Unit conventions (SI versus others)

[Mark44]

If we go about 6 MPH, we can do about 30 acres per hour....

Yeah, but what would that be in square furlongs per fortnight? (I.e. in $\frac{\text{furlong}^2}{\text{fortnight}}$)

 

[DennisN]

This is best answered by The Frantics.

 

[DennisN]

I did not see this image in the thread, so I'll go right ahead and post it... (even though "The rest of the World" is not entirely accurate)

Bonus joke:

 

[SW VandeCarr]

I did not see this image in the thread, so I'll go right ahead and post it... (even though "The rest of the World" is not entirely accurate)

Yes, but none of this is worth damaging the great institution of AMERICAN FOOTBALL. As I said in post 35, we cannot have 9.144 meter intervals nor can we have a 100 meter long fields. The game is sacrosanct. Nor is it acceptable to make an exception for football by allowing the game to keep its beloved "yard" That will cause gas stations, supermarkets, horse traders and who knows who else to demand exceptions. Our Congress cannot resist making exeptions to the point where it might even abolish the metric labels that already exist on canned soup.

 

[DennisN]

Yes, but none of this is worth damaging the great institution of AMERICAN FOOTBALL. As I said in post 35, we cannot have 9.144 meter intervals nor can we have a 100 meter long fields. The game is sacrosanct. Nor is it acceptable to make an exception for football by allowing the game to keep its beloved "yard" That will cause gas stations, supermarkets, horse traders and who knows who else to demand exceptions. Our Congress cannot resist making exeptions to the point where it might even abolish the metric labels that already exist on canned soup.

Hmm, maybe the president could ask Congress to redefine the conversion to 1 yard = 1 meter. Just an idea. (sorry, I could not resist )

 

[olivermsun]

I did not see this image in the thread, so I'll go right ahead and post it... (even though "The rest of the World" is not entirely accurate)

It's ironic that the graphic on the right is subtitled 'Logical Smooth Sailing,' since SI units are actually not very convenient for that! (Try looking at navigational charts and you'll know what I mean.)

Bonus joke:

 

[WWGD]

Would be interesting to know, where Joule is actually used instead of calories. I mean, they don't get even this right, as usually all talk about cal and mean kcal. And I can't remember a correct weather report. I usually don't get understood very well, if I complain about temperatures above 300. But °C is SI you could argue. Well, that's right, although not logical, but e.g. °mC isn't allowed here. I vote for Delisle!

P.s.: I counted 89 (sic!) different miles on the Wiki page!

This miles : https://en.wikipedia.org/wiki/Myles_Standish is called "Kilometre Deboutish" In France *

* Not my original.

 

[WWGD]

I don't understand the benefits. Scientists and engineers are already free to use SI if they want, and so are manufacturers. So what are the benefits?

A very minor one: in supermarkets, in order to compare products you must compare gallons with pounds, ounces, etc. Some products include measures of both volume and weight, maybe others. So , unless you know the density and can do things in your head, or you carry a calculator, it becomes a mess.

 

[Dr.D]

People forget that the SI is mainly a practical system for use in our everyday lives (which is why the Candela is a base unit) ; meaning practical considerations are more important that what is most satisfying from a philosophical point of view.

0000

That's really funny, LOL! The unit sizes, in many cases, are completely absurd. Measure a small distance in meters (say the diameter of a pencil); much easier in inches. Measure your weight in Newton? Its already too much in pounds. Measure your tire pressure in Pascals? It takes 100000 of them to get up to atmospheric pressure.

Sure, I know about prefixes. Handy stuff like, for example, a (kilo-)^2 in the numerator will cancel a mega- iin the denominator.

Please don't try to sell SI based on practicality; it does not work.

 

[fresh_42]

Please don't try to sell SI based on practicality; it does not work.

I prefer a metric system over any of such absurdities like inches and miles. Do you know how many versions a mile has? Dozens! The centimeter and the kilometer do far better jobs than inches and miles. It is practical. The reason that some refuse to use it cannot be accounted as an argument against practicability. And if you don't like Pascal, then use bar, it's that easy. MKSA is a reasonable system and it is proven practical for the large majority of countries.

 

[davenn]

Please don't try to sell SI based on practicality; it does not work.

it does work and very well

I prefer a metric system over any of such absurdities like inches and miles. Do you know how many versions a mile has? Dozens! The centimeter and the kilometer do far better jobs than inches and miles. It is practical. The reason that some refuse to use it cannot be accounted as an argument against practicability. And if you don't like Pascal, then use bar, it's that easy. MKSA is a reasonable system and it is proven practical for the large majority of countries.

Hear Hear ! totally agree

 

[jtbell]

People forget that the SI is mainly a practical system for use in our everyday lives (which is why the Candela is a base unit) ; meaning practical considerations are more important that what is most satisfying from a philosophical point of view.

That's really funny, LOL! The unit sizes, in many cases, are completely absurd.

I think "practical" is better understood here as "practical to accomplish precisely and repeatably in the laboratory." It would be appealing from a philosophical or aesthetic point of view to define the unit of charge in terms of the charge on some number of protons or electrons, but that requires some way to count the number of protons or electrons in a sample.

Those who want elegance can use Gaussian units instead, as theorists in fact generally do, in which the unit of charge (the statcoulomb) is defined using Coulomb's law, by setting the proportionality constant to 1 instead of the "wierd" 1/4πε₀.

Any size unit is going to be "practical" in terms of its numeric values only in some contexts.

 

[sophiecentaur]

Any size unit is going to be "practical" in terms of its numeric values only in some contexts

That's very obvious but it clearly needed saying. Not many people have problems with c,M,k,m and μ etc..
Something that does make me smile is how the odd intermediate multipliers like d and D are used in some businesses - just because they were established a while ago. And then there's Angstroms. How can that possibly be a serious unit these days?

 

[davenn]

Angstroms. How can that possibly be a serious unit these days?

indeed, but astronomers and a few other still seem to have a fond attachment to it
All these optical filters, like my solar filter" are all rated with an Angstrom bandwidth rather than using nanometres

 

[fresh_42]

indeed, but astronomers and a few other still seem to have a fond attachment to it
All these optical filters, like my solar filter" are all rated with an Angstrom bandwidth rather than using nanometres

It is still a meter, so the confusions are limited. AU, ly and kpc are not, but also in use and nobody complains. Maybe it's a bit of a silent revenge, because Americans can't write Ångström.

 

[sophiecentaur]

It is still a meter, so the confusions are limited. AU, ly and kpc are not, but also in use and nobody complains. Maybe it's a bit of a silent revenge, because Americans can't write Ångström.

Americans would like the distances in yards, probably.

 

[sophiecentaur]

indeed, but astronomers and a few other still seem to have a fond attachment to it
All these optical filters, like my solar filter" are all rated with an Angstrom bandwidth rather than using nanometres

An astronomer friend of mine explained that Angstroms were and are used in Xray Crystallography because Crystallographers can't use decimal points. I suppose that 10^-10 would have seemed to be a nice round figure to work with, at a time when Micro was about the smallest prefix in common use.

 

[davenn]

An astronomer friend of mine explained that Angstroms were and are used in Xray Crystallography because Crystallographers can't use decimal points. I suppose that 10^-10 would have seemed to be a nice round figure to work with, at a time when Micro was about the smallest prefix in common use.

ohhh …. that's something I didn't know …. cheers

 

[f95toli]

0000

That's really funny, LOL! The unit sizes, in many cases, are completely absurd. Measure a small distance in meters (say the diameter of a pencil); much easier in inches. Measure your weight in Newton? Its already too much in pounds. Measure your tire pressure in Pascals? It takes 100000 of them to get up to atmospheric pressure.

Sure, I know about prefixes. Handy stuff like, for example, a (kilo-)^2 in the numerator will cancel a mega- iin the denominator.

Please don't try to sell SI based on practicality; it does not work.

Sorry, but that is just silly. Prefixes is a part of the the SI; meaning the "magnitude" of the base units is pretty much irrelevant.
Moreover, "day-to-day" use is a very small part of what a system of units is used for; high accuracy and precision is mainly important in industrial applications as well as science/engineering where 1 part in 10^6 is fairly typical of what is needed in the calibration lab of a factory; you don't need that precision when inflating a tire.

All the units in the SI can -and are- used to calibrate equipment that spans many, many orders of magnitude. "1 Pascal" might seem like very small pressure for everyday use but is quite a high pressure if you are working with ultra-high vacuum (~10^-8 Pa) or acoustics ( ~10^-6 Pa) ; if you are doing work with a diamond anvil you can reach pressures of several hundred GPa. Hence, we frequently use the Pa in applications covering 20 orders of magnitude in pressure.
It should be fairly obvious that the same is true of for all the base units such as the meter.

Most calibration labs can calibrate over several orders of magnitude of for all the units they cover; simply because that is what industry -i.e the customers- needs.

 

[sophiecentaur]

ohhh …. that's something I didn't know …. cheers

It's only hearsay, remember.
Edit: PS not heresy!

 

[Dr.D]

Sorry, but that is just silly. Prefixes is a part of the the SI; meaning the "magnitude" of the base units is pretty much irrelevant.
Moreover, "day-to-day" use is a very small part of what a system of units is used for; high accuracy and precision is mainly important in industrial applications as well as science/engineering where 1 part in 10^6 is fairly typical of what is needed in the calibration lab of a factory; you don't need that precision when inflating a tire.

If that be true, why does the largest national economy in the world still hold to US Customary units? Surely practicality has a role in this choice. This is nothing really imprecise about an inch; it could be defined with the same approach as used for a meter.

Recently, I had the opportunity to visit the UK for the first time. I was fascinated to discover that this 100% SI country still posts point-to-point road distances in miles, gives road warnings (like a coming merge, intersection, etc) in yards, and yet they sell gasoline by the liter. If SI is so very practical, why isn't the country, that has completely adopted the SI system, fully accepted it ?

I started college in the late 1950's, shortly after the Russians put up the Sputnik. There was a general panic in US technical education, with new textbooks using only vector notation, and an emphasis on the MKS system (SI had not yet been defined). I learned MKS, CGS, and US Customary (IPS and FPS) units. I've spendthe last 60 years teaching engineering and working in a variety industrial positions. I have used both USC and SI units as the need arose, but far more often things came to me in USC units. Occasionally, a problem would appear in mixed units, partly USC and partly SI. Then a choice had to be made, and I always found it best to convert as few items as possible to minimize the chance of a conversion error. What I found overall was that, for mechanics problems and everything related, the choice of units makes no difference at all if you use the given system properly. The same equations hold in any consistent system, so F = m*a is good in any consistent system of units.

I am constantly amazed at the passion with which SI advocates pursue their dream. What difference does it make? Oh, I know, it makes the US look like an out-lier, standing by ourselves. Well, so what? All that really matters is that we have a well defined system of units that covers all of the necessary things to be measured. Why must we all be bound to the same system of units? As long as we clearly communicate the system we use, that should be enough.

 

[fresh_42]

Why must we all be bound to the same system of units?

At least it would have avoided a giant crash on Mars.

 

[Dr.D]

At least it would have avoided a giant crash on Mars.

That crash could have easily been avoided by paying attention to what they were doing. There are any number of ways to foul up, and the only sure way to avoid them is to do nothing at all (and accept the consequences of that as well). Did you see my final sentence, "As long as we communicate the system we use, that should be enough." I suppose I should add to that the requirement that everybody be awake to what they are doing.

 

[Nugatory]

If that be true, why does the largest national economy in the world still hold to US Customary units?

Because it is the only one that is so large that cost of switching over the domestic economy exceeds the frictional cost of not being aligned with its trading partners. Even then, the more globalized sectors of the American economy have switched - you're not going to get very far wrenching on a new Ford or Chevy without a complete set of metric tools.

 

[fresh_42]

Did you see my final sentence

I did, but lack of communication is almost always the basic reason for mistakes. My main argument against miles is: which one? Those yards, miles and inches are simply so old, that it appears as if we still had a prototype attached on each town hall, and of course in each town a different one. Mile is simply not unique (Wikipedia lists 90) and I guess it's similar with inches.

 

[rootone]

SI units are just more sensible.
For example the Centigrade scale of temperature has 100 units of difference between the freezing and boiling temperature of water.
(Yes I know it is better defined now, but still same idea.)
What is the point of something like Farenheight degrees which use 212 units for no particular reason?.

 

[Nugatory]

Mile is simply not unique (Wikipedia lists 90) and I guess it's similar with inches.

The American Standards Institute has defined the inch to be exactly 2.54 centimeters - it's been that way since at least the second world war.

This definition has the nice property that if your gearset includes a 127-tooth gear you can use the same lathe and tooling to cut both metric and inch threads.

 

[Tom.G]

defined the inch to be exactly 2.54 centimeters - it's been that way since at least the second world war.

Almost.
https://www.britannica.com/science/inch
Since 1959 the inch has been defined officially as 2.54 cm.

 

[DrDu]

I am constantly amazed at the passion with which SI advocates pursue their dream. What difference does it make? Oh, I know, it makes the US look like an out-lier, standing by ourselves. Well, so what? All that really matters is that we have a well defined system of units that covers all of the necessary things to be measured. Why must we all be bound to the same system of units?

Maybe the buzzword "globalization" is the correct answer, here. Outside US and GB, since French revolution, at least meters and kg are a standard and nobody complains about it. Nevertheless, all computer screens are labelled in inches and pressure on car tyres in pound force per square inch. Obviously, the pound in "PSI" is not 0.5 kg, as in the rest of the world, but 0,453 592 370 kg which makes conversion a snap when you have a flat tyre.
Do you remember the reactor accident in Chernobyl? It coincided with the change from old units of radioactivity like rem, Ci to SI based ones like Sievert or Becquerel. The resulting chaos in communication did much harm to the credibility of the involved scientists and radiation specialists. I was very astonished when more than a quarter of a century later, the same mix of units appeared after the catastrophe in Fukushima.
If you got the impression that I am a big fan of the SI system in general, you are wrong though. Especially as far as radiation protection is concerned, the system is ridiculous. Why is Bq not called Hz? And why do we have to different units (Si and Gy) for two types of doses, although they both are J/kg? Apparently, radiation scientists haven't learned much since babylonian times.

 

[f95toli]

If that be true, why does the largest national economy in the world still hold to US Customary units? Surely practicality has a role in this choice. This is nothing really imprecise about an inch; it could be defined with the same approach as used for a meter.

Recently, I had the opportunity to visit the UK for the first time. I was fascinated to discover that this 100% SI country still posts point-to-point road distances in miles, gives road warnings (like a coming merge, intersection, etc) in yards, and yet they sell gasoline by the liter. If SI is so very practical, why isn't the country, that has completely adopted the SI system, fully accepted it ?

Firstly, USA adopted the SI a very long time ago (I can't remember the year, but >100 years). If you talk to someone at NIST you will find that everything they do internally (including calibration for customers) uses the SI; if they are asked to calibrate something in e.g. inch they will calibrate in meters and then convert using a defined constant (which is why the inch is defined via the meter).
Again,. what you as the "end-user" end up using is not really that important since you do not need a consistent system of units whereas an National Measurement Institute (like NIST) does. The "everyday" units in the USA are a mixture of customary units for length and weight (and derived units) and SI for everything else (second, mole, Ampere, Candela). Hence, the US does NOT have its own system of units; it i part of the SI just as every other country on the planet.

The situation here in the UK is a bit weird. The SI is used WAY more than most visitors would realize. The old units are basically used for transportation (distances between towns, speed limits), amounts of beer (pints) and the human body (length in feet and weight in stones; although the SI is of course used in hospitals etc), but the SI are is used for everything else (including in everyday use); you would certainly never use old units in engineering or science. Moreover, the UK started switched to teaching the metric system in schools in the 70s meaning people under the age of 50 are likely to use SI for just about everything (with the exceptions listed above). If you meet someone older than that they might ask you how many square feet your house is (which I wouldn't know).
Also, unless you drive it is perfectly possible to get manage without EVER having to use old units (my GP won;t mind if I tell them how tall I am in cm since this this is what they would record anyway).
The UK will -probably- change completely the the SI eventually, I would expect people's length and weight to become "metric" quite soon (my step-son certainly uses cm and kg for this). However,. replacing all road signs and speedometers is a logistical nightmare so that won't happen anytime soon (although signs showing both units are becoming more common for things like maximum heights(

 

[sophiecentaur]

I was fascinated to discover that this 100% SI country still posts point-to-point road distances in miles,

And fuel is supplied in litres so mpg no longer means anything! And we still drive on the left.

What is the point of something like Farenheight degrees which use 212 units for no particular reason?.

Two particular reasons, actually. 0°F is the temperature of a frigolithic mixture of ice, water, and ammonium chloride and 32°F is Defined by the freezing point of water (a fixed point). Rumour has it that Fahrenheit chose his own body temperature to be 100°F. Near enough to be believable perhaps. Maybe he was just hot blooded. 212°F would had sort of hung on the end as a consequence of that. I am not justifying it at all but someone must have been thinking it through somehow.

 

[A.T.]

Measure a small distance in meters (say the diameter of a pencil); much easier in inches.

How is inch easier than cm or mm here?

Please don't try to sell SI based on practicality; it does not work.

It is easier to convert mm to km, than inch to mile, because we use the decimal system.

 

[sophiecentaur]

It is easier to convert mm to km, than inch to mile, because we use the decimal system.

When I was at primary school (ten years of age), we were taught to do "Compound Practice". This consisted of what was effectively, mixed-base arithmetic and was used to solve problems like "what is the cost of 8cwt, 2qrs, 1st, 5lb of cement at 5s6d per stone." This was achieved without converting the weight to pounds and the money to pence. A nightmare and we moved straight into the gram centimetre system in secondary school so it was a total waste of time.

 

[russ_watters]

As an American engineer, my biggest unit annoyance has got to be the anachronistic fractions. We've mostly converted to decimals (spreadsheets, instruments), but still have to scale drawings at 1/4", 3/16", 1/8" or 3/32"=1'

 

[russ_watters]

I know it is old, but since the thread is resurrected;

In the USA, the SI system is already used where it is convenient and not used where it is not.

I don't agree with that. From what I see, the IP system is primarily used where customary, even where it is not convenient, such as in the scaling example I gave above. Or, rather, where the inconvenience of using it is not greater than the inconvenience of switching, as previously pointed out.

Using base-2 fractions probably had a practical purpose at one point, but today it doesn't outside of computer programming. While it looks silly, there isn't much pain in a 3/32" scale drawing or a triangular cross section scale ruler, so we still use it. But there's just no way I'd use that in a spreadsheet over base-10 unless absolutely forced.