Unit conventions (SI versus others)

[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.

 

[russ_watters]

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.

As a mid-career American engineer who has had to use both since elementary school, it is difficult for me to see how you could not see the objective superiority of the SI system. If you were designing a system from scratch, would you not use a system with fewer units and fewer unique conversions? Isn't there objective superiority in that setup of a system?

I respectfully submit that what you are perceiving as the objective superiority of the IP system is just your comfort with having used it preferentially for so long...which also largely answers the question of why the USA hasn't fully switched.

In the examples you gave, though, it almost seems like you are purposely misusing/abusing the units.

 

[russ_watters]

It also might be instructive to think of specific tasks or situations where you use SI or IP and should use the opposite base number system, to illustrate the power of custom and conditioning over practicality.

If I need to estimate, for example, a static pressure for an HVAC system, I might say "three quarters of an inch" even though I know (if I put a second thought into it) that the control system and the balancer's digital manometer use decimals and I've just made other peoples' lives harder by not rounding it.

But how much does a soda cost? "Three quarters of a dollar"? No, it's "seventy-five cents."

So, SI-only people; do you ever say "three quarters of a kilometer"? Or is it always "seven hundred fifty meters"? For driving, IP people use both: "three quarters of a mile" or "seven tenths of a mile" are both common.

 

[fresh_42]

Oh, I know, it makes the US look like an out-lier, standing by ourselves. Well, so what?

I'm the first in line to defend cultural specialties, as they make our world colorful and interesting. However, I doubt that a measuring system should be part of it. It's simply far easier to use the same conventions instead of country by country systems. And it would be cheaper. It's similar to the calendar. Personally I would go even further as I consider the entire concept of nationalities outdated, but that's a different discussion.

To appreciate different kinds of music or food, to dance in different ways and to speak different languages is one thing, to measure a length is in my opinion not worth protection.

 

[Dr.D]

As a mid-career American engineer who has had to use both since elementary school, it is difficult for me to see how you could not see the objective superiority of the SI system. If you were designing a system from scratch, would you not use a system with fewer units and fewer unique conversions? Isn't there objective superiority in that setup of a system?

I respectfully submit that what you are perceiving as the objective superiority of the IP system is just your comfort with having used it preferentially for so long...which also largely answers the question of why the USA hasn't fully switched.

In the examples you gave, though, it almost seems like you are purposely misusing/abusing the units.

Russ, you asked if there is not an objective superiority to the SI system. I don't think so. After a long career working engineering problems in both systems, I don't see any difference at all.

I do not wish to claim that US Customary units are superior to SI; only that they are just as good and that there is no justification for demanding that we all use one or the other. When I started college back in the 1950's, I thought for sure that the US would adopt the MKS system within a decade. When I finally finished school in the late 1960s, I was even more sure. Yet when we look around us today, the history is obvious. I have a daughter who is a structural engineer (designing building structures), and I've discussed this with her several times. She works entirely in US Customary units, and would not touch a design in SI units. Why is this? Its because that is what people are accustomed to working with, and fewer construction errors result. I am pretty sure that a French structural engineer would see things the same way, only flipped. There is nothing evil in familiar units, and in fact, there is a lot of good.

Regarding your last comment, I really don't understand what you mean by "abusing units." Is using the base units for a quantity an abuse? I don't think so. The base unit is the basis for all multiples of that unit.

 

[diogenesNY]

I read a great deal about nineteenth century seafaring, and I have to frequently contend with distance described in terms of 'cable lengths' for which there are actually several differing definitions, somewhat dependent on context. They range from 185.2M ( 1/10 nautical mile) to 219.5M (120 fathoms - 720 feet) ... actually, the length of a fathom has tended to vary as well.

https://en.wikipedia.org/wiki/Fathom

https://en.wikipedia.org/wiki/Cable_length

The fun really begins when ships are described in terms of Gross Tonnage.

https://en.wikipedia.org/wiki/Gross_tonnage

The modern U.S. Navy supposedly has a whole (somewhat mythical) category of non-standard units of measure used by members of individual departments (e.g. describing how much of a liquid to pour in units denominated by 'glugs'.) Any retired Navy Chiefs here who could shed some light on this?

diogenesNY

 

[russ_watters]

Russ, you asked if there is not an objective superiority to the SI system. I don't think so. After a long career working engineering problems in both systems, I don't see any difference at all.

What about the example I gave, though? I'll be more specific: isn't a system where the conversion factors are 10, 10, 10, 10, 10 etc. inherrently simpler and therefore easier to learn and use than a system where the conversion factors are 12, 3, 1760 (5280), 16, etc.?

Following an example given earlier, I have no idea how big an acre is. I'd have to look it up in order to use it. Issues like that are much less likely to come up in SI because there aren't as many different units for the same thing. You mostly just have to know the one base unit and the prefixes.

I do not wish to claim that US Customary units are superior to SI; only that they are just as good and that there is no justification for demanding that we all use one or the other.

While I get/said practical issues have gotten in the way, the reason for standardizing is this:

I have a daughter who is a structural engineer (designing building structures), and I've discussed this with her several times. She works entirely in US Customary units, and would not touch a design in SI units...

Its because that is what people are accustomed to working with, and fewer construction errors result.

Inability or lack of confidence to read each others' work is a problem. Errors are a problem. Redudnant: Standardizing would eliminate errors caused by lack of standardization.

Regarding your last comment, I really don't understand what you mean by "abusing units." Is using the base units for a quantity an abuse? I don't think so. The base unit is the basis for all multiples of that unit.

Others largely addressed this, but yes, it's abuse. I think you know that what you said is not how people actually use the units because - as you correctly point out - it would be absurd. It's like you are saying a location is a mile away when in reality it is across the street and you purposely drove in the opposite direction and then in a circle to get there.

 

[gmax137]

isn't a system where the conversion factors are 10, 10, 10, 10, 10 etc. inherrently simpler and therefore easier to learn and use than a system where the conversion factors are 12, 3, 1760 (5280), 16, etc.?

Well, I'd say yes and no to that. How likely is the need for these conversions? How often do you hear "the store is a mile and a half up the road" and wonder to yourself, "let's see, how many inches is that?" It just doesn't come up in daily life.

OTOH, anyone who has had to do take-offs from isometric piping drawings knows how to hate fractions ("the total pipe length is 5 feet 6 3/8 inch plus 18 inch plus 4 foot 3 5/16 inch plus..."). I don't know why they still draw them that way, maybe its the triangular rulers...

Where I work the component design drawings are all in decimal inches (the steam generator tubesheet to nozzle dimension is 236.73 inches...). I do sometimes have to divide by 12 to get feet.

Rambling, but I think part of the reason the US didn't switch to metric is the installed base of inch-based manufacturing machinery (lathes and mills, etc) at the end of WWII. Most everyone else had to buy new machines, where the US factories had not been bombed. Mentioned above, the 100/127 tooth gear combination let's you turn metric threads on an inch lathe, but it still isn't as straightforward as a metric lathe because the leadscrews are 8 threads per inch, not 2 mm (or whatever) pitch.

One last thing, also mentioned above, was the "weight in kg" thing. Please don't preach the purity of the SI system and then tell me "I've lost weight, down to 68 kilos" -- when you guys start reporting your weight in Newtons then I will start listening ha ha.

 

[russ_watters]

Well, I'd say yes and no to that. How likely is the need for these conversions? How often do you hear "the store is a mile and a half up the road" and wonder to yourself, "let's see, how many inches is that?" It just doesn't come up in daily life.

Constantly (and I see you provided an example of your own, later). Not that specific example, perhaps, but most people who don't do much math see issues like it every day (I weigh 160 lb and just drank 12 oz of soda...). For engineers (depending on the industry), many times a day. Calculations we could do in our head we have to use a calculator for because of the units, all the time.

One issue I see almost every day is whether to measure the dimensions of a room in feet or inches, and converting when multiple people on the same project aren't following the same convention. It may not be a BIG problem, but it is an issue that simply wouldn't exist if we used meters.

Another one I use a lot: CFM and inches of water gauge to horsepower (to watts). I have the rolled-up constant(s) for that memorized, but for SI you don't need one/them. Not having the weird unit conversions memorized is the type of thing that causes good engineers to fail the professional engineer exam!

One last thing, also mentioned above, was the "weight in kg" thing. Please don't preach the purity of the SI system and then tell me "I've lost weight, down to 68 kilos" -- when you guys start reporting your weight in Newtons then I will start listening ha ha.

I was going to give you that one, but I'm not sure if in other countries they tell each other their mass or if they say weight like we do. It's possible the mix-up comes from us (the US or UK).

 

[jbriggs444]

I was going to give you that one, but I'm not sure if in other countries they tell each other their mass or if they say weight like we do. It's possible the mix-up comes from us (the US or UK).

Body weight is a mass measurement and is properly reported in pounds mass or kilograms mass. Anyone who thinks it should be reported in Newtons has taken one first year physics course too many.

 

[russ_watters]

Body weight is a mass measurement and is properly reported in pounds mass or kilograms mass.

I agree. My question was why we call it "weight" and whether Europeans do too.

 

[Dale]

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.

Sure. The inconvenience of fighting long established customs is part of what I mean by “convenience”. If a manufacturer has a bunch of designs in inches then it would be very inconvenient to change their existing customary approach and use meters.

 

[Dr.D]

Body weight is a mass measurement and is properly reported in pounds mass or kilograms mass. Anyone who thinks it should be reported in Newtons has taken one first year physics course too many.

How strange! Body weight is not a weight at all, is that what Mr. Briggs is saying? Why not call it a body mass and not confuse the issue? Why would you think it is intended as a mass measurement when the label clearly says "weight"?

 

[fresh_42]

I agree. My question was why we call it "weight" and whether Europeans do too.

At least Germany does. We buy kg and the scales announce kg. I don't think this is different in other countries. I never know whether I've bought 3 kg of potatoes or if they sold me 3 N. I hope it are 30 N, but doubts remain. This all dates back to the days where scales have been balance scales with standard pieces of iron as weight. So 3 kg potatoes is what equals 3 kg pieces of iron. In this sense, it is natural to say kg instead of Newton, because it is based on a comparison.

The spring scale at school showed Newton.

 

[Dale]

Inability or lack of confidence to read each others' work is a problem. Errors are a problem. Redudnant: Standardizing would eliminate errors caused by lack of standardization.

This is a problem, but it is a problem whose costs are difficult to estimate. It can be very expensive, as NASA famously found out. It makes it hard to factor in when making a decision to switch or not.

 

[russ_watters]

At least Germany does. We buy kg and the scales announce kg.

Again, not what I'm asking. I'm asking about the word "weight" vs the word "mass". I think you inadvertently answered, though:

This all dates back to the days where scales have been balance scales with standard pieces of iron as weight.

"weight". Did you mean to say "mass"?

This is why I have sympathy for the other view even though I know it is wrong. When I say "I weigh 150 lb", there are no contradictions: the scale says lb, which without a suffix is lbf, a force; a weight. It's a contradiction to say "I weigh 60 kg". Even worse is to say "I weighed-out and bought a pound of olives". I said "weight", I measured and listed force, but I really meant mass.

I suspect history is the resolution, though, like you say: the dictionary definition of "weight" includes both weight and mass, even if scientifically they are different. The word likely originates from before the concept of mass was known, and thus is still used to mean both colloquially even though it is now defined scientifically to be just one.

 

[russ_watters]

This is a problem, but it is a problem whose costs are difficult to estimate. It can be very expensive, as NASA famously found out. It makes it hard to factor in when making a decision to switch or not.

Agreed, though $125 million (the cost of that one error) is a pretty big number to start with and [google] was on its own a third of the conversion cost for NASA.

I have a bias toward fixing problems though, so if a problem is known to be big but can't be proven to be bigger than the solution, I'd still rather fix it than risk it.

 

[jbriggs444]

How strange! Body weight is not a weight at all, is that what Mr. Briggs is saying? Why not call it a body mass and not confuse the issue? Why would you think it is intended as a mass measurement when the label clearly says "weight"?

In commerce, "weight" means what a scientist would call mass.

The same practice applies in medicine. Dosages are typically in mg dose per kg mass, not mg dose per Newton downforce on the floor.

 

[fresh_42]

"weight". Did you mean to say "mass"?

I guess it is a mass, but people say weight(s) and that is what counts: the amount of de-balancing.
This has been the first Google entry as I searched for "Gewichte" (= weights).

weight (n.) (https://www.etymonline.com/search?q=weight)
Old English gewiht "weighing, weight, downward force of a body, heaviness," from Proto-Germanic *wihti- weight "get thinner" is recorded from 1961.

The German dictionary says, it comes from "wegan" = weighing which originally meant "to move (oneself)".
They say mass is over old German and Latin from Greek mãza = dough made from barley flour, flat bread.

However, if I tell someone my weight, I end up with kg. Some people use pounds, which is 500 g. Nobody is talking about mass, nor do they use Newton. And to make confusion complete: If you order 1/4 of some kind of sausage at the butcher, you will get 125 g.

 

[Tom.G]

Does that smallest item really say 500kg?? Core of a Neutron star maybe?

 

[OmCheeto]

Does that smallest item really say 500kg?? Core of a Neutron star maybe?

It appears to say "50dkg"

50 dekagrams = ½ kg