems for welding machines for instance. A knurled set of rolls or a V-groove set of rolls wouldn't be affected a lot by variations from our American system of call out for wire diameters versus a metric diameter standardization. .045 U.S. diameter wire is called out as 1.2mm, a true metric type of system would likely change that diameter to 1mm period, if you look at a U-groove type of roll system designed for that same .045 diameter wire and then possibly tried to use it on a new metric designation of 1mm wire it might not work so well. I believe this is the type of thing that could bring up some real issues and problems in conversion of systems.
In a sense the metric system and the U.S. decimal system share a common structure in using 10's, 100's, 1000's, and 10,000's etc. for division and specification of measure. Obviously where they differ greatly is in the feet, yards, miles, etc. versus the millimeters, decimeters, centimeters, and other like measures.
I believe bozaktwo1 hit on the real issues when he brought up the problems of trying to incorporate metric replacement parts for U.S. standard type equipment and other items. You might just have to scrap things and start over to institute those types of changes. As with many things however, niche markets have been created by doing just those sorts of things and I would suspect that might be the very case should a complete conversion ever take place. Just a few more $.00's Regards, aevald

By jon20013 (*****)

Date 07-17-2007 18:28

Here's another example:

A number of variations on the metric system have been in use. These include gravitational systems, the centimetre-gram-second systems (cgs) useful in science, the metre-tonne-second system (mts) once used in the USSR and the metre-kilogram-second system of units (mks) most commonly used today.

The current international standard metric system is the International System of Units (Systeme international d'unites or SI) It is an mks system based on the metre, kilogram and second as well as the kelvin, ampere, candela, and mole.

The SI includes two classes of units which are defined and agreed internationally. The first of these classes are the seven SI base units for length, mass, time, temperature, electric current, luminous intensity and amount of substance. The second of these are the SI derived units. These derived units are defined in terms of the seven base units. All other quantities (e.g. work, force, power) are expressed in terms of SI derived units.

By HgTX

Date 07-17-2007 19:18

The U.S. hasn't converted because manufacturers don't want to retool and there's enough of a domestic market that they don't have to. That's pretty much it. There's not all that much variation out there within SI-based systems.

Hg

By jon20013 (*****)

Date 07-17-2007 19:27

I agree, there's not that much variation WITHIN the SI based systems and if the entire world were using the same units it would be a no-brainer but thats my issue and hoping to hear some differences experienced from the members regarding units of measurement, etc., from OUTSIDE of the SI based systems.

By thcqci

Date 07-17-2007 21:42

I agree, tooling is no doubt the overriding reason we don't convert with Americans wanting to do it our way a close second. In my past life in calibrations, the SI system rules. It is definitely easier to use and has definitive definitions of units. Simple examples are pounds. Pounds what, force or mass? Tons, short or long? Ounces, troy or avoirdupois? SI is kilograms for mass and Newtons) for force. No confusion. Calculations are way easier. 10 is way easier to multiply and divide by than 25.4 or 2.2939. In general terms, 1000cc of water at 20°C has a mass of 1kg and a volume of 1L.

FYI. NIST, has several documents with high precision conversions.

http://ts.nist.gov/WeightsAndMeasures/Metric/mpo_pubs.cfm
http://physics.nist.gov/Pubs/SP811/contents.full.html
http://ts.nist.gov/WeightsAndMeasures/Metric/upload/SP1038.pdf

By 803056

Date 07-17-2007 21:54

Years ago I took a physics course where the professor introduced the SI system of measures. Of course there were moans and protestations from those in the class. The professor took it in stride and never missed a beat of his lecture and simply added that he had no preference whether we used the customary units of measure or the SI. It was entirely up to us. Within three days the entire class was using SI units and the professor never gave us one "I told you so."

Best regard - Al

By HgTX

Date 07-17-2007 22:11

Just how much non-SI "metric" have you gotten the impression is out there? Meters, centimeters, kilometers, millimeters, grams, kilograms, milligrams, liters, milliliters, there aren't different versions of these out there. If some countries have by convention still a few non-metric units remaining (like stones and furlongs), that's neither part of nor a reflection on the metric system. It's not like you have the German centimeter vs. the French centimeter vs. the Egyptian centimeter.

Hg

By Lawrence (*****)

Date 07-17-2007 22:46

Egyptian centimeter...... I beleive that was the distance equivilant to the span of the Pharoah's pinkie toenail.
>G<

By jon20013 (*****)

Date 07-18-2007 00:09

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.

Comments?

By HgTX

Date 07-18-2007 18:12

Some variation in how people choose to combine the units doesn't change what the units are. There is NO confusion about what a kilogram, a meter, or a second are. Or a centimeter, or a gram. On the other hand, an ounce or a pint can mean different things in different contexts.

Not everything on the list you copied from Wikipedia is a current concern and source of confusion--like a system in use in the Soviet Union before 1955. Or the gravitational systems--are those widely used enough to become a source of confusion? They're in your wikipedia list *below* the pre-1955 Soviet system, so I'm thinkin' they're not all that popular.

It is simply not the case that U.S. industry as a whole didn't want to convert because they were afraid of confusion between kg/mm^2 and MPa (both easily understood for what they are, as compared to the relationship between cubic feet and pints) or because some physicists in particular fields use angstroms and blood pressure (another very specific application) is measured in mm Hg or because some English-speaking countries maintained a couple of weird conventions for tradition's sake like discussing people's height in feet or weight in stones.

"Gosh, I don't know whether to say one meter or one hundred centimeters" is *not* the reason not to convert. Anyone living with the metric system understands full well that there are multiple ways to express the same measurement--just as we can handle the concept that 100 yards is the same thing as 300 ft. U.S. industry as a whole didn't want to convert because it was going to cost them $$, more $$ than they thought they might gain through the added simplicity of working with a base 10 system (or with the same system in use in 99.44% of the rest of the world). Since people in the U.S. have relatively little contact with other countries (compared to people in said other countries), they're not feeling the pressure to move on, and they're not getting exposure and familiarity.

That's it. Economics and social comfort.

Hg

By jon20013 (*****)

Date 07-18-2007 19:20

Thanks hg, I do realize it's largely a matter of economics and agree with you in that matter but (remember, I'm not claiming to be anywhere close to an expert on this, thats why I'm asking) it's still my opinion that the "localized" differences around the world that remain as strongholds against strict compliance with SI remains a big problem.

I hope the forum members don't think me just being a smart a$$, I am really trying to fully grasp the situation. I also know for a fact that there is large disagreements between "our" manufacturers and SI compliance manufacturers over how to nicely merge and simplify some of the terminolgies and units.

By makeithot (***)

Date 07-18-2007 23:27

I personaly can't wait for the day that everyone is on the metric system, having been forced to learn both I will argue that metric is not only easyer to add,subtract,divid,etc but is alot more accurate. The learning curve is also alot shorter. the base units are the base units and interpertation and which one you use is up to whatever secter you work in. Sadly though we continue to use both systems in Canada which is a real pain in the ars because a guy has to carry around two sets of tools, If our brothers to the south would only pull their head out of the sand long enough to see the advantages and the simplicity of the system life could be that much simpler. From the shop floor point of view I can go from mm to cm to meters and back alot faster then 1/32 to 1/16 to 1/8 to 1/4 to 1/2 etc haven't even got to an inch yet and if I want to convert from one to the other well you know that story. Just like the Robson screw not made in America how good could it be , but everyone that has adopted it, has also increased plant production by at least 30% you do the math. So the arguement that re-tooling will cost to much, is really getting old. Most likely not hard to see where I'm going with this one. But on a happy note you cannot belief the satisfaction one gets from useing robson screws in every piece of gear of we send south.

By DaveBoyer (*****)

Date 07-19-2007 03:40

When I was working for the Dana auto frame plant We built the frame tooling to the metric frame prints while the tool drawings were in inches and all the machine tools were graduated in inches allso. This is not an insurmountable problem. I don't buy the argument that metric is more acurate, but do agree that it is simpler, and errors may be less frequent. Re tooling is extremely expensive when You are talking about machine tools, but modern CNC machinery will generally accept inch or metric, so as older machines get replaced this problem will eventually largely resolve itself. We used inches and decimal inches, and it is expected that everyone knows decimal equivilence of fractional inches to the 1/16s at least.

By HgTX

Date 07-19-2007 16:45

So far, no one else is chiming in with how the distinction bewteen using centimeters and millimeters has been an obstacle for adoption of a metric system. People from various metric countries do fine with each other, far better than they do trying to deal with our system. Slight variations in convention are not a big deal; learning something based on 32 of this and 16 of that and 62.4 of this and 5280 of that is a problem.

I've lived in one metric country, dealt with lots of people from other metric countries, and the fact that people still measure time in hours is not a "big" problem as far as "big" problems go, especially put in the perspective of the enormous variations in non-metric systems. Remaining exceptions to the metric system are not a problem with the metric system but with not using the metric system. As long as someone is using a unit that is based on the units and prefixes of the SI, or something historical but with a clearly understood meaning such as "hours" or, within its own proper context, "mm Hg", there is no "big" problem, any more than the question of whether or not to draw a 7 with a horizontal line through it is the reason the Arab world has not adopted the so-called "arabic" numeral system (0-9).

You may have to accept that your metric friend is right, unless you can find other evidence to bolster your case. The way to retool metric is to pick whatever version of the system your biggest metric competitor uses, and use that. Once you've retooled to something based in mm and g, it's really not that big a change to adjust later paperwork to m and kg, if it comes to that.

Hg

Hg

By jon20013 (*****)

Date 07-19-2007 17:18

Hg, you seem to be correct. I guess it really does boil down to simple economics rather than deviations within the SI and non-SI system itself. Thanks for your input. Lesson learned, we can't always be right!!!