Units of measurement

Phil Corso

Senior Member
Larry.., thank you for asking for the rest of the story! Warning… it’s a 6-parter for trivia-buffs!
Once, for homework, I asked my physics students to research "horsepower!" A popular website was, http://www.absoluteastronomy.com

Part 1. History (at least the best story)
As part of a marketing ploy to sell his steam engine James Watt compared his invention's output to that of his biggest competitor at the time... the horse! He determined that the typical horse could turn a mill's 12-foot lever-arm 144 times in an hour. Furthermore, Watt found that the horse maintained a force of 180-pounds. Then, he calculated the "power" as force times distance divided by time as (180 x2x 3.14 x12) x (144/60.) The resultant, 32,572, was rounded off to 33,000 and was declared the Hp-unit of measurement or 33,000 ft-lb/min.

Part 2. Reconciliation with SI Units.
Let 1-foot = 0.3048 meter; and 1-lb = ma, for mass x acceleration, so,
1-lb(f) = (1lb)x(9.80665m/s^2)x(0.45359237 kg/lb), resulting in,
1-lb(f) = 4.44822 Newtons.

Part 3. Location Answer.
The English were responsible for determining "a", acceleration, thus, it was calculated for the longitude and latitude of Greenwich, England, Lon-0°, Lat-51.48°. Not to be outdone, the French calculated "a" at Lat-45°. Unfortunately, I don't know the nearest city to Lon-0°, Lat-45°! This led to Cv or "chevaux vapeur" for ‘steam-driven’ horses. Also known as the metric Hp, the Germans referred to it as Ps or "Pferdestarke!"

Part 4. "a" @ Lat-51.48°N (Greenwich, England) is used for Hp in SI-units, while "a" @ Lat-45°N (Someplace in France) is used for metric-horsepower Ps or Cv. (In fact ,there is a third designation, Ch.)

Part 5, Here's a trick to jog your memory... How is the discovery of America related to the conversion from kW to Hp? An easy way to remember the Watt per Hp constant, 746, is that it's half of 1492, the year of Columbus' (for which we celebrated his birthday, yesterday) great discovery. Well, I think of it as a great discovery! Of course, many wish he had gone east instead of west!

Part 6. The comment above reminds me of another "direction ploy" designed to make one's life easier: Back in the '60s NYC's Traffic-Commissioner Barnes, came up with a plan to eliminate NYC traffic snarls. He simply declared that at 5:00 pm, all north-south avenues would become one-way streets... north-bound. Then, traffic would become Westchester's problem!

Regards, Phil Corso
 
... One of the obstacles to changing the systems used in the US is cost ... You have say, a motor with a 2" drive shaft. To accommodate that, you need bearings that fit which won't be in SI units. ...
Not the greatest example, as 99% of ball & roller bearings have been integer-millimeter sizes for decades.

But to your point: There's no cost incurred by changing the label to read "50.8 mm" on replacement parts for antique machinery. And there's a lot of cost & chaos resulting from using two (or more) different measurement systems.
 

kwired

Electron manager
Location
NE Nebraska
Not the greatest example, as 99% of ball & roller bearings have been integer-millimeter sizes for decades.

But to your point: There's no cost incurred by changing the label to read "50.8 mm" on replacement parts for antique machinery. And there's a lot of cost & chaos resulting from using two (or more) different measurement systems.
That has been my observation also, most ball and roller bearings are cataloged by mm instead of inches, ID, OD and width, even those used on shafts that are cataloged by inches.

Even decades ago most of them were manufactured overseas AFAIK so probably not surprising they are designated in mm even if they are an exact inch/fraction of an inch measurement. But thousandth's of an inch are often critical on such things so precision matters regardless of which standard units are used.
 

Besoeker

Senior Member
Location
UK
Not the greatest example, as 99% of ball & roller bearings have been integer-millimeter sizes for decades.

But to your point: There's no cost incurred by changing the label to read "50.8 mm" on replacement parts for antique machinery. And there's a lot of cost & chaos resulting from using two (or more) different measurement systems.
The cost is in sourcing 50.8 mm units.
 

Phil Corso

Senior Member
Gentlemen... while still on the subject of "odd", "unusual", or "weird" unit-measurements, following are a few used in my career:

1) "Aluminum-Ball-Seconds"! A dimensional-unit used by Celanese Plastics Company, to measure the viscosity of melted cellophane!

2) "Tennis-Ball-Bounces"! A dimensional-unit used in an LNG-to-Gas plant to monitor the leak-rate in a cryogenic heat-exchanger's Perlite filled "cold-box" enclosure.


3) "Firkins per Fortnight"! Another dimensional-unit used in the LNG industry! But, description is not easy! It's "like describing a ratchet without using your hands",,, very difficult. So, contact me for details!


Regards, Phil Corso
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
3) "Firkins per Fortnight"! Another dimensional-unit used in the LNG industry! But, description is not easy! It's "like describing a ratchet without using your hands",,, very difficult. So, contact me for details!
Wait, someone actually uses that as a unit in practice, not simply as a joke??

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I know that.

But it has nothing whatsoever to do with selecting a system of measurement units.
I'd suggest that it does, in the sense that there is a very strong impetus to use the same system of units as your suppliers. If everyone around you uses inches, then you are pretty much forced to use inch measurements, or you will be subject to inflated costs associated with getting stuff custom made, smaller production runs, etc.

One of the reasons that the US gets away with sticking with inches is that they are such a large economy that it doesn't hurt them as much to buck the rest of the world.

There is a _huge_ political component in the selection of any standard. If we (meaning the members of this forum) got together and designed the 'perfect' system of units (simple physical relations (like SI), nice unit sizes, nice simple physical constants that were easy to measure for the fundamental units), it would go _nowhere_, because we don't have the political base.

-Jon
 

MAC702

Senior Member
Location
Clark County, NV
...The meter was redefined in terms of wavelengths of cesium light, the kilogram was defined in terms of a pure silicon sphere and so forth. ...
...and the inch was redefined to be 25.4 mm, EXACTLY. So we are on the Metric System, too...

Edit. Wow, this thread really got long while I was off. Someone beat me to it.
 

Besoeker

Senior Member
Location
UK
I'd suggest that it does, in the sense that there is a very strong impetus to use the same system of units as your suppliers. If everyone around you uses inches, then you are pretty much forced to use inch measurements, or you will be subject to inflated costs associated with getting stuff custom made, smaller production runs, etc.

One of the reasons that the US gets away with sticking with inches is that they are such a large economy that it doesn't hurt them as much to buck the rest of the world.

There is a _huge_ political component in the selection of any standard. If we (meaning the members of this forum) got together and designed the 'perfect' system of units (simple physical relations (like SI), nice unit sizes, nice simple physical constants that were easy to measure for the fundamental units), it would go _nowhere_, because we don't have the political base.

-Jon
Lot of common sense there, that man.....:thumbsup:
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Don't think so.
And the system in my original post is SI, not metric........:p
The US uses SI base units to define its system of measurements. This went back to the 1800s, where the US passed a law adopting the meter as the basic unit of length...but in the same law defined the yard as being 3600/3937 of a meter. (This made the inch 25.4000508....mm)

Later on the US changed the length definition to make the inch 25.4mm exactly. At the time this change was made the difference between the two definitions of the inch was small enough to not be significant in any industry...except surveying. So now we have the 'survey foot' which is tied to the meter using the relation 1200/3937 exactly and we have the regular foot which is tied to the meter by being 12*0.0254 exactly.

-Jon
 

Besoeker

Senior Member
Location
UK
The US uses SI base units to define its system of measurements. This went back to the 1800s, where the US passed a law adopting the meter as the basic unit of length...but in the same law defined the yard as being 3600/3937 of a meter. (This made the inch 25.4000508....mm)

Later on the US changed the length definition to make the inch 25.4mm exactly. At the time this change was made the difference between the two definitions of the inch was small enough to not be significant in any industry...except surveying. So now we have the 'survey foot' which is tied to the meter using the relation 1200/3937 exactly and we have the regular foot which is tied to the meter by being 12*0.0254 exactly.

-Jon
Yes, I recall the inch being redefined as 25.4 mm exactly. I seem to recall that and the back of out primary school arithmetic book it was defined as 2,5399...cm.
But it begs the question.......
Why define the units of one measurement system in terms of another instead of just using the other?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
But it begs the question.......
Why define the units of one measurement system in terms of another instead of just using the other?
Convenience and tradition.

The length of the meter is _defined_ by the speed of light. Why is the length selected to make the speed 299 792 458 m / s, rather than something nice like 300,000,000 m/s (or 1,000,000,000) meters per second?

Because once we learned how to define the length of the meter with the speed of light it was easier to make the new standard closely correspond to the old, rather than make everyone have to change their meter sticks.

-Jon
 
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