Unit conventions (SI versus others)

[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 Personally I would go even further as I consider the entire concept of nationalities outdated, but that's a different discussion.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

 

[fresh_42]

It appears to say "50dkg"

View attachment 227373

50 dekagrams = ½ kg

Still wrong, as $d$ means "deci" which is 0.1, so 50 dkg = 5 kg. Uranium maybe?

Edit: Just found out that $da$ for "deka" is relatively new and they formerly used indeed "(dk)g" for dekagram = 10 g. So it's all in all more a timestamp than a sample of interesting matter.

 

[gmax137]

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.

OK, I can't really disagree with you on that one.

How about world weightlifting records? Why is the record for the snatch (unlimited) shown as 220 kg? Surely that should be shown as 2150 N? I think even I could snatch 220 kg on the moon, for instance.

My real point is, arguments over which system of units is "better" are really just silly.

Just to round out the discussion of mechanic's tools, I have three sets - inch, metric, and Whitworth (for the old Brit motorcycles). The ironic thing is, people often point to Whitworth as "see, we have too many "standards'" while the fact is, Mr Whitworth was the guy who (in 1841) said "we need to standardize fasteners." https://en.wikipedia.org/wiki/Joseph_Whitworth

 

[DrDu]

I just found this hillarious example in an actual thread in the chemistry forum https://www.physicsforums.com/threads/volume-of-co2-from-tank-at-30psi.951374/:

Goal: How many gallons, at 30psi and 70°F, would a 20oz CO2 tank (assume properly filled with 20oz of gas) expand to fill? How many times could a 5 gallon tank be filled at 30psi from the 20oz tank?

 

[OmCheeto]

I just found this hillarious example in an actual thread in the chemistry forum https://www.physicsforums.com/threads/volume-of-co2-from-tank-at-30psi.951374/:

First time in a LONG time that I've seen the "Rankine" temperature scale used.

 

[fresh_42]

First time in a LONG time that I've seen the "Rankine" temperature scale used.

I like Delisle!

 

[Dr.D]

I spent at least a full day in total just by switching to or from foot, miles or gallons since I joined PF. What a waste.

Whatever in the world did you do this for? Why switch? If the information comes to you in SI, by all means, work the problem in SI. If the information comes to you in US Customary units, then converting it to SI is an utter waste of time. The point is simply that any coherent systems of units works, and there is no need to convert.

 

[OmCheeto]

... In this sense, yes, call me snobbish.

Me too. I do almost all of my calculations in SI now.
Perhaps sometime in the future, PF will require all problems to be presented in SI, just like we all have to speak English.
Maybe they can make an "Archaic Physics Units" sub-forum, just for the old-timers and historians.

btw, that was really interesting reading about the Delisle temperature scale at wiki, as I'm fairly certain I've never heard of it.
It was originally backwards!
Kind of like the "Apparent Magnitude" scale of celestial objects. I understand the historical reason for it, but it still annoys me.
They also pointed out 3 other temperature scales which I hadn't heard of: Newton, Réaumur, and Rømer.

The point is simply that any coherent systems of units works, and there is no need to convert.

If you're a savant, maybe, and can think in different systems.
I'm no savant, so I convert everything to SI. Base units. As missing a prefix can send your calculations off by hundreds of millions of years!

 

[fresh_42]

It was originally backwards!

Celcius was originally backwards! A friend of him turned it posthum!

 

[OmCheeto]

Celcius was originally backwards! A friend of him turned it posthum!

Someone should start a new thread; "When did science switch from the sport-like '#1 is the greatest! And higher numbers(2nd place, 3rd place) mean less. ' to whatever it is we have now?"

 

[dlgoff]

... while the real world would love to use e.g. Delisle

While doing a search on the Delisle scale, the Wikipedia information has this conversion chart I found interesting. Here's a scaled down image of it I made.

 

[Dr.D]

As missing a prefix can send your calculations off by hundreds of millions of years!

That right there points to one of my principle objections to SI as it is commonly (improperly) used. Proper SI says all lengths are in meters, and that works just fine. Right now, I'm dealing with a problem concerning machine parts with all dimensions in millimeters (yes, I can multiply by 10^3). The difficulties come when calculating mass moments of inertia, with density in kg/m^3, linear dimensions in mm, so suddenly there is a factor 10^15 that drops into everything. What a mess! If it was all dimensioned in inches with weights in pounds, then the MMOI calc would give lb-s^2-in in the usual way, with no massive fudge factors required. But its so much better, or so they tell me ...

 

[OmCheeto]

While doing a search on the Delisle scale, the Wikipedia information has this conversion chart I found interesting. Here's a scaled down image of it I made.

View attachment 228147

Doh! I guess I read through that section too fast. From that chart, the Delisle scale is STILL BACKWARDS!

The Celsius scale, like the Delisle scale, originally ran from zero for boiling water down to 100 for freezing water. This was reversed to its modern order after his death, in part at the instigation of Swedish botanist Carl Linnaeus and the manufacturer of Linnaeus thermometers, Daniel Ekström.​

 

[russ_watters]

That right there points to one of my principle objections to SI as it is commonly (improperly) used. Proper SI says all lengths are in meters, and that works just fine. Right now, I'm dealing with a problem concerning machine parts with all dimensions in millimeters (yes, I can multiply by 10^3). The difficulties come when calculating mass moments of inertia, with density in kg/m^3, linear dimensions in mm, so suddenly there is a factor 10^15 that drops into everything. What a mess!

This is nonsense. If all problems 'should' be done in the base units, there would be no reason for the prefixes to even exist. It's as if you are trying to prove your car is better than ours by beating ours with a baseball bat before comparing them. It says more about the chip on your shoulder than about the ease/difficulty of using the units.

If it was all dimensioned in inches with weights in pounds, then the MMOI calc would give lb-s^2-in in the usual way, with no massive fudge factors required.

Are you calling the prefix a fudge factor? Is 12 inches per foot a fudge factor? What if you did the problem in in^3 and you find the catalog for the part you need is in ft^3. Can you do that "fudge" in your head?

Caveat: Every industry has conventions for unit choices and rolled-up constants for common calculations. It may seem easier once you have them memorized, but it is only because they are memorized. A couple of days ago I had to dissect the rolled up constant 6356 for showing my work in a report. It took me an absurd amount of time.

But its so much better, or so they tell me ...

If you drive your car as actually recommended instead of driving it on the highway at 60mph in first gear, it will definitely work much better.

 

[russ_watters]

A bit more on that last example:
cfm x in wc / 6356 = horsepower

I literally used this equation for years thinking 6356 included a constant of nature* (like 1.082 does...) before I realized it was entirely comprised of IP unit conversions. And so I didn't realize the equation I was using was simply airflow x pressure = power. No conversions inherrent!...

...until I did a project in SI and found that m^3/s x Pa = Watts

It was a revelation! Physics makes sense as applied to engineering!

Two minor caveats:
-You of course need to know the named units. But it is as if ft-lb=horsepower, but without the conversion factor. Still easier.

-Can't avoid conversions with time: Often the flow rate is expressed per hour. But again, that issue is worse in IP. I once showed my boss a calculation with ton-hours per year as the unit and he kicked me out of his office! (it has three different units of time in it).

*An not for nothing, but SI is also designed to eliminate conversion factors with some constants of nature as well.

 

[jim hardy]

As a fellow who still keeps Whitworth wrenches
and uses Imperial unit steam tables
and considers himself practical
I agree with Dr D - we should be fluent in both systems.

i appreciate some of the simplicities of the Metric system
but i curse my Oldsmobile with its fasteners that are SAE on the drive train and Metric on the body.
The eye can pretty easily discern the 1/16 inch increments in SAE bolt heads , but a mm being only ~2/3 of that fools me most every time.

We EE's have it easy because our units are already SI (Amps Volts Ohms Farads Henries).
but I've always pitied ME's because their Ohm's Law involves local gravity, Reynolds Number, viscosity, density, a square root, and a discharge coefficient.

So i think you guys ought to ease up on Dr D . His point is simply that we should be multilingual if we wish to consider ourselves well rounded.

When nobody can read cursive anymore how will we proof-read printed versions of the Bill of Rights ?

Magnetics is a place you really need to be fluent - there's still Oersteds and Gilberts and Maxwells floating around. Every specialty develops its own shorthand with implicit 'fudge factors' .